Brennan D Soane

Bruce C Ball
email: bruce.ball@sac.ac.uk

Johan Arvidsson

Gottlieb Basch

Felix Moreno

Jean Roger- Estrade

No-till in northern, western and south western Europe: A review of problems and opportunities for crop production and the environment

Recent literature on no-till is reviewed with particular emphasis on research results and commercial uptake in northern, western and southwestern Europe. Increased interest in notill, as well as minimum or reduced tillage, is the result of changes in the economic circumstances of crop production, the opportunity to increase the area of more profitable autumn-sown crops and increased concern about environmental damage associated with soil inversion by ploughing. Highly contrasting soil and climate types within and between these regions exert a strong influence on the success of no-till. While no-till may often result in crop yields which equal or exceed those obtained after ploughing, modest reductions in yield may be tolerated if production costs are appreciably lower than with ploughing. The relative costs of fuel and herbicides have changed appreciably in recent years making no-till more attractive commercially. While effective weed control is an essential aspect of no-till, current herbicide technology may not yet fully achieve this.

No-till soils will usually have lower temperature and higher moisture content at the time of drilling, delaying drilling of spring-sown crops in northern regions. Their bulk density and bearing capacity are greater than for ploughed soils but the pronounced vertical orientation of macroporosity will allow encourage penetration of roots and water, especially in view of the increased population of deep-burrowing earthworms. Particular care must be taken to minimise soil damage at harvest and to ensure the even distribution of crop residues prior to drilling.

Reduced erosion and runoff under no-till are widely observed and are of particular importance in southwestern Europe. No-till reduces losses of phosphorus in runoff and the loss of nitrate through leaching. Emissions of greenhouse gases CO 2 and N 2 O from no-till soils are highly variable and depend on complex interactions of soil properties. Emission of CO 2 from fuel during machinery usage is always appreciably reduced with no-till. Increased soil organic carbon in surface layers of no-till soils may not be associated with increased carbon sequestration throughout the profile. All relevant factors must be included in the evaluation of the relative overall climate forcing effects of no-till and ploughing. Adoption of no-till could be encouraged by government financial assistance in recognition of environmental benefits, although future restrictions on the use of herbicides may be a deterrent. Opportunities for further research on no-till are outlined.

Introduction

Definition of no-till

No-till (also known as direct drilling and zero tillage) is a system in which crops are sown without any prior loosening of the soil by cultivation other than the very shallow disturbance (< 5 cm) which may arise by the passage of the drill coulters and after which usually at least 30% of the surface remains covered with plant residues.

Geographical and time limits of review

Priority is given to literature published since 2000 but much earlier research work was undertaken in Europe and is considered in previous reviews [START_REF] Cannell | The suitability of soils for sequential direct drilling of combine-harvested crops in Britain: a provisional classification[END_REF][START_REF] Basch | Alternativen zum traditionellen Landnutzungssystem im Alentejo/Portugal, unter besonderer Berücksichtigung der Bodenbearbeitung[END_REF][START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF][START_REF] Rasmussen | Experiments with no-inversion tillage systems in Scandinavia -Impacts on crop yields, soil structure and fertilization[END_REF][START_REF] Massé | Feasibility of minimum tillage practices for annual cropping systems in France[END_REF][START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF][START_REF] Linke | Direktsaat -Eine Bestandesaufnahme unter besonderer Berücksichtigung technischer, agronomischer und ökonomischer Aspekte[END_REF]Tebrügge and Böhrnsen, 1997a;[START_REF] Fernández-Quintanilla | Historia y evolución de los sistemas de laboreo. El laboreo de conservación[END_REF][START_REF] González | No tillage crop production in Spain[END_REF][START_REF] Tebrügge | Reducing tillage intensity -a review of results from longterm study in Germany[END_REF][START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF][START_REF] Mikkola | Direct drilling in Finland, a review[END_REF]. Although no-till research has been conducted in virtually all parts of Europe, we have concentrated attention on northern Europe (Finland and Scandinavia), western Europe (Netherlands, UK, France and Germany) and southwestern Europe (Spain and Portugal). This provides a transect from a boreal to a Mediterranean climate (Table 1) and covers a wide range of crops and production systems. The average growing season in Finland is limited by frost to May-September (Personal communication: L. Alakukku) while in northern Spain it is limited by shortage of available soil water to October-July (Fernández-Ugalde et al., 2009b).

The climate of much of northern and western Europe is broadly characterised by long, wet winters, late and often wet springs, cool moist summers, and an early return of autumn rains, sometimes before crop harvests have been completed or even commenced. Rainfall intensities are generally low and soil and water conservation problems rarely have a widespread influence on farming practice although, on susceptible soils and sloping ground, erosion can be serious locally as a result of occasional local high intensity rain or wind storms. Interest in no-till in this region has therefore been primarily to reduce costs and to increase area capability rather than the need to reduce soil erosion and runoff. No-till also has the ability to reduce the surface movement into water courses of particulate P and herbicides as pollutants.

These conditions contrast very markedly with those in southwestern Europe (roughly defined as latitudes below 43 o N) in which much of the summer growing season is hot and semi-arid and rain falls mainly in winter (Table 1), with some summer rain storms which can be highly erosive. In these areas soil and water conservation, with the maintenance of a crop residue mulch on the surface, have assumed a dominant role in the adoption of no-till, especially in seasons of unusually low (<300 mm) or high (>800 mm) annual rainfall [START_REF] Giráldez | Conservation tillage under extreme meteorological conditions in Southern Spain[END_REF]. Due to drought cropping in southwestern Europe in summer is often impossible in the absence of irrigation.

The values for water deficit (potential evapotranspiration -rainfall) shown in Table 1 are a useful guide to the likely problems arising with no-till. In northern and western Europe, water deficits during winter are usually negative indicating the high risks of soil anaerobiosis in poorly drained no-till soils. In south western Europe the high positive water deficits during the summer months confirm the benefits to be gained by water conservation opportunities with no-till. However, in some parts of south western Europe winter rainfall, though very unpredictable, may exceed evapotranspiration (Table 1) and soils will suffer from water logging (Basch and Carvalho, 1997).

Present state of no-till in Europe

It is now widely recognised that no-till can offer a number of economic and environmental advantages compared to mouldboard ploughing for the preparation of soils prior to crop establishment [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF]. The global uptake of no-till in 2009 was 105 Mha, an increase of 233% over the previous 10 years [START_REF] Derpsch | Development and current status of no-till adoption in the world[END_REF]. However, in spite of extensive pioneering research in Europe on no-till, particularly during the period , the uptake of no-till in Europe is currently very limited (Table 2) compared to its widespread use in crop production throughout North and South America and elsewhere [START_REF] Derpsch | Historical review of no-tillage cultivation of crops[END_REF][START_REF] Triplett | No-tillage crop production: A revolution in agriculture! Agron[END_REF][START_REF] Derpsch | Development and current status of no-till adoption in the world[END_REF]. In view of this, Europe is considered to be a "developing continent" with respect to no-till [START_REF] Basch | Europe: The developing continent regarding conservation agriculture[END_REF], a view supported by [START_REF] Derpsch | Development and current status of no-till adoption in the world[END_REF] in their survey of no-till world-wide. Nevertheless, this review will show that no-till research is now active in several countries of Europe and that there has also been a recent resurgence of commercial interest. [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF] report that the area of no-till in Finland increased rapidly from 1998 to reach 8-12% of the total area of cereals and oilseed crops by 2005 and 13% by 2008 (Alakukku et al., 2009b). [START_REF] Basch | No-tillage in Europe -State of the Art: Constraints and Perspectives[END_REF] Intermediate forms of non-ploughing or non-inversion tillage have often been adopted more readily than no-till [START_REF] Cannell | Reduced tillage in north-west Europe -A review[END_REF][START_REF] Tebrügge | Reducing tillage intensity -a review of results from longterm study in Germany[END_REF][START_REF] Munkholm | Minimum tillage demand for Danish sandy loams[END_REF][START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF] and are sometimes recommended as an introductory treatment before starting no-till or, along with ploughing, as an occasional remedial treatment if problems have arisen during a period under no-till. Surveys made in 2003/2004 and reported by IRENA (undated) indicate that the incidence of "conservation tillage" (reduced tillage and no-till) throughout the EU-15 countries varied between <5% in Ireland to 10-40% in Austria, Belgium, Germany, Finland, and France. Particularly rapid increases were noted in Portugal and Germany. We are here concerned primarily with continuous no-till. 1.4. Relative advantages and disadvantages of ploughing and no-till most likely to influence the successful uptake of no-till are shown diagrammatically in Fig. 1. The opinions and choices of farmers related to tillage will be dictated primarily by agronomic factors (Table 3), whereas environmental factors will be relevant to general concerns about soil and landscape protection and climate change. Ploughing may continue to be attractive, especially on smaller farms [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF] and where mixed husbandry of crops and animals is practiced, whereas large arable farms may become increasingly well suited to notill, as well as to intermediate forms of non-ploughing or non-inversion tillage involving reduced depth or intensity of disturbance [START_REF] Cannell | Reduced tillage in north-west Europe -A review[END_REF][START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF]. Fig. 1. A summary of climate (Sections 1 and 2), crop (Section 2), soil (Section 3) and environmental (Section 4) factors related to the sustainable uptake of no-till within European regions, with relevant sections in the paper identified. We review recent literature on no-till research and commercial uptake in northern, western and southwestern Europe and attempt to examine the reasons for the limited current commercial interest in no-till in these regions and to what extent this may be influenced in future by the application of new technology and anticipated changes in economic and environmental circumstances. The problems and opportunities of no-till are examined in the light of recent research findings and farmer experience, with emphasis on crop and soil responses and the environmental implications of adopting no-till in the contrasting climates and soils within these three regions of Europe.

Crop production with no-till

Crop residues and crop establishment

The presence of crop residues on the soil surface is a universal component of no-till now that the burning of residues is no longer practiced in Europe, the only exception being for forage crops for which residues on the surface may be virtually nil. Although the term "crop residues" normally applies to above ground portions of the crop excluding the harvested component, the below ground residues also have an important but often overlooked role in no-till studies.

The quantity of crop residues left on the surface after harvest varies with different crops. Straw yields for cereals are approximately equal to the grain yield, with average straw yields of 6.6 and 6.1 t ha -1 for winter wheat and winter barley, respectively in Germany but may reach 10 t ha -1 for high yield crops [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. In a normal year in Denmark the average production of cereal straw is about 3.5 t ha -1 [START_REF] Rasmussen | Straw management in various tillage systems in Denmark[END_REF] whereas [START_REF] Christian | Straw incorporation by different tillage systems on heavy clay soils in southern England[END_REF] report straw residues of 9 t ha -1 and [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF] mention straw yields in England and Wales in 1981 as high as 12.6 t ha -1 . Spring wheat and oats were found to have appreciabley larger amounts of crop residues than spring barley and gave problems for no-till sowing subsequently in Finland [START_REF] Känkänen | Growth and yield of different spring cereal species in zero tillage, compared to conventional tillage[END_REF].

A number of problems have been found in northern Europe when cereal crops were drilled in the presence of crop residues on the surface in contrast to the situation in south western Europe and other semi-arid areas where surface residues serve an essential function in soil and water conservation and promote higher yields [START_REF] Børresen | The effect of straw management and reduced tillage on soil properties and crop yields of spring-sown cereals on two loam soils in Norway[END_REF]. Importance is attached to the amount and form of residues, the height of cut, whether the residues are still attached to the crop roots, and whether still standing [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF]. These factors affect the drilling operation and also reduce access of solar radiation to the soil surface and the evaporation of water from the surface. These effects may cause serious delay in the drilling of spring-sown crops as the soil surface may be appreciably wetter and colder than when soils are ploughed prior to winter [START_REF] Hay | The effects of tillage, direct drilling and nitrogen fertiliser on soil temperature under a barley crop[END_REF], especially in Scandinavia or at high elevations as in Switzerland [START_REF] Anken | Long-term tillage system effects under moist cool conditions in Switzerland[END_REF]. In Finland importance is attached to the moisture content of the soil surface at the time of drilling into no-till soils. If it is high then drilling may have to delayed by up to a week after drilling into ploughed soils is possible, or even several weeks if soils are particularly wet [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF]. Such delays are often considered unacceptable by farmers [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF]. However, delayed crop germination with no-till in some years in Norway has been attributed to a marked reduction of topsoil temperatures under a surface mulch of crop residues [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF].

Seeds which have been forced into close contact to straw, a problem with some direct disc drills, can suffer from fungal phytotoxicity problems when the surface soil is wet. The results of extensive research on this problem in many parts of the world (reviewed by [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF] has led to the conclusion that drills used in no-till should ensure that crop residues and the planted seed are not in close proximity. Under dry conditions the close proximity of crop residues with seed may delay germination due to lack of seed/soil contact. Although now not practiced in Europe, the burning of crop residues was found to be advantageous in many experiments in the UK [START_REF] Cannell | Reduced tillage in north-west Europe -A review[END_REF] and Scandinavia [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF] and in practice by many farmers leading to blockage-free drilling operations, reduction in slug infestation, destruction of weed seeds on the surface and improvement in surface tilth. [START_REF] Cedell | Direktsådd av oljeväxter[END_REF] presented results from 47 experiments in Sweden with oilseed rape after spring barley 1980-1987. With burning of the straw (28 experiments), crop yield was 13% higher for no-till than for mouldboard ploughing, with removal of the straw 2% higher and with straw left on the soil surface 4% lower. Only in one experiment with burnt straw were crop yields lower with no-till than for mouldboard ploughing, suggesting that on the soils tested compaction was not a major factor affecting yields with no-till. In one long-term experiment on a clay soil, no-till was combined with different straw treatments during the period 1993-2009 [START_REF] Rydberg | Direct drilling[END_REF]. On average, crop yield for no-till with chopped straw was 16% lower than for mouldboard ploughing. Removing the straw and a shallow stubble cultivation before drilling resulted in the same yield as for mouldboard ploughing. In some areas, cereal straw is used as a source of biomass or, as in Scotland, used for animal bedding and feeding. In such situations stubbles in no-till are cut short and the straw removed prior to drilling [START_REF] Holmes | Effects of tillage, direct drilling and nitrogen in a long term barley monoculture system[END_REF]Ball et al., 1994b). Where animal production is not practiced, the residues of no-till cereal crops are best handled by chopping and spreading very evenly [START_REF] Böhrnsen | Influence of straw management by no-tillage winter rape and winter wheat production[END_REF][START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF]Böhrnsen, 1997a, Mikkola et al., 2005).

In south western Europe crop yields are generally appreciably lower than in western and northern Europe (see Table 4) and surface residue amounts under no-till will be correspondingly lower. [START_REF] Lampurlanés | Hydraulic conductivity, residue cover, and soil surface roughness under tillage systems in semi-arid conditions[END_REF] reported that the amount of surface residues plays an important role in soil water conservation under no-tillage in northeast Spain. Thus, when no-till fallow is used, a greater quantity of straw should be left on the soil at harvest of the previous crop to maintain a satisfactory residue cover for moisture conservation during the entire fallow period.

Crop yields and quality

Intensive research on crop yields with no-till has been conducted in most countries in Europe. An extensive list of quantitative and qualitative indicators comparing no-till with ploughing, based on the results of a large number of research projects, was presented by [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF]. Examples of crop yields observed in no-till research in various parts of Europe are summarised in Table 4. In general no-till gives crop yields within 5% of those obtained with ploughing but soil, crop and weather factors exert important influences. Yields of no-till crops tend to approach or exceed those after ploughing as the rainfall decreases from northern to southwestern Europe. For example, in northern Europe no-till yields rarely exceed those after ploughing [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF]Arvidsson, 2010a) and for spring-sown barley on a clay soil in Finland no-till yields were on average 5% lower than after ploughing (Alakukku et al., 2009b) although in many experiments in Finland no-till yields for spring barley, oats and wheat on clay soils were as low as 60-80% of yields after ploughing [START_REF] Känkänen | Growth and yield of different spring cereal species in zero tillage, compared to conventional tillage[END_REF]. In contrast, within areas of extreme aridity in northern Spain barley yields with no-till were sometimes twice those with conventional tillage (Fernández-Ugalde et al. (2009b). (1) J. Labreuche Personal communication, 2011, Arvalis Institute du Vegetal, Boigneville, France (2) The higher yields are for 2007 when conditions were wet, the lower yields are for 2008 when conditions were very dry (3) The upper yields refer to the average over 15 y for Spring barley and the lower yields refer to the average over 9 y for Winter barley

Extensive reviews of crop yields under no-till have been given for the UK by [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF] (175 experiment years) and for Germany by Tebrügge and Böhrnsen (1997a). In Spain trials of no-till compared to ploughing have been conducted in Andalucía [START_REF] Giráldez | II Jornadas Técnicas sobre cereales de Invierno[END_REF][START_REF] Pelegrín | The influence of tillage methods on soil physical properties and water balance for a typical crop rotation in SW Spain[END_REF], in central Spain (Hernánz andSánchez-Girón, 1981, 1994), and later in other areas of Spain [START_REF] Cantero-Martínez | Effect of different tillage systems in fallow and continuous crop of barley on soil water content in Mediterranean conditions[END_REF][START_REF] López | Soil and plant response to conservation tillage in rainfed barley production systems of Aragón, NE Spain[END_REF][START_REF] Cantero-Martínez | Effect of three different tillage systems on water use and root development of barley in a semiarid environment[END_REF]. Comparative studies of different tillage systems in Portugal, including no-till, started in 1984 in the Alentejo region [START_REF] Basch | Alternativen zum traditionellen Landnutzungssystem im Alentejo/Portugal, unter besonderer Berücksichtigung der Bodenbearbeitung[END_REF][START_REF] Carvalho | Experiences with direct drilling in Portugal[END_REF] and were later were extended to other regions (Basch and Teixeira, 2002). It was concluded that minimum tillage systems, but especially no-till, are capable of providing similar yields.

Mechanisms for yield reductions under no-till vary according to local conditions. On lighttextured soils in Denmark, compaction has been identified as the primary problem whereas in Sweden and Norway crop residues generally cause greater problems than compaction while in Finland no-till yield reductions have been attributed to crop residues, weeds and compaction under wet conditions. Crop yields immediately after adopting no-till are often appreciable lower than after ploughing but improve after about three years of no-till as soil structural conditions improve [START_REF] Ball | Cultivation and nitrogen requirements for continuous winter barley on a Gleysol and a Cambisol[END_REF][START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF][START_REF] Six | The potential to mitigate global warming with no-tillage management is only realized when practised in the long term[END_REF][START_REF] Anken | Nitrate leaching and soil structural properties under conventionally cultivated and no-till crops[END_REF]. [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF] report that winter barley yields during the first year of no-till were 90% of those after 17 years of no-till in the season, whereas the mean yield during the 2 nd to 7 th years of no-till averaged 98% of yields obtained after 18 to 23 years.

A number of reasons have been advanced to explain frequent conspicuously lower yields after the first one or two years of no-till, when compared to yields after ploughing, than observed subsequently. Among these are: 1) compaction from previous harvest traffic before soil strength and bearing capacity had increased; 2) limited time for build-up of soil structure improving factors under no-till (e.g. accumulation of organic matter, vertically orientated structure, stabilising influence of roots and fauna; 3) reduced N availability (see Section 2.5); 4) lack of knowledge.

The quality of harvested crops may be of equal economic importance to the yield yet aspects of quality are rarely reported in no-till studies. Stringent quality factors are relevant for grain crops grown for milling, brewing and animal feed [START_REF] Ball | Weed and pest control in various systems in Scotland[END_REF]. Under partially unsuitable conditions for no-till with spring barley they report that all grain quality indices were significantly lower with no-till than after ploughing. In two seasons grain water content at harvest was respectively 28 and 27 % for no-till and 19 and 21% after ploughing. This large difference would have resulted in an appreciable extra cost for drying the grain from no-till.

Crop and rotation suitability for no-till

In northern Europe no-till research has mainly concentrated on growing winter wheat, winter-and spring-sown barley. Although spring cereals are the dominant crop in boreal conditions in Finland (Alakukku et al., 2009b), winter-sown crops are generally more suited to no-till than spring-sown crops and no-till has the advantage of allowing these more profitable crops to be established when weather conditions often do not allow opportunity time for conventional ploughing (Soane and Ball, 1998;[START_REF] Mikkola | Direct drilling in Finland, a review[END_REF]. [START_REF] Rasmussen | Experiments with no-inversion tillage systems in Scandinavia -Impacts on crop yields, soil structure and fertilization[END_REF] reported results of no-till experiments throughout Scandinavia on soils ranging from 3-4% clay in western Denmark to 60-70% clay in Sweden and Finland. Yield reductions for spring-sown crops were often observed whereas yields of winter-sown crops generally were equal to those after ploughing. Many of these experiments were long-term (>5 years) in which cumulative effects of compaction could be identified and separated from seasonal variation of soil moisture [START_REF] Rasmussen | Pløjning, direkte såning og reduceret jordbearbetning till korn. (Ploughing, direct drilling and reduced tillage for cereals)[END_REF][START_REF] Munkholm | Spatial and temporal effects of direct drilling on soil structure in the seedling environment[END_REF].

In northern and western regions there are also considerable areas of other crops grown on wider row widths (mainly potatoes and sugar beet). Potatoes and sugar beet usually require deep and intensive tillage for high yields and their harvest, using heavy machinery, often occurs when soils have returned to a high water content approaching field capacity, causing considerable soil damage. However no-till production of potatoes [START_REF] Ekeberg | Tillage intensity effects on soil properties and crop yields in a long-term trial on morainic loam soil in southeast Norway[END_REF] and sugar beet (Personal communication: J.Labreuche, (2011), Arvalis Institute du Vegetal, France) may be possible in certain favourable conditions but on less stable soils in the Netherlands reductions in marketable yields of these crops of 20 and 15%, respectively, have been reported under no-till [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF], while in Germany.sugar beet under no-till on a Eutric Cambisol with 6% clay yielded 15% less than after ploughing (Tebrügge and Böhrnsen, 1997). The yields of non-graminaceous crops, such as fodder rape, cabbage and potatoes, in southeast Norway all increased in no-till and reduced tillage compared to ploughing on a morainic loam, rich in humus with high gravel and high porosity [START_REF] Ekeberg | Tillage intensity effects on soil properties and crop yields in a long-term trial on morainic loam soil in southeast Norway[END_REF].

In western Europe the more favourable weather conditions permit additional spring-sown crops such as maize for silage and grain and sunflowers to be grown and in Germany, the yields of field beans, rape seed and maize were equal for no-till and after ploughing [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF]. Grassland is an essential crop for many northern and western European countries and the re-seeding of pasture grasses often presents problems which may be avoided by no-till [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF]. There are also opportunities to establish arable crops after perennial pasture using no-till provided that the soil surface is not too uneven. Barley yields with no-till were almost equal to those after ploughing a pasture in Finland [START_REF] Lötjönen | Direct drilling of cereals after ley and slurry spreading[END_REF] though in Scotland [START_REF] Vinten | The effect of cultivation method, fertilizer input and previous sward type on organic C and N storage and gaseous losses under spring and winter barley following long-term leys[END_REF] found consistently lower barley yields after grass with no-till than after ploughing due to difficulties in weed control and frequent anaerobic conditions in the soil.

Where rotations are commonly used, the inclusion of a crop unsuited to no-till will make it impossible to gain the advantages of an uninterrupted sequence of no-till operations. However, the use of rotations in no-till cropping has particular relevance for weed control [START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF]. Crops which require much traffic of heavy harvesting machinery, such as potatoes and sugar beet, often when soils are wet in northern Europe, may cause difficulties for no-till establishment of the following crop. However, farmers in Germany and Switzerland are able to plant winter wheat after sugar beet with no-till provided that the previous harvest takes place when soils are not excessively wet.

Crop sequences may influence the choice of crops for no-till. For instance, in France the extension of maize growing to more northerly regions left less opportunity time for ploughing and the drilling of wheat. This favoured reduced tillage after maize before wheat but no-till did not prove so attractive to farmers for technical reasons. More recently trials have been conducted in France to test the suitability of no-till for sugar beet, rapeseed, potatoes, wheat and maize [START_REF] Massé | Feasibility of minimum tillage practices for annual cropping systems in France[END_REF][START_REF] Arrouays | Stocker du carbone dans les sols agricoles de France? Contribution à la lutte contre l'effet de serre[END_REF]. Winter wheat, winter barley, rapeseed and peas gave equal or improved yields under no-till but maize and sugar beet showed variable results.

In Germany crops such as legumes, sugar beet, rapeseed and silage maize account for some 30% of the arable land (Tebrügge and Böhrnsen (1997a) and leave very little residue on the surface after harvest and may thus present few problems for drilling subsequent crops provided that soil damage after harvest is not excessive. Winter wheat and rapeseed gave generally equal yield to that after ploughing in Switzerland but silage maize yield after no-till was only 87% of the ploughed yield [START_REF] Anken | Long-term tillage system effects under moist cool conditions in Switzerland[END_REF].

Soil suitability for no-till

No-till may be successful on certain soil types and sometimes quite unfavourable on others. Soils in the UK with imperfect drainage and weak structure generally led to lower yields with no-till than after ploughing, especially for spring-sown barley after wet winters. In recognition of the importance of soil characteristics, [START_REF] Cannell | The suitability of soils for sequential direct drilling of combine-harvested crops in Britain: a provisional classification[END_REF] classified the soils of the UK into three classes according to their perceived suitability for no-till and a map was produced showing their distribution. Class 1 soils were most suitable and included the selfmulching calcareous clays derived from limestone or chalk. Later, a Scottish group used a combination of inherent soil wetness and compactability to determine the choice of tillage to be used to establish winter barley in Scotland [START_REF] Ball | Provisional land grouping for selection of cultivation requirement for winter barley in Scotland[END_REF]. Throughout the UK good internal drainage was deemed a pre-requisite for reliable success with no-till, as in Germany [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. On undrained clay soil in England the no-till yield of oats was 3.4 t ha -1 compared to 6.1 t ha -1 after ploughing. The corresponding yields on drained plots were 7.0 and 7.2 t ha -1 , respectively [START_REF] Cannell | A study of mole drainage with simplified cultivations for autumn-sown crops on a clay soil. IV. A comparison of direct drilling and mouldboard ploughing on drained and undrained land on root and shoot growth, nutrient uptake and yield[END_REF]. The importance of soil type and permeability for successful no-till is emphasised in wet seasons. On a clay soil in Finland the yield of spring barley was 12% lower for no-till than after ploughing in wet seasons whereas on a silty clay no differences in yield were observed, even in wet seasons (Alakukku et al., 2009b).

Relatively unstable sands, silts, loams and clays, often of marine origin, in the Netherlands were found to be largely unsuited for no-till [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF]. Sandy soils under no-till in the Netherlands showed increased penetration resistance in the 5-30 cm depth due to compaction damage, reducing root penetration and yields, especially of root crops. On weakly structured coarse-textured soils in Denmark soil compaction acts as a severe limitation to the success of no-till. Periodic non-inversion loosening and cautious control of vehicle traffic is recommended for no-till soils [START_REF] Munkholm | Spatial and temporal effects of direct drilling on soil structure in the seedling environment[END_REF]. Sandy and sandy loam soils, especially if low in organic matter, may lack the ability to acquire a stabilised structure under no-till, and require regular loosening and are therefore excluded from soils considered to be suitable for no-till in Germany [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF].

Crop responses to soil nutrients with no-till

The responses of crops to the application and availability of plant nutrients on no-till soils have not been fully examined. Available P and K tend to become highly stratified near the soil surface under no-till but this does not apparently reduce their supply function [START_REF] Ehlers | Optimizing the components of the soil water balance by reduced and notillage[END_REF]Claupein, 1994, Peigné et al., 2007). Enhanced nutrient accumulation near the surface of notill soils can be due to decomposition of crop residues and to increased microbiological activity. [START_REF] Van Den Bossche | Effect of tillage intensity on N mineralization of different crop residues in a temperate climate[END_REF] studied the influence of the decomposition of crop residues as a source of nitrogen for crop plants with no-till compared to that of ploughing.

It has been widely observed that during the first 2-3 years of no-till crops require a higher level of N fertiliser to achieve the same level of yield as obtained with ploughing [START_REF] Frankinet | Regional experiences with reduced tillage in Belgium[END_REF][START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. Possible explanations for this effect are: (a) loss of available N due to denitrification to N 2 O (becoming less as structure improves; (b) reduced mineralization of organic matter in spring/autumn/winter (but maybe increased mineralization in summer [START_REF] Bäumer | Effects of nitrogen fertilization[END_REF][START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF]; (c) immobilisation of N in crop residues and;(d) limited uptake of soil N due to restricted root growth; (e) compensation effect which overcomes the effects of sub-optimal soil conditions leading to poor establishment [START_REF] Bäumer | Effects of nitrogen fertilization[END_REF][START_REF] Rasmussen | Experiments with no-inversion tillage systems in Scandinavia -Impacts on crop yields, soil structure and fertilization[END_REF]. After this initial period of no-till there may be opportunities to reduce fertilizer applications which would contribute to reduced overall production costs [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF].

Response to nitrogen fertilisers in no-till crops vary widely. [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF] showed that the yield of no-till oats was very much lower at low N applications than the yield for ploughing with no difference at high levels of N application, whereas the responses of winter wheat to applied N were virtually identical for no-till and ploughed treatments at all levels of N application. [START_REF] Massé | Feasibility of minimum tillage practices for annual cropping systems in France[END_REF] claimed that under dry soil conditions the yield of no-till winter wheat at high N applications could exceed those after ploughing as the reduced evaporation of moisture from the no-till soil gave the potential for higher optimum yields. Mean N offtakes by spring-sown no-till cereals on clay soils in Finland were 13-14% lower than for autumn-ploughed soils, even though applied fertilizer-N was 9% higher than on the ploughed soil (Alakukku et al., 2009b). This may have been due to greater denitrification in the no-till soil.

N mineralization from crop residues on or near the surface of no-till soils is slower and N immobilization is greater than when the residues are incorporated by ploughing (Van Den Bossch et al., 2009). These effects, in contrast to the above, have the potential for mineral N content to be higher with less nitrate leaching under no-till giving rise to greater N efficiency, and suggesting that after several years of no-till such soils may need less N fertilisation than ploughed soils [START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF]. However, urea applied to the surface of no-till soils was found to result in rapid and considerable volatilization of ammonia [START_REF] Rochette | Ammonia volatilization following surface application of urea to tilled and no-till soils: A laboratory comparison[END_REF]. This was attributed to the 4.2 times higher concentration of urease in no-till than in ploughed soil as well as the crop residues allowing decreased access of the urea to the soil surface. This discourages the surface application of urea on no-till soils. With increasing duration of no-till the amount of potentially minerizable N organic matter will increase which will compensate for the lower mineralization rate (and thus increase N supply to the crop. This is supported by results they reported for responses to N fertilizer in 1972-76 and 1980-81 for oats and winter wheat respectively, with very much higher yields at low N applications being obtained in the earlier period than in the later period in which there was no difference in response for no-till and ploughing.

For spring-sown no-till crops it has been widely considered that a lack of disturbance from cultivations in autumn will reduce nitrogen mineralization and nitrate leaching during the winter [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF] while the addition of crop residues to the soil surface will tend to reduce the level of soil mineral nitrogen due to their high C/N ratio encouraging immobilisation. In this way nitrate losses by leaching through the profile at times when the crop is neither present nor actively growing will be reduced [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF]. However, for winter-sown crops nitrate leaching on loamy sand and sandy loams in Denmark was not decreased by no-till or the presence of straw on the surface [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF].

Crop reliability under no-till in variable seasons

There is considerable evidence to show that in northern and western Europe crop yields under no-till are frequently lower than those after ploughing in wet seasons while there may be little or no difference than dry seasons [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF]Alakukku et al., 2009b). The weather throughout the year will influence soil water content, aeration and temperature and can thus be expected to have a marked influence on crop responses and yields in different seasons. Any lack of yield reliability will strongly influence farmer acceptability of no-till [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF]. Exceptionally wet seasons resulted in reduced yields of spring-sown cereals with no-till on clay soils in Finland (Alakukku et al., 2009b). Over four seasons on a Stagnogley clay soil in England the range of winter wheat yields for drained and undrained ploughed treatments were 54 and 75% of average while the corresponding range of values for no-till were 56 and 110% respectively, confirming that no-till on such a soil would result in very wide yield variation, especially when undrained [START_REF] Cannell | A study of mole drainage with simplified cultivations for autumn-sown crops on a clay soil. IV. A comparison of direct drilling and mouldboard ploughing on drained and undrained land on root and shoot growth, nutrient uptake and yield[END_REF]). There are a number of reasons why seasons of high rainfall tend to decrease the success of no-till in northern European conditions. However, difficulties have been found in interpreting the highly important influence of weather conditions on yield results under no-till, even in longterm trials (Soane and Ball, 1998). Apart from the importance of long-term trends in climate, there are often sequences of years in which, for instance, rainfall may be well above or below average rather than following a truly random distribution about a mean. If the duration of an experiment lies mainly within an "abnormal" sequence of weather, the results and their interpretation may be seriously biased.

In Scandinavia very dry spells at the time of sowing in autumn or spring may lead to poor establishment with no-till [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF], Schjønning et al., 2010a). During a 6-year period of comparing no-till (NT) with ploughing (CT) on a soil with 13% clay in Denmark, the yield ratio (NT/CT) of winter wheat varied from 55 to 121%, and the variation of NT and CT yields about the mean were 53 and 42%, respectively. This higher variability with NT, plus the fact that the mean yield with NT was only 84% of the CT mean yield does not encourage commercial uptake of no-till for that location.

In south western Europe no-till can result in an appreciable increase in available soil water in dry seasons. In southern Spain an almost linear inverse relationship has been found between annual rainfall and the ratio of sunflower yield for no-till to that for ploughing. This ratio varied from about 2.5 to about 0.9 for annual rainfalls of 240 and 470 mm, respectively [START_REF] Giráldez | Conservation tillage under extreme meteorological conditions in Southern Spain[END_REF].

Weeds. diseases and pests

Adoption of no-till introduces important changes to the incidence of weeds, crop diseases and pests, as well as the problem of volunteer cereals [START_REF] Ball | Weed and pest control in various systems in Scotland[END_REF][START_REF] Bräutigam | Influence of long-termed no-tillage on soil borne plant pathogens and on weeds[END_REF][START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF][START_REF] Christian | Strategies to control volunteer cereals in cereal rotations[END_REF]. Successful and economic control of these problems is a vital component in ensuring the commercial acceptability of no-till.

Incidence of weeds under no-till

Weed populations under no-till show marked differences from those after ploughing with new or previously unimportant weeds often becoming dominant after a period of no-till in which weed seeds are retained near the surface and their dormancy and germination characteristics will be quite different than if buried during ploughing [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF]. Perennial grass weeds (e.g. Elytrigia repens) are likely to be favoured by a no-till regime although in Finland E. repens is sometimes not a problem on heavy clay soil if there is little infestation initially whereas it normally becomes a serious problem on coarse textured soils [START_REF] Lötjönen | Direct drilling of cereals after ley and slurry spreading[END_REF]. Other grass weeds (e.g. Bromus sterilis and Alopecurus mysuroides) may present considerable difficulties in control due to their unusual reproductive mechanisms. In Scotland after 20 years of no-till the incidence of Bromus sterilis in a winter barley crop in ploughed and no-till plots was 2.5 and 59.7 plants m -2 respectively [START_REF] Ball | Weed and pest control in various systems in Scotland[END_REF]. The population of dicotyledenous weeds after no-till are generally similar to that after ploughing and can usually be effectively controlled by herbicides. The situation in southern Europe appears to be different from northern Europe as the long, dry summers restrict perennial weed growth and most of the weed species present are annual [START_REF] Basch | Conditions and feasibility of no tillage in Portugal[END_REF].

The shading provided by a heavy layer of crop residues with no-till can act as a deterrent for germination of weed seeds on the soil surface as well as inhibiting the early growth after germination of certain weed species on the soil surface. The growth of weed seedlings in spring with no-till winter wheat has been observed to be much slower than on ploughed soils suggesting that herbicide applications could be reduced in such circumstances [START_REF] Bräutigam | Influence of long-termed no-tillage on soil borne plant pathogens and on weeds[END_REF]. A surface mulch of crop residues can also reduce the temperature at or just below the soil surface which may adversely affect weed seed germination [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF].

Volunteer cereals, causing contamination and reduced quality of harvested grain, can be a problem with no-till in northern and central Europe, especially when winter crops are drilled soon after harvest [START_REF] Melander | A review of the major experiences with weeds in non-inversion tillage systems within the European Community (EEC)[END_REF]. [START_REF] Ball | Weed and pest control in various systems in Scotland[END_REF] report that volunteer barley in a following winter barley crop for ploughed and no-till land were 70 and 363 plants m -2 respectively. They can often be better controlled by ploughing than by the herbicides used in no-till [START_REF] Christian | Strategies to control volunteer cereals in cereal rotations[END_REF].

Many no-till research projects have been conducted with monoculture crop systems which have accentuated weed and pest problems. Rotations and cover crops are considered to be essential components in reducing weed problems and the dependence on herbicide usage in no-till systems [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF]Ball et al., 1994b;[START_REF] Derpsch | Development and current status of no-till adoption in the world[END_REF].

Herbicide usage

The availability of herbicides suitable for control of a wide range of dicotyledonous and monocotyledonous weed species is a paramount requirement for any no-till system. The introduction of glyphosate (Roundup) in 1971 brought many advantages but its effectiveness is reduced by low temperatures and frequent rainfall after application and additional herbicides may be needed. In a long-term no-till trial in Switzerland [START_REF] Anken | Long-term tillage system effects under moist cool conditions in Switzerland[END_REF] the number of herbicides used in addition to glyphosate for maize and wheat were four and eight, respectively.

In south western Europe, weed control under no-till with a pre-emergence herbicide application is effective in years with normal rainfall distribution as most of the weed seeds remain at the soil surface and germinate before the establishment of autumn-sown crops [START_REF] Calado | Weed management in no-till winter wheat (Triticum aestivum L.)[END_REF]. Thus, no-till crop establishment also reduces post-emergence infestation as buried seeds remain below the surface soil layer and do not germinate. Furthermore, the reduced weed pressure, together with a much better soil bearing capacity, which allows early herbicide application timings even in wet winters, enables the use of reduced herbicide rates to guarantee effective post-emergence weed control [START_REF] Barros | Effect of reduced doses of a post-emergence herbicide to control grass and broad-leaved weeds in no-till wheat under Mediterranean conditions[END_REF][START_REF] Barros | Effect of reduced doses of a post-emergence graminicide to control Avena sterilis L. and Lolium rigidum G. in no-till wheat under Mediterranean environment[END_REF]. However, a delay of the autumn rains may cause problems through retarded weed germination in already established winter crops. Also the lack of effective herbicides for spring-sown crops in southern Europe (for example, sunflowers and soya) may limit uptake of no-till for these crops.

Herbicide resistance

Herbicide resistance has been identified in several species of grass weeds in the UK such as black grass (Alopecurus myosuroides), wild oats (Avena fatua), and Italian ryegrass (Lolium perenne L) which could threaten the effectiveness of no-till systems dependent on herbicides [START_REF] Davies | Reduced Cultivations for Cereals: Research, Development and Advisory Needs under Changing Economic Circumstances. Home-Grown Cereals Authority[END_REF]. Resistance to atrazine, simazine and glyphosate has been widely reported for numerous weed species on no-till farms in the USA [START_REF] Triplett | No-tillage crop production: A revolution in agriculture! Agron[END_REF]. These problems, if widespread, may reduce the opportunities for adoption of no-till [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF]. There are thus opportunities for improvements in the use of existing herbicides and the adoption of new ones.

Crop pests

In humid climates the maintenance of crop residues on the surface tends to increase the slug population causing appreciable damage to young seedlings in winter-sown wheat and barley [START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF]. The use of mulluscicides may control slug populations but will increase production costs and may adversely affect beneficial soil biota which can play an important part in biological pest control mechanisms which could result in reduced pesticide requirement in no-till cropping [START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF]. The incidence of shoot and root damage in sugar beet by springtails (Onychiurus spp.) is reduced by no-till whereas the incidence of European corn borer (Ostrinia nubilalis) in maize after maize tends to increase with no-till [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF].

Crop diseases

Tillage practices in general have a limited effect on crop diseases [START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF] although the presence in no-till of a surface layer of crop residues has potential for the carryover of pathogens, such as leaf spot and root diseases from one season to the next [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF] especially when the same crop is grown consecutively. In Scandinavia leaf net blotch (Pyrenophoros teres) has increased after no-till [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF]) but generally increased crop diseases have not been widespread with no-till. In Scotland fungus snow-rot (Typhula), which affects winter-sown cereals, was less prevalent after no-till than after ploughing [START_REF] Ball | Weed and pest control in various systems in Scotland[END_REF]. Infestation by eyespot (Pseudocercosporella herpotrichoides) in winter wheat has been found to be appreciable lower with no-till than ploughing after 8 years but no differences were observed after 3 years [START_REF] Bräutigam | Influence of long-termed no-tillage on soil borne plant pathogens and on weeds[END_REF]. The concentration of organic matter near the surface seems to suppress some soilborne fungal crop diseases [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. Similar results were observed for take-all (Gäumannomyces graminis). Reduced tillage and no-till resulted in marked decreases in the incidence of clubroot (Plasmodiophora brassicae) in brassica crops in Norway but without explanation [START_REF] Ekeberg | Tillage intensity effects on soil properties and crop yields in a long-term trial on morainic loam soil in southeast Norway[END_REF]. [START_REF] Jordan | Influence of cultivation practices on arable crop pests, diseases and weeds and their control requirements[END_REF] report that the incidence of Barley Yellow Dwarf Virus on winter barley sown after no-till is only 2.7% compared to 57.3% after ploughing. They suggest two explanations, (1) the aphids which spread the virus do not recognise the young barley plants in the presence of residues and (2) the surface soil in no-till will harbour greater numbers of beneficial polyphagous predators (insects and spiders).

Economics and energy use with no-till cropping

Economics will dictate whether farmers find no-till an attractive alternative to ploughing but the components contributing to the overall decision are complex and research has seldom been undertaken to make a full analysis on a whole farm basis. Some of the relevant factors are listed in Table 5. With no-till the greater opportunity to grow more profitable winter-sown crops may be an important economic advantage. Tebrügge and Böhrnsen (1997b) surveyed the opinions of 111 farmers in 7 European countries and found that reduced working time and lower costs were the dominant reasons for adopting no-till. Examples of labour, herbicide and mechanization costs for crop establishment by no-till and ploughing (Table 6) given by [START_REF] Crochet | Simplification du travail du sol: un intérêt économique variable d'une exploitation à l'autre[END_REF] showed the total of these costs for no-till was 50% of that for ploughing.

Table 6

Cost of crop establishment for conventional and the no-till treatments in France (after [START_REF] Crochet | Simplification du travail du sol: un intérêt économique variable d'une exploitation à l'autre[END_REF].

Tillage system

Labour cost

(€ ha -1 ) Herbicide (€ ha -1 ) Mechanization 1 (€ ha -1 )
Plough, cultivate, drill 31 2 134

Spray, direct drill 10 7 67 1 Mechanisation cost includes equipment depreciation, maintenance and operating cost.

Reduced yields with no-till compared to ploughing are recognised as being acceptable if appreciable reductions in production costs can be achieved. Tebrügge and Böhrnsen (1997a) suggest that yield reductions with no-till up to 12-28% would be acceptable on a 500 ha farm. On 100-150 ha in Finland a yield depression of 10-15% with no-till would be economically acceptable [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF]. However, [START_REF] Khaledian | The inpacts of direct seeding into mulch on the energy balance of crop production system in the SE of France[END_REF] point out that sometimes the economic loss arising from any yield reduction in adopting no-till may well be greater than the reduced costs of production. Yield responses to no-till obtained in experiments may be biased since the quality of management on comparatively small plots may not be achievable in commercial farming (Soane and Ball, 1998;Ball et al., 1994a).

Studies on the economic implications of adopting no-till for winter wheat, winter rape, winter barley, and maize grain over a period of 4 years in the relatively dry, cool climate of northeast Germany (100 km north of Berlin) are reported by [START_REF] Verch | Comparing the profitability of tillage methods in Northeast Germany -A field trial from 2002 to 2005[END_REF] using experimental field data applied to a hypothetical 1000 ha farm and taking into account all relevant expenses. The calculated profit (averaged over crops and a 4-year period) for ploughing, reduced tillage and no-till were -7, 111 and 55 euros ha -1 respectively. The negative profit with ploughing arose because of the high costs of land preparation in relation to the prices obtained for crops. Wheat grown with no-till in one year resulted in the highest profit (468 euros ha -1 ) but large yield variation with no-till in some seasons reduced overall potential profit.

The costs of adopting a no-till system relative to those for ploughing will vary with time. Over the past 30 years the cost of gyphosate relative to the cost of diesel has decreased considerably. Details of savings in fuel consumption obtainable with no-till are given in Section 4.6.2.

Soil responses to no-till

Structure and composition

At the start of no-till experimentation in Europe there was no way of anticipating whether soil quality would improve as a result of increased biological activity and accumulation of organic matter or, on the other hand, deteriorate as a result of cumulative increases in compaction and reduced permeability. There were clear opposing mechanisms for improvement or deterioration of structure. As a result of much research [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF][START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF], a range of soil responses can now be anticipated after introduction of notill (Table 7). The stratification ratio, defined by [START_REF] Franzluebbers | Soil organic matter as an indication of soil quality[END_REF] as the ratio of a parameter value in the soil surface with that at a lower depth, may be used as an indicator of soil quality. Such indicators of soil quality were studied in long-term tillage studies in Spain by [START_REF] Imaz | Soil quality indicator response to tillage and residue management on semi-arid Mediterranean cropland[END_REF]. The most sensitive indicators were penetration resistance, particulate organic matter (POM), total organic matter and aggregate stability for the 0-5 and 5-15 cm layers. These factors were all positively correlated to soil water retention, earthworm activity and organic matter stratification, which were all greater with NT than with conventional tillage. The percentage of water resistant aggregates (at 0-20 cm, averaged for three crops over 6 years) for a clay loam soil in the Czech Republic was 27.8 % for ploughed soils and 45.9% for no-till [START_REF] Javůrek | Impacts of long-term reduced tillage use on soil carbon content and water stable aggregates in Ortic Luvisol, loam soil[END_REF]).

An accumulation of organic matter near the surface after a relatively short period of no-till was widely reported in early experiments [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF] and subsequent development of vertically oriented soil structural characteristics are attributed to increased earthworm population, increased aggregation, stability of old root channels, and natural pedological activity due to shrinkage and swelling in clays rich in montmorillonite. Such soils are essentially self-mulching, especially when calcareous. No-till also results in soils gaining bulk density, mechanical strength and aggregate stability. These cumulative effects, with an equilibrium usually not being reached in less than three years (Soane and Ball, 1998), mean that extrapolation from the results for the first year or two of experiments may be highly misleading. The accumulation of organic C near the surface of no-till soils is matched by a similar accumulation and stratification of total N.

Long-term adoption of conservation tillage (reduced and no-tillage) in Mediterranean areas of Spain has proven to be an effective way to increase soil organic matter and, especially, to improve biochemical quality at the soil surface [START_REF] Madejón | Effect of long-term conservation tillage on soil biochemical properties in Mediterranean Spanish areas[END_REF]. Changes in soil biochemical properties, and their distribution with depth, with conservation tillage will complement physical and chemical tests to evaluate the effect of conservation tillage in Mediterranean semi-arid conditions. [START_REF] Madejón | Effect of long-term conservation tillage on soil biochemical properties in Mediterranean Spanish areas[END_REF] have shown the importance of the stratified characteristics of the total organic carbon (TOC) with depth since the climatic conditions of the semi-arid Mediterranean areas are an important limiting factor for the accumulation of organic carbon in the topsoil layers.

Structure orientation

The development of increased vertical orientation of macro-porosity (30-300 μm) is an important characteristic of no-till soils. This arises, partly from the preservation of root and earthworm channels and partly from natural structure development in a vertical orientation in the absence of disruption from ploughing. Vertically orientated macro-porosity with no-till gives rise to increased air and water permeability throughout the profile even when the differences in macroporosity volume show little differences [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF].

Rate of structural change

Changes in some structural properties after the introduction of no-till may be measurable within a few months (bulk density, soil strength) but may not be fully developed for 3-5 years or longer. After the introduction of no-till on loess soil in Germany, the increase in organic C content in the 0-30 cm layer continued until a maximum was reached at 8 to 10 years [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. [START_REF] Munkholm | Spatial and temporal effects of direct drilling on soil structure in the seedling environment[END_REF] found that on a sandy loam in Denmark, bulk density and penetration resistance below 4 cm were appreciably higher for no-till than ploughed soil within the first year and further increased during the second year but differences remained stable after three years. Initial increases in bulk density near the surface of no-till soils may subsequently (after 6 years) decline to values equal to or even less than that after ploughing [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF].

Changes in soil acidity

Increases of acidity in surface layers of soils under no-till have been widely reported and are usually associated with the acidifying effect of nitrification of ammoniacal fertilisers and the decomposition of crop residues. On well-drained morainic loam in Norway, the pH of topsoil (0-5 cm) after 13 years of no-till was 6.25 compared to 6.48 for ploughing and the lime requirement to rectify this was estimated to be 130 kg CaO ha -1 y -1 [START_REF] Ekeberg | Tillage intensity effects on soil properties and crop yields in a long-term trial on morainic loam soil in southeast Norway[END_REF]. Such increases of acidity are approaching the levels at which reductions in the availability to crops of soil N, P and K would occur. Reductions in pH may also occur on alkaline soils in south western Europe. For instance, after 6 years of no-till on a vertic Cambisol in Portugal a drop in pH from 8.1 to 7.5 was found for the 0-10 cm depth [START_REF] Carvalho | Long term effects of two different soil tillage treatments on a Vertisol in Alentejo region of Portugal[END_REF].

Soil hydrology

After the introduction of no-till, changes can be expected in evaporation of water from the surface, infiltration rate and hydraulic conductivity as a result of the different soil physical properties, particularly increased organic matter near the surface and increased vertically orientated macrostructure throughout the profile [START_REF] Strudley | Tillage effects on soil hydraulic properties in space and time: State of the science[END_REF].

Infiltration

The infiltration rate of no-till soils is usually, but not always, found to be appreciably higher than in ploughed soils. Infiltration rate increases have been observed and have been attributed to protection by residues of the surface from raindrop impact, the stability of aggregates near the surface and continuity between the surface and the sub-surface layers of vertically orientated macroporosity [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF][START_REF] Ehlers | Optimizing the components of the soil water balance by reduced and notillage[END_REF]. Infiltration rate may increase with time after adoption of no-till, along with markedly higher values on no-till than on ploughed soil [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF]. These effects are usually attributed to the greater aggregate stability, SOC and protective mulch normally found on no-till soils [START_REF] Lampurlanés | Hydraulic conductivity, residue cover, and soil surface roughness under tillage systems in semi-arid conditions[END_REF]. However, infiltration rate depends closely on the permeability of the surface few mm and the connectivity of pores in that layer with that of lower layers. The continuity of earthworm burrows from the surface to the subsoil under notill contributes to increased infiltration and reduced runoff. Any type of "pan" or crust at the surface of a no-till soil, perhaps induced by wheel traffic when the surface layer was wet or by raindrop impact when the surface mulch was incomplete, will much reduce the infiltration rate [START_REF] Armand | Runoff generation related to intra-field soil surface characteristics variability. Application to conservation tillage context[END_REF]. After 18 years of no-till on silt loam soil in Poland, [START_REF] Lipiec | Soil porosity and water infiltration as influenced by tillage methods[END_REF] found that the cumulative infiltration for no-till was reduced by 61% of the corresponding value for ploughed soil. The greater infiltration on the ploughed soil correlated to greater pore continuity than in the no-till soil. This unusual result, which they attributed to a stable aggregate structure and optimum moisture conditions at the time of tillage, illustrates the variability of conditions found in no-till soil. To gain more appropriate infiltration data, measurements on no-till soils may have to be sited in relation to the horizontal distribution of mulch, surface soil exposure, and any evident wheel tracks [START_REF] Armand | Runoff generation related to intra-field soil surface characteristics variability. Application to conservation tillage context[END_REF].

Water retention

After 6 years of no-till on a silty loam soil in Germany, differences in water retention between no-till and ploughed land were very small and crop yields were identical [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF]. However, in the semi-arid climate of northeastern Spain, the much higher barley yields under no-till (2000 kg ha -1 ) than conventional tillage (1000 kg ha -1 ) in dry years is attributed by Fernández-Ugalde et al. (2009b) to the ability of no-till to increase plant available water (held between -33 and -1500 kPa). Plant available water at 0-5, 5-15 and 15-30 cm was 11.7, 18.1 and 26.6 m 3 100m -3 for no-till and 7.9, 14.8 and 20.9 m 3 100m -3 for soil chisel-ploughed to 15 cm.

Permeability and deep drainage

After a preliminary period of establishment, the vertically orientated structure and stabilised earthworm and root channels in no-till soils contribute to increased hydraulic conductivity. Measurements of hydraulic conductivity after 8 years of no-till on a silty loam soil in Germany were higher than after ploughing [START_REF] Vogeler | Effect of tillage systems and P-fertilization on soil physical and chemical properties, crop yield and nutrient uptake[END_REF]. Increased downward movement of water under no-till and decreased runoff during periods of high rainfall may result in water tables being higher and oxygen concentrations being lower than for ploughed soils under certain circumstances (Basch and Carvalho, 1997).

Damage to un-cultivated soils from vehicle traffic

In the absence of any loosening from tillage, the physical properties of the topsoil after a period of no-till will be influenced by the passage of loaded wheel or tracks, particularly during harvesting operations. The amount of damage during such traffic is a product of the soil bearing capacity (or compactability) and the mechanical characteristics of the tyres or tracks. Excessive compaction of topsoil is considered a major problem in adoption of no-till in Denmark [START_REF] Munkholm | Spatial and temporal effects of direct drilling on soil structure in the seedling environment[END_REF]. Soils under no-till may show the influence of residual ploughpans attributable to the passage of the ploughshare and in-furrow tractor wheel traffic prior to the adoption of no-till. If not loosened prior to commencement of no-till this effect may still be observed even after 5 years from the cessation of ploughing [START_REF] Capowiez | The effect of tillage type and cropping system on earthworm communities, macroporosity and water infiltration[END_REF] and will influence crop and soil responses to no-till.

Bearing capacity and strength

No-till soils initially will be readily compacted and will, within a year or two, show appreciable higher bulk density and strength (penetration resistance) as a result of vehicle traffic and have a much greater bearing capacity than ploughed soils [START_REF] Tebrügge | Soil structure and trafficability after long-term application of no-tillage[END_REF][START_REF] Ehlers | Optimizing the components of the soil water balance by reduced and notillage[END_REF]. This effect usually occurs throughout the depth at which loosening would have occurred during ploughing [START_REF] Boguzas | Effect of reduced and notillage, straw and green manure management on soil physical properties and earthworms[END_REF].

Unless very wet, no-till soils may show an elastic non-compactive behaviour at normal levels of loading and after a number of years acquire some of the properties of grassland soils with a high bearing capacity [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. However, periods of high soil moisture and much reduced bearing capacity occur quite frequently in northern and western Europe at the time of harvest for several crops, particularly late harvested sugar beet, potatoes and silage maize. During such periods soil compaction on no-till and very shallow cultivated soils may be considerable and subsequent ploughing may be necessary to achieve maximum crop yield [START_REF] Koch | Cumulative effect of annually repeated passes of heavy agricultural machinery on soil structural properties and sugar beet yield under two tillage systems[END_REF].

3.4.2. Amount and nature of traffic over no-till soils [START_REF] Tebrügge | Soil structure and trafficability after long-term application of no-tillage[END_REF] showed that the amount of traffic employed for crop establishment (expressed by driving distance per ha, % area covered and load index) is appreciably greater for a ploughed than a no-till system. Wheel traffic of heavy machinery over moist, uncultivated soils, especially at harvest of crops such as maize, sugar beet and potatoes, and of cereals in wet autumns, can cause substantial compaction to a depth of 20-30 cm and sometimes deeper. The risks of serious soil damage at depths up to 30 cm during sugar beet harvesting was demonstrated by [START_REF] Koch | Cumulative effect of annually repeated passes of heavy agricultural machinery on soil structural properties and sugar beet yield under two tillage systems[END_REF] after ploughing and shallow cultivation using a 6-row self-propelled harvester with a total weight of 34-40 t and wheel loads of 7.9 to 11.7 t. Although a no-till treatment was not included, it seems very unlikely that a no-till soil could resist such loading without damage.

There may also be considerable churning and rutting of surface soils at harvest, leaving some areas of fields prone to flooding. These conditions, more prevalent in northern Europe, may render a soil quite unsuitable for subsequent no-till drilling and have tended to promote a "vicious cycle of ploughing-compaction and more ploughing" [START_REF] Lal | Thematic evolution of ISTRO: transition in scientific issues and research focus from 1955 to 2000[END_REF]. However, a period of no-till may appreciably increase the trafficabilty of some wet soils through the maintenance of vertically orientated macroporosity [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF], and thus reduce soil damage at harvest. If the surface is level the high strength of no-till soils can allow faster drilling.

In spite of the benefits of increased organic matter near the surface, a prolonged regime of no-till in humid climates renders the surface soil liable to suffer from repetitive loading from vehicle traffic. [START_REF] Ball | Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments[END_REF] found that soil strengths near the surface of no-tilled soils in the UK were high enough to restrict root growth and seedling emergence. The effect on winter barley grain yield of different numbers of wheal passes over no-till and ploughed soil [START_REF] Campbell | Controlled traffic after ploughing or direct drilling under winter barley in Scotland 1980-1984[END_REF] showed that yield depression at 4 and 6 passes was less for no-till soils than for ploughed soils suggesting that the no-till soil was more resistant to damage from heavy traffic than ploughed soils.

There is a continuing trend for farm machinery to become larger and heavier especially for harvesting equipment, with the weight of sugar beet harvesters reaching 60 tons. The loads imposed by potato and sugar beet harvesters, with contact pressures often in excess of 200 kPa, are highly damaging, in spite of the use of dual or wide, low pressure tyres. In western Norway potato harvesting operations have to be followed by remedial deep ploughing to avoid erosion risks due to soil impermeability [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF]. The harvesting of silage maize in Switzerland caused structural damage to the topsoil of a Luvisol when under no-till which was not relieved by natural regenerative processes and resulted in anaerobic conditions during periods of high rainfall and greatly reduced yield relative to ploughing [START_REF] Anken | Nitrate leaching and soil structural properties under conventionally cultivated and no-till crops[END_REF].

The use of any form of traffic control for sowing, spraying or at harvest is even more appropriate under no-till than under ploughing, especially on soils prone to wetness [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF]. Tramlines are now widely adopted as a means to control postdrilling traffic distribution in cereals. Wide-frame tool carriers running on permanent "wheelways" or modular controlled traffic farming (CTF) systems to reduce soil compaction may have particular relevance under no-till systems. Several experiments on traffic-free systems have shown appreciable added benefits in terms of soil quality, crop yields and soil biota where no-till has been employed rather than ploughing [START_REF] Campbell | Controlled traffic after ploughing or direct drilling under winter barley in Scotland 1980-1984[END_REF][START_REF] Chamen | Prevention strategies for traffic-induced subsoil compaction: a review Part 2. Equipment and field practices[END_REF][START_REF] Tullberg | Controlled traffic farming -From research to adoption in Australia[END_REF]. The adoption of combined no-till and controlled traffic farming has become the norm in certain parts of Australia (Personal communication: J.Tullberg) but CTF systems have a number of problems still to be elucidated in Europe (Schjønning et al., 2010b).

No-till crop production is not necessarily excluded on farms having an animal component but there will be additional risks of soil damage due to vehicle traffic during the distribution of slurry and solid manure and also from animal grazing, especially in northern regions where high soil moisture levels may be common during winter months. In Denmark slurry distribution is commonly undertaken using a tractor/trailer system having a combined weight as high as 50-55 t with axle lads of 13-15 t which would be expected to cause appreciable compaction in the subsoil as well as throughout the topsoil (Schjønning et al., 2010b). It seems unlikely that such a system would be appropriate within a no-till regime. [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF] report that slurry was applied on no-till soils by trailing hoses over winter cereals and was injected prior to sowing of spring crops. Both operations represent additional wheel traffic over no-till soils when, because of high moisture content, they are liable to be readily damaged. The application of slurry on no-till coarse textured soil in Finland often resulted in lower barley yields with no-till than after ploughing [START_REF] Lötjönen | Direct drilling of cereals after ley and slurry spreading[END_REF]. This was partially attributed to ammonia volatilization due to lack of incorporation with the topsoil which could be overcome by using a slurry injector or shallow cultivation.

Soil biodiversity

In addition to soil microbial activity, soil macro-, meso-and micro-fauna are now recognised as important components in soil health and no-till regimes have widely been found to increase the biodiversity of soils [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF].

Microbial population and activity

No-till affects the habitat and activity of a large range of soil micro-flora and -fauna and enzyme concentrations. [START_REF] Brito | Effects of soil management on the arbuscular mycorrhizal fungi in fall-sown crops in Mediterranean climates[END_REF] report that in a Mediterranean climate the roots of wheat plants grown under no-till had a 6-fold greater level of mycorrhizal colonization than did those grown under a ploughing regime. [START_REF] Javůrek | Changes of biological actiovity in soil after 10 years of minimum and no soil tillage for stand establishment[END_REF] studied the distribution of microbial biomass, number of bacteria, azotobacter, dehydrogenase, urease, invertase, and oxidizable C over a 5-year period after 10 years of no-till and ploughing. Microbial biomass and enzyme activity of β-glucosidase and urease have all found to be higher on silt loam notill soils in Belgium than on ploughed soils [START_REF] Van Den Bossche | Effect of tillage intensity on N mineralization of different crop residues in a temperate climate[END_REF]. The value of soil respiration rate as an index of microbial activity in no-till soils was reviewed by [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF]. At shallow depths higher microbial activity in no till soils is associated with higher SOC, while at deeper depths the causative factor may be greater earthworm activity.

Earthworms and other macro-fauna

Earthworm populations are invariably higher under no-till than under ploughing and increase with the duration of no-tillage [START_REF] Gerard | The effect on earthworms of plouging, tined cultivation, direct drilling and nitrogen in a barley monoculture system[END_REF][START_REF] Kladivko | Tillage systems and soil ecology[END_REF][START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF][START_REF] Anken | Long-term tillage system effects under moist cool conditions in Switzerland[END_REF][START_REF] Boguzas | Effect of reduced and notillage, straw and green manure management on soil physical properties and earthworms[END_REF]Pelosi et el. 2009;[START_REF] Peigné | Earthworm populations under different tillage systems in organic farming[END_REF]. In Italy and Germany the numbers of earthworms was found to be up to 8 times higher after a period of no-till while total earthworm biomass was 38 and 176 g m -2 for ploughing and notill, respectively [START_REF] Ehlers | Approaches toward conservation tillage in Germany[END_REF]. Significant differences are found in the species and habits of earthworms. In Italy the proportion of Allolobophora spp. and Octodrillus spp.

were much greater than in Germany [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF]. In a three-year experiment northern France, [START_REF] Pelosi | Earthworm community in conventional, organic and direct seeding with living mulch cropping systems[END_REF] found deep burrowing anecic earthworms were 3.75-fold more numerous in the no-till treatment, while endogeic species were 2.82-fold more numerous in the conventional treatment. After a period of no-till, the anecic group make a major contribution to improvements of soil structure, particularly infiltration and hydraulic conductivity. Species of earthworms feeding on surface residues and living near the surface may be strongly influenced by the type and amount of residues and by the type of direct drill employed [START_REF] Baker | No-tillage Seeding Science into Practice[END_REF].

The casting activity of earthworms below the soil surface contributes to greater aggregate stability, especially at about 12 cm depth in no-till soils [START_REF] Bottinelli | Abundance and stability of belowground earthworm casts influenced by tillage intensity and depth[END_REF][START_REF] Melero | Long-term effects of conservation tillage on organic fractions in two soils of southwest Spain[END_REF] have shown that long-term soil conservation management (reduced and no-tillage) in southern Spain improved the quality of Entisols and Vertisols through enhancing the soil organic carbon fraction and soil biological status, especially in the upper layers. These changes are of great importance to organic matter turnover and nutrient availability under semi-arid Mediterranean climatic conditions and can have a major influence on the upward and downward movement of water, gases and chemicals, the growth of roots and the metabolism of soil biota. Earthworm populations and their effects on macrostructure are best studied in long-term experiments [START_REF] Peigné | Earthworm populations under different tillage systems in organic farming[END_REF].

Under semi-arid Mediterranean conditions in Tunisia quadrat sampling after 3-5 years of no-till after ploughing showed that the number of species of invertebrates increased from 26 to 34 species and that the total population numbers increased from 61 to 319 individuals [START_REF] Errouisi | Soil invertebrates in durum wheat (Triticale durum L.) cropping system under Mediterranean semi arid conditions: A comparison between conventional and no-tillage management[END_REF]. In particular, the protected environment of the no-till soil encouraged major increases in the numbers of earthworms and arthropods and hence contributed to enhanced breakdown of organic matter.

The populations of beetles and a wide range of other soil fauna [START_REF] Derpsch | Implications of no-tillage versus soil preparation on sustainability of agricultural production[END_REF] are associated with the stabilised structure of no-till soils, the greater SOM and moisture content near the surface and the lack of destruction of bio-structures such as nests, channels, fungal hyphae networks and pathways which occurs with a ploughing regime [START_REF] Triplett | No-tillage crop production: A revolution in agriculture! Agron[END_REF]. The numbers of beetles was found to increase under no-till in Scotland [START_REF] Ball | Influence of no-tillage on physical, chemical and biological soil properties[END_REF].

4.

Environmental effects of no-till

Increased importance of environmental factors

No-till has been found to have a number of environmental advantages in certain circumstances [START_REF] Düring | Environmental aspects of no-tillage application -Erosion and leaching of agrochemicals[END_REF] which are not necessarily directly related to the immediate economic factors which influence commercial uptake. Nevertheless, they are likely to have increasing importance as concerns about soil and landscape protection assume greater significance. Of particular importance are herbicide dispersal, erosion, P dispersal, eutrophication, nitrate leaching and greenhouse gas emissions [START_REF] Davies | Reduced Cultivations for Cereals: Research, Development and Advisory Needs under Changing Economic Circumstances. Home-Grown Cereals Authority[END_REF].

Environmental consequences of herbicide usage

Opportunities for reducing herbicide use

The widespread dependence of no-till on high and regular applications of herbicides has raised apprehension as to the fate of applied herbicides and the environmental consequences. Herbicide usage should be reduced to the minimum consistent with desired level of weed control. Split applications may be more effective than a single application of the same amount. In France, following the publication of "Grenelle de L'Environnement", there is a national programme with the objective of reducing herbicide use by 50% by 2018 and there are various possibilities The weed seed bank on the soil surface can be reduced by employing a chaff catcher below combine harvesters and occasional remedial tillage operations may overcome certain weed species which have become dominant after several years of no-till. Herbicide usage can be minimised by the greater use of suitable crop rotations.

Herbicide dispersal and degradation in no-till soils

Gyphosate, the most widely used herbicide in no-till systems, has a low ecotoxicity [START_REF] Düring | Glyphosate: Importance in no-till agriculture with regard to environmental side-effects[END_REF] and is usually strongly absorbed on contact with soil and is then subject to microbial degradation but adsorption on to soluble humic acids may result in some leaching within profiles. Herbicides are more actively biodegraded during summer in no-till soils than in ploughed soils due to a greater quantity and activity of microorganisms and increased organic matter near the surface after no-till [START_REF] Düring | Environmental aspects of no-tillage application -Erosion and leaching of agrochemicals[END_REF].

The widespread use of glyphosate in no-till practice would not seem to present an environmental problem but other herbicides are less strongly adsorbed, and the presence of large macropores in no-till soil profiles may increase the risk of herbicide leaching [START_REF] Greener | Soil tillage effects on surface water pesticide concentrations[END_REF]. Studies on the transport and persistence of herbicides in soils under conservation tillage (reduced tillage and no-till) in Spain [START_REF] Cox | Mobility of metamitron in soils under conventional and reduced tillage[END_REF][START_REF] Cox | Leaching of clopyralid and metamitron under conventional and reduced tillage systems[END_REF][START_REF] Cuevas | Assessing herbicide leaching from field measurements and laboratory experiments[END_REF] showed that the mobility and persistence of certain herbicides (e.g., trifluralin and metmitron) were lower under conservation tillage than under conventional tillage.

Herbicides may enter water courses both by seepage in ground water and by transfer adsorbed on to soil particles and leaving fields through water or wind erosion. In France, [START_REF] Réal | Transfer de produits phytosanitaires par drainage, ruissellement ou percolation[END_REF] analyzed the presence of many pesticides in field drainage networks in four areas of France. They found that when persistent herbicides and fungicides were applied in spring to maize and wheat respectively, the transfer by drainage in winter was less after notill than after ploughing. [START_REF] Basch | Displacement of herbicides under different tillage systems[END_REF] measured the concentrations of atrazine, metolachlor and isoproturon in runoff in Portugal. With the exception of the first runoff event, the amount of herbicides in the runoff from conventional tillage was always higher than in the runoff under no-till and the time between application and the first heavy rain is crucial for the risk of herbicide dissipation in runoff, especially under no-till where a considerable proportion of the herbicide is retained in the surface residues. Similar results were found at other experimental sites in Europe which participated in an EU project on "Effects of tillage systems on herbicide dissipation" [START_REF] Borin | Effects of tillage systems on herbicide dissipationan experimental approach at field scale[END_REF] but there is a need to recognise the widely differing amounts of herbicides used in no-till and ploughing systems.

Erosion and runoff

Erosion and runoff have been identified as important problems throughout Europe needing control, especially in the Mediterranean region [START_REF] Düring | Environmental aspects of no-tillage application -Erosion and leaching of agrochemicals[END_REF][START_REF] Montanarella | European Soil Protection Strategies[END_REF][START_REF] Cerdà | Soil erosion and agriculture (Editorial)[END_REF], although the intensity is generally appreciably less than that experienced in North and South America and the tropics and erosion was not an incentive for adopting no-till in the Netherlands [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF]. Recent studies, however, suggest that risks of erosion and compaction of European soils have increased as a result of continuing conventional inversion tillage, reductions in soil organic matter and increased mechanisation [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF]. Erosion risks may increase with anticipated changes in climate throughout Europe, with a greater number of high intensity rainstorms and a greater likelihood of severe off-site pollution problems from silt, hericides and nutrients conveyed into water courses and other places. Flood control is an increasing concern in many areas and the maintenance of crop residues on the surface of no-till soils of good structure may offer a successful way of reducing runoff during storms. [START_REF] Troeh | Soils and Soil Fertility[END_REF] estimate that the average annual soil loss within the EU is 17 t ha -1 yr -1 , considerably exceeding the rate of soil formation of 1 t ha -1 yr -1 , while losses of 20-40 t ha -1 may occur in individual storms [START_REF] Montanarella | European Soil Protection Strategies[END_REF]. The soil lost in erosion with no-till at a site in Finland [START_REF] Puustinen | Influence of cultivation methods on suspended solids and phosphorus concentrations in surface runoff on clayey sloped fields in boreal climate[END_REF] was 29% of that with ploughing (570 instead of 2100 kg ha -1 ). Average runoff in Scandinavia, under current traditional tillage practices, varies greatly from 270 mm y -1 in Denmark to about 580 mm y -1 in Norway. No-till in place of autumn ploughing on sloping clay soils in western Norway, where runoff may sometimes exceed 1000 mm y -1 , can reduce particle erosion by 79% [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF].

Wind erosion can be a problem on cultivated sandy soils in the Netherlands and some other countries. No-till could overcome that problem but might result in a loss of crop yields on unstable soils [START_REF] Van Ouwerkerk | Experiences with minimum and no-tillage practices in the Netherlands. I: 1962-1971[END_REF]. Both water and wind erosion are major problems in the Saxony region of Germany and have been found to be effectively controlled by conservation tillage and no-till (IRENA, undated). The increase in soil erosion in northern France is attributed to intensive cultivation practices [START_REF] Massé | Feasibility of minimum tillage practices for annual cropping systems in France[END_REF] but no-till has not been adopted there to any extent to counter this problem.

Evidence for reduced runoff from no-till soil compared with ploughed soil has been reviewed by [START_REF] Armand | Runoff generation related to intra-field soil surface characteristics variability. Application to conservation tillage context[END_REF] and compared to observations made at two sites in the Upper Rhine valley where runoff typically occurs from maize fields between the end of April and July. They attached importance to the type of management of crop residues and the lateral distribution of crop residues over the surface of no-till fields which may be far from uniform, strongly influencing the incidence of surface crusting and runoff.

In southern European countries such as Italy, Spain and Portugal, soil and water conservation have also been found to be enhanced by the surface residue mulch with no-till due to increased infiltration during winter rainfall and reduced evaporation from the surface in dry summers. [START_REF] Hummel | Future needs[END_REF] considered that "Soil conservation must be the compelling and overriding selling point" for no-till in Mediterranean countries. [START_REF] Bonari | Soil erosion and nitrogen loss as affected by different tillage systems used for durum wheat cultivated in a hilly clayey soil[END_REF] working on clay soil at a hilly site near Pisa, Italy, found that annual runoff (averaged over two years) was 94 mm for ploughing and 57 mm for no-till. Corresponding results for soil loss were 28 and 3 t ha -1 . The reduced soil loss was accompanied by less loss of N attached to soil particles as ammonium or organic compounds. In Spain no-till has the ability to conserve water in low rainfall seasons and to protect the soil against erosion in wet seasons [START_REF] Giráldez | Conservation tillage under extreme meteorological conditions in Southern Spain[END_REF].

Losses of P

The eutrophication of water courses attributable to increased P loading is a serious environmental problem in Finland [START_REF] Muukkonen | Effect of no-tillage and ploughing on the risk of phosphorus load from clay soils[END_REF]. Risks of pollution from loss of particulate P during erosion can also be high on sloping ground in Sweden and Norway. High losses of particulate-bound P (PP) in runoff occur in Scandinavia from cultivated clay soils during warm, wet winters but losses of P can also occur as dissolved reactive P (DRP) which can account for 9-93% of the total P lost in runoff [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF]. No-till can greatly reduce losses of PP but a stratified surface layer rich in P can develop in no-till soils from which an appreciably higher loss of DRP can occur than on ploughed soils. [START_REF] Puustinen | Influence of cultivation methods on suspended solids and phosphorus concentrations in surface runoff on clayey sloped fields in boreal climate[END_REF] found that the loss of PP on no-till soil was 30% of that from ploughed soil (1.13 instead of 3.71 kg ha -1 ) but DRP increased by 348% under no-till (2.02 instead of 0.58 kg ha - 1 ). Such very considerable increased losses of DRP in runoff after no-till compared to ploughing have also been attributed to the release of DRP from dead weeds following glyphosate application [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF] and to P leaching from fertilizers retained near the surface [START_REF] Puustinen | Influence of cultivation methods on suspended solids and phosphorus concentrations in surface runoff on clayey sloped fields in boreal climate[END_REF]. Another explanation is that the increased organic C in the surface layers of no-till soils enhances the lability of the P already at high concentration from unincorporated fertilizer applications [START_REF] Muukkonen | Effect of no-tillage and ploughing on the risk of phosphorus load from clay soils[END_REF]. While the total loss of P will be lower after no-till than after ploughing, the loss of the dissolved reactive P fraction may be higher than after ploughing [START_REF] Puustinen | Influence of cultivation methods on suspended solids and phosphorus concentrations in surface runoff on clayey sloped fields in boreal climate[END_REF] raising doubts as to the relative value of no-till as a technique for reducing eutrophication of water courses. The risks of losses of PP and DRP in runoff need to be minimised by the maintenance of a high infiltration rate in no-till soils [START_REF] Muukkonen | Effect of soil structure disturbance on erosion and phosphorus losses from a Finnish clay soil[END_REF]) and increasing the internal drainage of the profile by tile drainage. The relative importance of P losses in the form of particulate P or as DRP will depend on the infiltration capacity and incidence of surface runoff (Alakukku et al., 2009a).

4.5

Losses of N Losses of N can occur not only as nitrate leaching but also as nitrogen compounds attached to soil particles during runoff. Nitrate leaching is a serious problem in many parts of Europe and any crop/soil management practices which can reduce its extent, such as omitting autumn ploughing, will have particular importance [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF]. There is a lack of consensus in the literature on the effect of no-till on nitrate leaching (Oorts et al., 2007c;[START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF]. This variability appears to depend on soil type, the use of catch crops before spring-sown crops and the various pathways for water movement in structured soils. Nevertheless, [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF] reported that a lack of soil loosening in autumn leads to less mineralization of nitrogen and reduced leaching of nitrate into ground water in some notill soils with well-developed vertically orientated macroporosity through which excess rain water is conducted as bypass flow. This may be of major environmental importance in areas designated by the EU as Nitrogen Sensitive Areas. [START_REF] Düring | Leaching and decomposition of agrochemicals in no-tilled soils[END_REF] reported lower nitrate concentrations in topsoil in autumn after no-till than after ploughing, but since greater downward drainage in no-till soils, especially in macro-pores, the amount of nitrate leached may be similar. The lack of consistent reductions in nitrate leaching for no-till compared to ploughing for autumn-sown crops on two sandy loam soils in Denmark [START_REF] Hansen | Can non-inversion tillage and straw retainment reduce N leaching in cereal-based crop rotations?[END_REF] was attributed to more rapid root growth on ploughed soil and thus a greater ability of crops to absorb nitrate before leaching could occur. Poor establishment of autumn-sown crops on no-till soil due to dry spells led to increased nitrate leaching compared to that on ploughed soil.

Greenhouse gas emissions

Soils are now recognised as having an important role in the emission of greenhouse gasses, studies on the influence of no-till have usually been restricted to only one gas and a limited range of soils. Results therefore tend to be tentative and to be highly dependent on local conditions and vary widely with changes in weather and crop management practices. Though rarely attempted, there is a need to study all three greenhouse gases (CO 2 , N 2 O and CH 4) simultaneously, as was undertaken by Regina and Alakukku (2010) over a 10-month period at 6 sites representative of typical soil types in Finland which had been under no-till for 5-7 years.

N 2 O emissions

No-till influences the dynamics of soil nitrogen in several ways [START_REF] Conen | Nitrous oxide emissions from land-use and land-management change[END_REF]. Factors influencing N 2 O emissions from no-till include soil wetness and compaction status (which together influence soil aeration and the N 2 O production mechanism through denitrification), soil temperature, nitrogen fertiliser type and application method, crop type and the extent of crop residues cover. Of these, soil moisture status is probably the most important [START_REF] Ball | Dynamics of upward and downward N 2 O and CO 2 fluxes in ploughed or no-tilled soils in relation top water-filled pore space, compaction and crop presence[END_REF][START_REF] Rochette | No-till only increases N 2 O emissions on poorly-aerated soils[END_REF][START_REF] Almaraz | Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec[END_REF]. The higher moisture content and reduced aeration under no-till, especially after rainfall on heavier poorly drained soils in northern Europe, leads to greater denitrification and emission of N 2 O than under ploughing [START_REF] Christian | Reduced cultivations and direct drilling for cereals in Great Britain[END_REF][START_REF] Vinten | The effect of cultivation method, fertilizer input and previous sward type on organic C and N storage and gaseous losses under spring and winter barley following long-term leys[END_REF]Oorts et al., 2007b;[START_REF] Ball | Dynamics of upward and downward N 2 O and CO 2 fluxes in ploughed or no-tilled soils in relation top water-filled pore space, compaction and crop presence[END_REF][START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF], while very wet soils favour the reduction of N 2 O to N 2 . Highest N 2 O fluxes typically occur at water-filled pore space in the range of 60-80%. The emissions of N 2 O from 6 contrasting soils was strongly correlated with the total C and N contents of the 0-20 cm layer six contrasting soils in Finland [START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF] which may explain higher N 2 O fluxes from no-till soils. [START_REF] Six | The potential to mitigate global warming with no-tillage management is only realized when practised in the long term[END_REF] observed increased N 2 O emissions only during the first 10 years of no-till followed by a decrease after 20 years. This effect might be attributable to slow changes resulting in the improvement of internal structure and drainage of no-till soils. Increased N 2 O formation in no-till soils may also be associated with higher numbers of earthworms since the gas is formed in their intestine [START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF].

On well-aerated soils the impact of no-till on N 2 O emissions is small [START_REF] Rochette | No-till only increases N 2 O emissions on poorly-aerated soils[END_REF] and some studies have indicated lower N 2 O emissions with no-till than for ploughed soils [START_REF] Almaraz | Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec[END_REF][START_REF] Mutegi | Nitrous oxide emissions and controls as influenced by tillage and crop residue management strategy[END_REF]. The considerable level of variability is illustrated in Table 9. On a well-drained sandy loam soil in Denmark [START_REF] Mutegi | Nitrous oxide emissions and controls as influenced by tillage and crop residue management strategy[END_REF] found that N 2 O emissions from no-till were lower for no-till than for ploughing when residues are retained (as is usual), with no-till and ploughed soils showing N 2 O emissions of 258 and 551 μg N m -2 day -1 respectively. This was attributed to complex interactions between the effects of residue-N transformation, C availability and soil physical conditions. Where legumes are incorporated by ploughing, N 2 O emissions tend to be appreciably higher than with no-till, indicating a further advantage for no-till by including legumes in a rotation [START_REF] Almaraz | Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec[END_REF].

It is clear that further research is needed to establish the interacting role of several factors in addition to tillage, such as soil type, soil N status, soil aeration, soil temperature, crop type and residue management in influencing N 2 O emissions and that a general conclusion about the influence of no-till is not possible. 4.6.2. CO 2 emissions from fuel use . Fuel used in no-till operations is invariably less that than used with normal ploughing systems but the difference will depend strongly on the soil type, the depth of ploughing, the number and type of secondary cultivations. Tebrügge and Börhnsen (1997a) reported the average fuel consumption for growing small grains over a range of soils in Germany to be 43.55 l ha -1 for stubble cultivation, ploughing, secondary cultivation and sowing as compared to 6.8 l ha -for no-till (sowing and plant protection), giving a potential saving of 37 l ha -1 equivalent to 84%. Other estimates of fuel saving with no-till compared to ploughing and secondary cultivations vary widely, for example: 50% [START_REF] Khaledian | The inpacts of direct seeding into mulch on the energy balance of crop production system in the SE of France[END_REF], 68% [START_REF] Koeller | Machinery requirements and possible energy savings by reduced tillage[END_REF], 75-80% [START_REF] Riley | Trends in reduced tillage research and practice in Scandinavia[END_REF]; 83% (Arvidsson, 2010b).

The effect of soil type on fuel savings with no-till is of importance. For a clay soil in Sweden fuel consumption for ploughing and cultivating and no-till treatments was 54 and 9 l ha -1 , respectively, while on a silty loam the corresponding values were 27 and 7 l ha -1 (Arvidsson (2010b), giving savings of 45 and 20 l ha -1 on the clay and silty loam soils respectively. Similarly in Germany fuel consumption for ploughing, cultivating and sowing were reported to be 44, 54 and 78 l ha -1 for sand, loam and clay soils in Germany [START_REF] Koeller | Machinery requirements and possible energy savings by reduced tillage[END_REF]. The corresponding fuel usage for no-till and very shallow cultivation were 17, 20 and 25 l ha -1 giving savings with no-till of 27, 34 and 53 l ha -1 , respectively. The emission of CO 2 from the production and consumption of tractor fuel is approximately equal to 376 kg CO 2 per 100 l diesel [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF] Therefore, for a range of soil types, an average saving of 40 l ha -1 by using no-till in place of ploughing would achieve a reduced emission of 376 x 0.4 x 12/44 = 41 kg CO 2 -C ha -1 for each crop season. [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF] claims that no-till has the potential, if adopted on 40% of the EU land area (the proportion considered to be suitable without major problems), of reducing CO 2 emissions by 4.2 Mt y -1 as a result of lower fuel consumption alone. 4.6.3. CO 2 emissions from soil CO 2 emissions from soil are attributable to a number of different processes which can be divided into short-term effects immediately after ploughing and longer-term effects during the main growing season. Ploughing tends to produce a marked short-term flux of CO 2 in the first few days after soil disturbance with longer-term differences between tillage treatments being minor [START_REF] Vinten | The effect of cultivation method, fertilizer input and previous sward type on organic C and N storage and gaseous losses under spring and winter barley following long-term leys[END_REF][START_REF] Chatskikh | Soil tillage enhanced CO 2 and N 2 O emissions from loamy sand soil under spring barley[END_REF]. Similarly [START_REF] López-Garrido | Carbon losses by tillage under semi-arid mediterranean rainfed agriculture (SW Spain)[END_REF] found, for conditions in south west Spain, that tillage caused a sharp increase in soil CO 2 emissions immediately after tillage. Throughout the year, cumulative losses of carbon through CO 2 emissions were higher under conventional tillage than under no-till and reduced tillage. [START_REF] Álvaro-Fuentes | Effect of tillage on short-term CO 2 emissions from a loam soil in semi-arid Aragón (NE Spain)[END_REF]Álvaro-Fuentes et al. ( , 2007ab, 2008) studied short-and long-term CO 2 fluxes in north east Spain. Soil CO 2 emissions just after tillage were 40% higher under conventional tillage than under no-till as the CO 2 accumulated in soil pores was released to the atmosphere after the tillage event. At the same time, tillage has an accumulative effect during the whole growing season in increasing microbial decomposition resulting in 20% higher soil CO 2 emissions under conventional tillage than under no-till. Part of this effect can be attributed to greater root respiration under ploughing, especially in warmer months [START_REF] Almaraz | Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec[END_REF]. Reduction of CO 2 fluxes under reduced tillage compared with conventional tillage have also been reported by [START_REF] Sánchez | Soil CO 2 fluxes in cereal land use of the Spanish plateau : influence of conventional and reduced tillage practices[END_REF][START_REF] Sánchez | Soil CO 2 fluxes beneath barley on the central Spanish plateau[END_REF] in soils of the Spanish plateau.

Under certain conditions, however, higher CO 2 emission rates have been observed after no-till than after ploughing [START_REF] Almaraz | Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec[END_REF]. For instance over a 331-day period in Northern France CO 2 emissions from long-term no-till and ploughing were 4064 and 3160 kg CO 2 -C ha -1 respectively (Oorts et al. 2007b). The greater emission under no-till was considered to be associated with decomposition of old weathered residues present at the surface as a result of the unusually warm weather during the monitoring period. Another example of increased CO 2 efflux under no-till (Table 8) was observed by [START_REF] Yamulki | Short-term effects of tillage and compaction on nitrous oxide, nitric oxide, nitrogen dioxide, methane and carbon dioxide fluxes from grassland[END_REF], though they offered no explanation for the difference between tillage treatments. In Table 9 the periods of measurement are mostly quite short so that flux variation at different periods of the crop cycle may not be fully reported in contrast to the studies reported by [START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF] who measured CO 2 and N 2 O fluxes during a 10-month period at approximately monthly intervals. Considerable variability of CO 2 emissions has been found in Finland where Regina and Alakukku (2010) report higher fluxes with no-till than with ploughing at 2 out of 6 sites while at one site the opposite effect was found and no differences were observed at 3 sites, with CO 2 emissions being strongly correlated with total C and N and mineral N. There are, therefore, no consistent effects of tillage on CO 2 emissions, these being related to water content, climate and the amount, type and stratification of organic matter. An understanding of these effects requires further research.

Differences in CO 2 emissions between tillage systems are likely to result from both shortterm and long-term effects (Oorts et al., 2007b). Short-term effects are due to the physical disturbance of soil and crop residues. Long-term effects include the effects of changes in soil qualities over several years. Most studies concentrate on short-term effects. Long-term CO 2 emissions involve complex interactions between the different factors determining emissions (temperature, rainfall, water content, SOM, crop residues) (Oorts et al., 2007b).

CH 4

There is a lack of European data on methane emission or absorption by the soil. [START_REF] Ball | Field N 2 O, CO 2 and CH 4 fluxes in relation to tillage, compaction and soil quality in Scotland[END_REF] showed that methane oxidation was greater under no-till than under ploughing, but the oxidation rates were low and unlikely to contribute substantially to the greenhouse gas budget. In the USA, no-till has been shown to increase methane oxidation in comparison to minimum and conventional tillage [START_REF] Ussiri | Nitrous oxide and methane emissions from longterm tillage under a continuous corn cropping system in Ohio[END_REF]. Other work has shown that no-tillage can emit methane and at a greater rate than from conventional tillage [START_REF] Alluvione | Nitrogen, tillage and crop rotation effects on carbon dioxide and methane fluxes from irrigated cropping systems[END_REF] but most studies suggest that tillage has a minor influence on the exchange of methane. [START_REF] Regina | Greenhouse gas fluxes in varying soils types under conventional and no-tillage practices[END_REF] conclude that CH 4 fluxes were not affected by no-till but suggest that, depending on soil conditions, a weak CH 4 flux with no-till might be negative or positive.

Sequestration of carbon

The sequestration of carbon by soils represents an important component in the balance of carbon within the environment [START_REF] Lal | Farming carbon[END_REF]. Article 3.4 of the Kyoto Protocol refers to the desirability of promoting C sequestration by soils [START_REF] Wereszczaka | The effect of 15 years of direct drilling application: Part I -S index and carbon sequestration[END_REF]. It is therefore important to establish the effect of no-till on C sequestration and what factors may influence this.

Although there is no consistent effect of tillage on long-term CO 2 (Section 4.6.2.), early studies established that the SOC content of no-till soils, at a depth of 0-30 cm or less, frequently exceeded that after ploughing and did so almost invariably at 0-15 cm depth with a pronounced stratification of SOC with depth usually apparent within three years of commencing no-till [START_REF] Yang | Impacts of long-term and recently imposed tillage practices on the vertical distribution of soil organic carbon[END_REF].

A summary by [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF] of early long-term studies in Canada, Germany, Italy, Spain and Portugal indicated that no-till can be expected to show an additional accumulation compared to ploughing of 1100, 1500, 800, 800 and 1000 kg C ha -1 yr -1 , respectively. Data from long-term trials summarised by of sampling not given). Similarly in France, measurements made on the Boigneville long-term field trial (32 years) have shown that the difference between C stocks in ploughed and no-till treatments within the 0-28 cm depth during a 32-year period was +5.2 Mg C ha -1 in favour of no-till (Oorts at al., 2007a) which is equivalent to 162 kg C ha -1 yr -1 . In a semi-arid area in northern central Spain the soil carbon accumulation to a depth of 30 cm after 10 years of no-till was 25% higher than after mouldboard ploughing [START_REF] Sombrero | Carbon accumulation in soil. Ten-year study of conservation tillage and crop rotation in a semi-arid area of Castille-Leon, Spain[END_REF].

A global assessment of 67 long-term experiments, involving 276 paired no-till and ploughed treatments [START_REF] West | Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis[END_REF] indicated, at depths usually of 30 cm or less, a mean increase of 570 kg C ha -1 yr -1 for no-till compared with ploughing but with considerable variation as has been found in Europe (Table 9).

Table 9

Examples of average annual change in soil organic carbon (SOC) after no-till compared to ploughing in Europe (in ascending order of SOC change). In general these initial research results supported the hypothesis that higher SOC at 0-30 cm depth is a valid indication of greater C sequestration after no-till than after ploughing and led to confident predictions that no-till offered an important method to mitigate anthropogenic CO 2 emissions [START_REF] Spargo | Soil carbon sequestration with continuous no-till management of grain cropping systems in the Virginia coastal plain[END_REF]. For instance, [START_REF] Smith | Preliminary estimates of the potential for carbon mitigation in European soils through no-till farming[END_REF] estimated that 100% conversion to no-till farming in the EU would sequester, through fuel savings and soil organic matter increases, about 26 Mt C yr -1 or 4% of the total man-made carbon released annually as CO 2 in Europe.

Country

Recent research has shown, however, that this hypothesis is not universally correct and may be invalid and that very wide differences can be expected according climate, soil type, cropping system and duration of no-till and ploughed treatments and also that sampling to a depth of 40 cm, 60 cm or even 1 m [START_REF] Lal | The plow and agricultural sustainability[END_REF], may be necessary to establish fully the role of no-till in C sequestration. Sampling at depths of 0-30 cm or less has resulted in over-estimation of soil C sequestration under no-till [START_REF] Baker | Tillage and carbon sequestrationwhat do we really know?[END_REF] since considerable accumulations of C have been found below 30 cm for ploughed soils which were absent at this depth after no-till [START_REF] Snyder | Review of greenhouse gas emissions from crop production systems and fertilizer management effects[END_REF]. [START_REF] Luo | Can no-tillage stimulate carbon sequestration in agricultural soils? A meta-analysis of paired experiments[END_REF] showed that on 69 paired studies (mostly in North America) the adoption of no-till resulted in an increase of 3.15 t C ha -1 at 0-10 cm but declines of 2.40 t C ha -1 and 0.90 t C ha -1 at 20-30 cm and 30-40 cm respectively with no differences below 40 cm. No-till did not increase overall soil C except where double cropping per year was used. They suggest that ploughing will result in deeper root growth than in no-till. Pooling the results of surveys conducted in the USA, Canada and Brazil by [START_REF] Blanco-Canqui | No-tillage and soil-profile carbon sequestration : An on-farm assessment[END_REF] and [START_REF] Christopher | Regional study of no-till effects on carbon sequestration in midwestern United States[END_REF] shows that the gain in C sequestration by no-till over ploughing varied from +4.94 Mg C ha -1 yr -1 to -11.0 Mg C ha -1 yr -1 . Both these extreme values, which exceed the limits yet reported within Europe, were found on silt loam soils within Pennsylvania.

Among the possible factors influencing the relative C sequestration under no-till and ploughing are soil factors, rainfall, temperature, crop characteristics (such as the amount and fate of crop residues applied and root distribution). [START_REF] Snyder | Review of greenhouse gas emissions from crop production systems and fertilizer management effects[END_REF] consider that no-till will show positive results only if crop productivity (i.e. biomass) with no-till is equal to or greater than that for ploughing which is not always the case. Higher C sequestration under no-till relative to ploughing will be encouraged by a proneness to erosion and drought on ploughed soils, good drainage, silt loam texture and good earthworm activity, while lower C sequestration after no-till can be expected on poorly drained soils with slow internal drainage, heavy textured soils with a proneness to smearing leading to poor germination, suboptimal soil temperatures in spring, and a proneness to compaction (Personal communication: R. Lal). Lack of differences in C sequestration after no-till and ploughing in Europe (Table 9) tend to be in cool, wet climates where the oxidation of incorporated crop residues proceeds only slowly, viz. Switzerland [START_REF] Anken | The effects of tillage systems on soil organic carbon content under moist, cold-temperate conditions[END_REF] and Scotland [START_REF] Sun | Distribution of soil carbon and microbial biomass in arable soils different tillage regimes[END_REF]. [START_REF] Anken | Long-term tillage system effects under moist cool conditions in Switzerland[END_REF][START_REF] Anken | The effects of tillage systems on soil organic carbon content under moist, cold-temperate conditions[END_REF] compared SOC after 19 years of no-till and mouldboard ploughing at a cool, moist site (average annual rainfall 1183 mm) in north-eastern Switzerland. The total SOC within 0-40 cm depth was 65 Mg C ha -1 with no significant evidence that no-till was responsible for increased C sequestration. In dry climates the concentration of crop residues near the surface may stimulate their decomposition at a faster rate than if they had been incorporated at depth [START_REF] Six | The potential to mitigate global warming with no-tillage management is only realized when practised in the long term[END_REF].

The rate of any enhanced carbon storage under no-till has been found to peak at about 5 to 10 years with SOC reaching a new equilibrium in 15 to 20 years [START_REF] West | Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis[END_REF]. [START_REF] Bhogal | The Effect of Reduced Tillage Practices and Organic Matter Additions on the Carbon Content of Arable Soils[END_REF] suggest that enhanced carbon storage potential with no-till will decline after about 20 years as a new equilibrium carbon content is reached.

There is a need to take differences in bulk density within the profile into account when comparing SOC accumulation for different tillage treatments. [START_REF] Lee | Determining soil carbon stock changes: simple bulk density corrections fail[END_REF] made a comparison of several methods to correct total soil carbon data for tillage responses coupled to changes in bulk density and they suggested the use of an equivalent soil mass method.

The very wide, and not fully understood, variation in C sequestration after no-till compared to ploughing suggests that further research is needed over a range of soils, crops and climates before any general conclusions can be made. It should not be overlooked that stratified increases in C near the surface in no-till will have numerous benefits to soil quality, even if the overall increase of C within the no-till profile is small or non-existent.

4.8

Net effects of no-till and ploughing on net climate forcing

True mitigation of global warming potential (GWP) by no-till is only possible if any increased carbon sequestration effect exceeds the GWP attributable to the net upward fluxes of the three major biogenic greenhouse gases (i.e. CO 2 , N 2 O and CH 4 ) (Fig. 2.). Several authors indicate that higher greenhouse gas emissions, N 2 O in particular, from notill may counterbalance greater C sequestration (Oorts et al., 2007b;[START_REF] Smith | Impacts of land management on fluxes of trace greenhouse gases[END_REF] with the effect being greatest on poorly drained, fine-textured agricultural soil [START_REF] Rochette | No-till only increases N 2 O emissions on poorly-aerated soils[END_REF]. Thus improved nitrogen management to reduce N 2 O emissions is essential to realize the full benefit of any increased C storage with no-till [START_REF] Six | The potential to mitigate global warming with no-tillage management is only realized when practised in the long term[END_REF], particularly on poorly aerated soils. The typical rate of C storage suggested by [START_REF] Bhogal | The Effect of Reduced Tillage Practices and Organic Matter Additions on the Carbon Content of Arable Soils[END_REF] for England and Wales is 310 (±180) kg C ha -1 y -1 . A nitrous oxide emission of only 1 kg N 2 O ha -1 y -1 greater from no-till than from ploughing would balance this GWP. Further examples of the CO 2 (equivalent) components in the budget for net forcing effect after no-till and ploughing (excluding CO 2 emissions in tractor fuel) are shown in Table 8 and emphasise the importance of N 2 O emissions in influencing the net forcing budgets of no-till. [START_REF] Snyder | Review of greenhouse gas emissions from crop production systems and fertilizer management effects[END_REF] concluded that there was no clear responsepositive or negativefor the mitigation of greenhouse gas emissions using no-till compared to conventional tillage. In some regions where stored organic matter increases, the net GWP can decrease whereas in other areas the GWP can increase slightly when changing from conventional tillage to no-tillage. Further research is needed to improve confidence in predictions of the benefits of no-till in reducing the overall net climate forcing and differences in usage of tractor fuel, N fertiliser and herbicides must be included.Particular problems arise because data on gas emissions are subject to large short-term variation and are often presented for appreciably different periods in the crop season (Table 8).

Prospects for increased uptake of no-till in Europe

Influence of climate changes

Stability of acceptable yields under a range of seasonal weather conditions and anticipated changes in climate is of major importance to farmers. Changing climate in northern Europe is tending to result in milder winters, wetter soils in autumn and spring and longer growing seasons which are already influencing the choice of tillage systems. In south-east England average rainfall during establishment of winter-sown crops (August-October) has increased from 61 mm (1971-1980) to 74 mm (1991-2000) [START_REF] Davies | Reduced Cultivations for Cereals: Research, Development and Advisory Needs under Changing Economic Circumstances. Home-Grown Cereals Authority[END_REF]. This trend is forecast to continue. In southern and central Europe climate changes are expected to increase drought problems during hotter, drier summers with wetter, warmer winters. To maintain yields in areas of low rainfall an increasing emphasis is anticipated on minimal soil disturbance and maximum residue coverage to keep evaporation to a minimum [START_REF] Birkás | Environmentally-sound Adaptable Tillage[END_REF]. If, as is expected, the winter climate in northern Europe becomes milder and wetter there will be enhanced risks of erosion, runoff and pollution of water courses from ploughed soils which might normally be frozen and snow covered, giving greater opportunities for notill on well drained soils [START_REF] Muukkonen | Effect of soil structure disturbance on erosion and phosphorus losses from a Finnish clay soil[END_REF].

Future crop and soil management policies

Environmental concerns related to greenhouse gas emissions and soil erosion may influence government policy regarding adoption of no-till in future. If no-till can reduce production costs while reducing global warming potential appreciably and meeting enhanced standards for environmental and food quality, it would have important advantages, even if yields were not higher, or even slightly lower, than following ploughing. [START_REF] Düring | Environmental aspects of no-tillage application -Erosion and leaching of agrochemicals[END_REF] consider that if the environmental advantages of no-till were considered to be sufficiently important then government subsidies could be awarded for the first three years of no-till when reduced yields may act as a deterrent to commercial adoption. To reduce erosion problems in Norway subsidies are paid to farmers who do not plough in autumn while in Denmark and southern Sweden subsidies are payable to promote spring ploughing to reduce N leaching [START_REF] Ulén | Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review[END_REF]. However, recent research seems to contradict earlier suggestions that no-till would result in appreciable carbon sequestration and thus it may not justify inclusion in trading credits for carbon within the concept of "farming carbon" [START_REF] Lal | Farming carbon[END_REF].

Although no-till offers reduced emissions of CO 2 through reductions in use of fuel it seems that no-till will never be acceptable within the standards for organic or biodynamic farming owing to the dependence on herbicides. The difficulties of employing reduced tillage systems within organic management are discussed by [START_REF] Berner | Crop yield and soil fertility response to reduced tillage under organic management[END_REF] and [START_REF] Peigné | Is conservation tillage suitable for organic farming[END_REF]. Restricted use of herbicides in no-till systems may be enforced as a result of recent EU Directives concerned with water quality (Drinking Water Directive) and Water Framework Directive). For instance the widespread use of metazachlor for oilseed rape is now under review within the Water Framework Directive [START_REF] Morris | The adoption of noninversion tillage systems in the United Kingdom and the agronomic impact on soil, crops and the environment -A review[END_REF]. The EU Agri-Environmental Programme may restrict the application of slurry or manure to no-till land unless it is injected or mixed with the topsoil.

The Soil Thematic Strategy was ratified by the European Commission on September 22, 2006. The strategy aims to address soil degradation throughout 27 EU Member States. The implementation of this as a Soil Directive could encourage the uptake of no-till as this farming practice addresses several of the identified soil threats (Commission of the European Communities, 2006). The Common Agricultural Policy (CAP), which controls government payments to farmers throughout the EU, is due for major reform in 2013 and it seems likely that environmental concerns will gain greater emphasis in influencing payments thereafter. Financial inducements are already available to encourage the uptake of conservation and notill systems in Saxony and after a ten-year programme area these practices amounted to 27% of the total arable area (IRENA, undated). However, a low degree of acceptance of alternative tillage systems by Spanish farmers is attributed to inadequate extension and technology transfer systems and lack of access to suitable machinery and equipment [START_REF] Cantero-Martínez | Evaluation of agricultural practices to improve efficiency and environment conservation in Mediterranean arid and semi-arid production systems. MEDRATE project[END_REF][START_REF] Angás | Crop Production Technologies in Mediterranean Region[END_REF].

Changes in production costs and crop prices

Increases in costs, particularly of fuel and machinery, may force farmers to give greater attention to abandoning mouldboard ploughing provided that no-till can achieve suitably high yields. Moves towards farm amalgamation and contracting out tillage operations, already widespread in Europe [START_REF] Mikkola | Direct drilling in Finland, a review[END_REF], will heighten demand for low cost, high capacity cropping systems which could be based on no-till. The machinery investment for ploughing systems exceeds that for no-till by a factor of 1.63, while maintenance costs are 4 times higher, fuel costs are 6.5 times higher and working time per unit area is 5 times higher and the combined performance costs are 4.2 times higher for ploughing than for no-till [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF]. These differences are likely to become even more important in future if profit margins for crop production decline and labour and fuel costs rise.

Mineral fertilizers also contribute significantly to overall production costs. Especially on soils having depleted organic matter in southern Europe, increased levels of soil organic carbon near the surface as a result of adopting no-till could significantly reduce the need for mineral N inputs in cereal production. [START_REF] Carvalho | A adubação azotada do trigo de sequeiro: o problema da sua eficiência[END_REF] reported that the increase from 1 to 2% soil organic matter, obtained under long-term no-till conditions, could account for a reduction of 62 kg N ha -1 as fertiliser to achieve the optimum economic yield level.

Water and energy costs are becoming increasingly important for the economic viability of irrigated field crops in the southern European countries. The practice of no-till is frequently mentioned as a water-saving technique, due to increased infiltration, reduced evaporation and increased storage. US studies clearly indicate that no-till could contribute to water savings when compared to conventional tillage [START_REF] Wagger | Corn yield and water-use efficiency as affected by tillage and irrigation[END_REF][START_REF] Norwood | Water use and yield of limited-irrigated and dryland corn[END_REF].

In spite of all the apparent economic advantages of a move away from ploughing, [START_REF] Tebrügge | No-tillage visionsprotection of soil, water and climate and influence on management and farm income[END_REF] finds it "difficult to understand farmer reluctance to accept no-till." However, although the area under no-till is still small, economic circumstances are increasingly favouring the uptake of no-till which is now actively promoted by the European Conservation Agriculture Federation (ECAF) and the Spanish Conservation Agriculture Association.

Conclusions

1. In northern and western Europe commercial uptake of no-till is currently very limited but has recently increased in Finland and other parts of Scandinavia. Initial hopes among certain research workers, often based on results of small plot experiments, in many northern regions of Europe for widespread commercial exploitation of no-till have proved to be unfounded, largely because of unanticipated problems with crop residues, seasonally variable yields and weed control. In general similar yields for winter-sown crops on well-drained soils have been obtained with no-till and ploughing but yields for spring-sown crops with no-till have often been somewhat lower. The interaction of factors related to soil properties, machinery usage, soil compaction, weather conditions, timeliness of sowing, weed control problems and crop residue handling have been found to be highly complex. Economic factors related to the production of winter-and spring-sown cereals are currently tending towards a more favourable basis for the adoption of no-till but a number of technical problems continue to restrict uptake in practice.

2. Throughout the wetter parts of northern Europe ploughing is still very widely adopted and is a particularly effective method of seedbed preparation on poorly drained soils because of its ability to provide surface drainage and aeration for the topsoil, especially in spring, to control weeds and to remediate surface compaction. The continued and widespread use of mouldboard ploughing or non-ploughing cultivations at reduced depth is anticipated in much of northern Europe unless economic incentives for no-till increase markedly.

3. In south western Europe the commercial uptake of no-till is now increasing as a result of perceived advantages in terms of soil and water conservation and reduced costs on larger farms. No-till has generally given equal or higher yields than after ploughing for winter-sown crops and savings in cost, especially on larger farms, may act as a powerful stimulus to its further adoption in those areas. In Mediterranean countries, no-till and the preservation of surface residues seem increasingly likely to become standard commercial practice because of better economics and improved soil and water conservation.

4. In all parts of Europe the commercial uptake of no-till depends closely on the successful handling of surface residues, weed control, compaction control and the correct selection and use of appropriate herbicides and direct drills. Controlled traffic farming and the use of rotations offer opportunities for enhanced commercial production with no-till. A high standard of managerial skills is essential, together with access to experienced advisors and consultants with up-to-date knowledge of all relevant environmental regulations and directives.

Environmental and economic factors may both favour the future adoption of no-till with financial support for no-till possibly becoming part of government environmental support policies.

5. Further quantification of all the environmental impacts of no-till is needed, particularly with respect to the influence on overall net climate forcing and pollution of water courses. The emission of greenhouse gases is extremely variable and the various factors involved, including fuel use, have not yet been fully evaluated for a wide range of crops, soils and climates. Carbon sequestration is not universally increased under no-till compared to ploughing and may vary widely with crop, soil and climate relations which are not yet fully established. Further research on the environmental aspects of the complete no-till system is needed.

6. Much early information on no-till in Europe was based on misleading results due to the use of short-term or monocultural experimentation, lacking full attention to economic implications. There is particular need for research into improved methods of non-herbicidal weed control in no-till systems using, for example, rotations and occasional shallow cultivations. Further research is also needed into the evaluation of the impact of no-till on grain quality (protein content, mycotoxins). No-till induces a dramatic change in the soil environment; this has consequences on the major plant nutrients (N, P, K) which still have to be better understood. The number of long-term no-till experiments (>5 years) in Europe over a range of soils, fertiliser applications and climate conditions with crops grown within rotations is still limited. Generalisation of conclusions from experiments of less than 3 years duration is unwise due to climatic variability and initial development of soil structural characteristics. The current economic and environmental potential of continuous no-till under rotational cropping has not been fully evaluated.

7. The success of no-tillage, both for crop production and for mitigating climate change is strongly related to soil wetness and susceptibility to compaction. Greater awareness and better interpretation of the spatial and time distribution of soil wetness and water deficits, as influenced by climate and soil type, will help improve the identification of soils and regions suitable for the successful uptake of no-till. A European-wide classification of the suitability of soils for no-till should be considered.

Fig. 2 .

 2 Fig. 2. Conceptual representation of generalised overall influence of no-till compared with ploughing on the components of net climate forcing (see text for discussion).

Table 2

 2 Commercial uptake of no-till in some Western European countries in 2007-2008. For sources see references cited in footnotes.

	_________________________
	Country	Area of no-till 1
		(kha)
	_________________________
	Finland*	200
	Germany*	5
	France*	200
	Switzerland*	12.5
	Spain*	650
	Portugal**	80
	Italy**	80
	Slovak Rep.**	37
	________________________
	1 Excluding orchard and tree crops
	*Derpsch and Friedrich (2009)
	**

Table 3

 3 Relative agronomic advantages and disadvantages of mouldboard ploughing and no-till in Europe, although not universally relevant to all regions.

	Ploughing

Table 4

 4 Examples of yields of crops in various locations in Europe.

	Reference	Country	Crop	Soil texture	No of	No-till	Ploughed	No-till as
				(drainage)	NT		yield	yield	% of
					years/		(t ha -1 )	(t ha -1 )	ploughed
					Sites
	Riley et al.	Norway	Winter	Various	27/n.k.		5.11	5.17	99
	(1994)		wheat,
			Barley
	Riley et al.	Sweden	Winter	Various	22/n.k.		4.45	4.89	91
	(1994		wheat,
			Barley
	Arvidsson	Sweden	Winter		127		6.26		95
	(2010a)		wheat
			Barley		44		4.25		88
	Alakukku et al.	Finland	Spring	Clay	1-8		4.1	4.3	95
	(2009b)			Moderate
			Barley	Drainage
				Vertic
				Cambisol
				Silty clay	1-7		4.3	4.3	100
				Well
				drained
				Eutric
				Cambisol
	Känkänen et al.	Finland	Spring	Clay	1-4		2.5	3.9	65
	(2011)			Well
			Barley	drained	1-4		4.6	5.2	88
			Oats	Vertic			2.4	3.4	72
			Wheat	Stagnosol
	Riley et al.	Denmark	Winter	Various	80/n.k.		4.54	4.91	92
	(1994		wheat,
			Barley
	Rasmussen	Denmark	Winter	Coarse	6 y		2.17	2.44	89
	(1994)		Oilseed	sand
			rape	Sandy	6 y		3.96	4.13	96
				loam
	Schjønning et	Denmark	Winter	13 % clay	6	y	7.15	8.57	83
	al., (2010a)		Wheat		(2003-8)
	Soane and Ball	Scotland	Spring	Sandy
	(1998) (3)		and	loam	24/1		4.82	5.00	96
			Winter	(good)
			barley	Cambisol
				Sandy clay	24/1		4.42	4.90	90
				loam
				(moderate)
				Gleysol
	Cannell et al.	England	Winter	Clay	1-4/1
	1986		wheat	Stagnogley	(Averag
				Drained	es over	8.80	8.40	105
				Undrained	4 years)	7.15	7.79	92

Table 5

 5 Factors of relevance in making whole-farm economic comparisons of tillage systems.

	___________________________________________________________________
	Size of farm	Costs of machinery requirements
	Role of animals (if any)	Value and use of crop residues
	Crop yield expectations (and variability)	Influence on choice of crops
	Availability and costs of herbicides	Fuel and labour costs
	Rotational practices	Farmer skills and knowledge
	____________________________________________________________________

Table 7 :

 7 Widely reported changes in soil properties within five or fewer seasons of no-till. Advantages Increased aggregate stability, especially near surface Increased organic matter content near surface Increased vertical and stable pore structure Increased biological activity, especially earthworms Increased infiltration rate Increased hydraulic conductivity in subsoil on well structured soils Increased soil strength and load bearing capacity with reduced damage from traffic Disadvantages Increased bulk density within previous ploughed layer (0-25 cm depth) Increased moisture content near surface in spring in northern regions delaying drilling Reduced soil surface temperature, especially in spring in northern regions delaying drilling Increased acidity near surface Increased accumulation of P near surface with risks of loss in runoff ____________________________________________________________________

Table 8

 8 Contributions of CO 2 (excluding in tractor fuel) and N 2 O emissions (expressed as CO 2 equivalent) to net climate forcing budgets after no-till and ploughing in several countries (fluxes relate to the stated time periods) .

	Country	Aeration status	Time period	N 2 O loss expressed as kg CO 2 -C ha -1	emission in no-Change in CO 2	in no-till relative to Overall CO 2 budget	References
						till relative to	ploughing
						ploughing (kg CO 2 -C ha -1 )	(kg CO 2 -C ha -1 )
				No-till	Ploughed
	France	Good	11 months 168±66	102±19	+904	+970	Oorts et al. (2007b)
	Denmark Medium	3 months	127	263	-990	-1126	Chatskikh and Olesen (2007)
	Scotland	Poor	200 days 1 7070	2180	+93	+4980	Vinten et al. (2002)
	England	Poor	20 days	1143	900	+700	+943	Yamulki and Jarvis (2002)

Comprising 4 post-sowing periods over 3 years

Acknowledgements

We are grateful for helpful comments received from Professor J.C. Holmes, Dr J.D. Pidgeon, and the three unknown referees which greatly contributed to improving the quality of the manuscript and to Professor Laura Alakukku for providing additional information on no-till research in Finland.