Adriano Sofo
email: adriano.sofo@unibas.it

Augusto Zanella
email: augusto.zanella@unipd.it

Jean-François Ponge

Prof Adriano Sofo

Prof Augusto Zanella

Ponge

Soil quality and fertility in sustainable agriculture, with a contribution to the biological classification of agricultural soils

Keywords: conservation and sustainable agriculture, humus, plant-microorganisms-soil interactions, soil classification, soil ecology, soil organic matter, soil quality and fertility. Highlights

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Soils are one of our most valuable resources and are fundamental natural capitals at the base of all trophic chains. In agricultural systems, management practices that ensure high production often have negative repercussions on soil health, quality and fertility (Fig. 1). A high number of physicochemical, microbiological and biochemical parameters are responsible for the fertility of a soil [START_REF] Cheik | Integrating local knowledge into soil science to improve soil fertility[END_REF][START_REF] Jiang | Bio-mediated soil improvement: The way forward[END_REF]. However, because of the challenge of considering them altogether, only the most informative and reliable ones can be selected. Soil biological properties are very sensitive to small-scale changes occurring in a soil compared to soil physicochemical parameters. To ensure the long-term sustainability of cropping systems, both the status of soil organisms and crops need to be taken into account [START_REF] De Vries | Below-ground connections underlying above-ground food production: a framework for optimising ecological connections in the rhizosphere[END_REF][START_REF] Pelosi | Enchytraeids as bioindicators of land use and management[END_REF]. In this commentary paper, the specific biological indicators of soil health will be discussed together with soil physicochemical parameters and suggestions for soil classification. The core assumption is that healthy soils provide an optimal environment for soil organisms that stimulate plant physiological and biochemical responses to stress [START_REF] Pinstrup-Andersen | Food security and sustainable use of natural resources: A 2020 vision[END_REF]Soussana et al., 2019).

The overall message is the urgency of better understanding the effects of soil management practices on the structure of soil microbial and animal communities, and on plant health and production. A high number of physicochemical, biological and biochemical parameters are responsible for the fertility of a soil. However, due to the impossibility of considering all of them, it is inevitable to select the most informative and reliable ones [START_REF] Gil-Sotres | Different approaches to evaluating soil quality using biochemical properties[END_REF]. Generally, the physicochemical parameters are of scarce utility as indicators, as they are altered often when soils are subjected to drastic disturbances [START_REF] Filip | International approach to assessing soil quality by ecologically-related biological parameters[END_REF]. On the other side, soil biochemical properties are more sensitive to smaller changes occurring in a soil [START_REF] Yakovchenko | A biologically based indicator of soil quality[END_REF][START_REF] Wallenstein | Quantitative analyses of nitrogen cycling genes in soils[END_REF][START_REF] Muscolo | Early warning indicators of changes in soil ecosystem functioning[END_REF]. The selection of biological indicators closely related to soil microbial dynamics is essential for the quantification of soil quality and its resilience to stresses, two basic requisites of soil fertility (Duru et al., 2015;[START_REF] Hopkin | Biological Indicators of Soil Health[END_REF][START_REF] Schloter | Microbial indicators for soil quality[END_REF][START_REF] Vogel | A systemic approach for modeling soil functions[END_REF][START_REF] Veen | Applying the aboveground-belowground interaction concept in agriculture: Spatio-temporal scales matter[END_REF]. The indicators adopted could provide reliable and easy-to-interpret information on soil and plant status, as they are little affected by the fluctuations related to the season and topographic effects.

4

Carrying out a representative soil sampling, measuring soil biological indicators, proposing a correct soil classification, and integrating all data in a holistic framework can facilitate the inclusion of soil health, quality and fertility in management decisions made by farmers, land managers, and crop advisers. Understanding the effects of agricultural management practices on soil health, soil microorganisms/animals, and plants is important, but soil health should be included as a factor when management decisions are made by farmers, land managers, and crop advisers.

[Figure 1. HERE] Despite significant data gaps, there is growing evidence that unsustainable agricultural practices not only negatively affect the health and quality of the soils needed to sustain healthy crops and provide nutrient-rich foods, but they can also significantly affect the integrity and resilience of the ecosystem as a whole. To 'break' the above-cited vicious circle, sustainable solutions are required to facilitate the conservation of soils. But, in order to understand which soils are or can be sustainably managed, soil classification criteria are essential. The founders of the soil classification linked the different units to precise pedogenetic processes (FAO- UNESCO, 1974) to be able to define the functioning of each unit. The advantages of knowing the position of a cultivated field on the map of the distribution of the world's soils remain minimal for the farmer. Scale and micromorphological or purely local factors intervene and affect the functionality of the soil map.

Generally, farmers do not know the name of the soils they are cultivating. Not even many scientists who study natural ecosystems know the name of the soils in which they grow plants or raise livestock. This article is focused on the most advanced and updated research on the processes occurring at the interface between soil physicochemical aspects, plant roots and soil microorganisms/animals in sustainable agrosystems, and on practices to establish sustainable soils and preserve their fertility. An examination of the published literature has revealed that: a) soil humus formation, soil compaction and degradation, soil-plant-atmosphere interactions, root development and rhizosphere processes, vegetation types and phytosociology, signaling among plants and organisms, and plant nutrient balance have all crucial roles in making a soil really sustainable; b) a conspicuous part of soil physical, chemical and biological fertility in agroecosystems can be attributed to the action of soil microorganisms, with a substantial contribution of soil animals; c) a new paradigm for soil classification is necessary for creating the conditions for soils to be sustainable and able to provide ecosystem services.

In this article, all these aspects are considered in a holistic view, with agrosystems considered as living systems where soil, vegetation, fauna and microorganisms co-evolve and are reciprocally influenced (paragraph 6). In living soils, the key role of microorganisms in agrosystems should be seriously taken into account in land management strategies, focusing not exclusively on crop yield and quality, but also on soil fertility restoration and environmental safety. Moreover, the role of soil fauna, especially considering their interactions with microorganisms and plant roots, can surely contribute to the long-term sustainability of agricultural soils.

The aim of this commentary paper is therefore to reveal the complex interactions between soil physical and chemical properties (on which classification is mainly based), soil microbial diversity, and plant health in sustainable agroecosystems. Nowadays, agricultural activity, rather than considering just productivity, is focused on the quality of products, natural resource stewardship and environmental aspects, moving toward sustainable management techniques. The sustainable, judicious and efficient use of soils is thus essential to support continued agricultural production and quality. Focusing on sustainable soil use management and food production, this new type of agriculture can perfectly fit within the background of natural resources challenge, where positive mutual interactions between soil microorganisms and cultivated plants play a key role.

2. The contentious nature of soil organic matter [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF] reject the concept of humus and claim that: "Governmentfunded research programs must therefore preferentially support science that bridges the gap between detailed and fine-scale mechanistic research at the plant-soil interface and fieldscale research relevant to those who manage soils for their multiple ecosystem services". In their "Soil continuum model -Consolidated view" (Fig. 2) they replaced "humification" and "mineralization" with the less precise terms of "formation" and " destruction". [START_REF] Stevenson | Humus chemistry: Genesis, composition, reactions, Organic Geochemistry[END_REF] founded the organic matter science accurately describing large to small molecules and their specific properties in a model very similar to the one presented as a novelty in 2015 by Lehman and Kleber. With the intention of strengthening our position in defense of humus, we report in Appendix 1 the thought of "The first major critique of the humification concept" [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF]. It seems to us that [START_REF] Waksman | HUMUS. Origin, chemical composition and importance in nature[END_REF] predicted soil depletion as a consequence of denying the concept of "Humus as an organic system". The recent acknowledgement of the supramolecular nature of soil humus substances (Drosos et al., 2018;[START_REF] Piccolo | The supramolecular structure of humic substances: A novel understanding of humus chemistry and implications in soil science[END_REF][START_REF] Pinton | The Rhizosphere: Biochemistry and Organic Substances at the Soil-Plant Interface, Second Edition[END_REF] allows to devise a fractionation procedure, called Humeomics, that enables a detailed characterization of the structure of humic molecules in soil. The molecular mass of humic molecules can be increased with repercussions on the amount of carbon stored in the soil [START_REF] Piccolo | The Soil Humeome: Chemical Structure, Functions and Technological Perspectives[END_REF].

[Figure 2. HERE]

As for "fine-scale mechanistic research", it reminds us of those who would like to eat the pills that astronauts carry into space, instead of real legumes or cooked meats. It may even happen that they do not understand why they do not feel good in the long term. The concept gets closer when we consider the mass of neurons composing our brains. Via the process of categorization, the brain transforms only some sensory stimulation into information. Only some of the wavelengths of light striking our retinas are transformed into seen objects, and only some of the changes in air pressure registered in our ears are heard as words or music. To categorize something is to render it meaningful [START_REF] Barrett | The Future of Psychology: Connecting Mind to Brain[END_REF]. The same neurons produce different thoughts in response to a specific evolution of the entire brain system, that becomes increasingly rich in interconnexions and information. In figurative terms, the functioning of the soil is similar to that of a brain, but instead of generating thought in a living net of neurons, it builds a real complex ecosystem of interconnected living beings in continuous coevolution.

Soil physicochemical quality and changes in soil organic matter

One of the main worldwide agricultural problems is the decline in soil fertility, mainly due to the reduction of soil biodiversity, and of nutrient and water content. Agricultural soils are particularly susceptible to this problem, as they are based on the purposeful simplification of the relationships between the plant and other components of the natural habitat. This simplification should make agricultural ecosystems easier to control, but instead it creates conditions of extreme weakness for plant life. Many studies have shown that the conversion of natural land to agriculture, together with the agricultural intensification that enhances soil organic matter (SOM) depletion, makes the greatest contribution to soil biodiversity loss [START_REF] Giller | Agricultural intensification, soil biodiversity and agroecosystem function[END_REF][START_REF] Six | A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics[END_REF]Sofo et al., 2014a;[START_REF] Vitti | Beneficial effects of Trichoderma harzianum T-22 in tomato seedlings infected by Cucumber mosaic virus (CMV)[END_REF]Sofo et al., 2020a).

The large-scale use of pesticides may also have direct or indirect effects on soil biodiversity, but the lack of data has resulted in contradictory research results (EASAC -European Academies Science Advisory Council, 2018; [START_REF] Silva | Distribution of glyphosate and aminomethylphosphonic acid (AMPA) in agricultural topsoils of the European Union[END_REF]. SOM, especially its stabilized fraction (humus in chemical sense), plays a crucial role in climate change mitigation and adaptation [START_REF] Shukla | IPCC -Summary for Policymakers[END_REF]. Amount and types of SOM are principally determined by the continuous physical and chemical action of soil organisms. Soil fauna and microbes are crucial for breaking down, transforming and decomposing SOM [START_REF] Filser | Soil fauna: key to new carbon models[END_REF]. For this reason, studies focused on understanding soil microorganism-SOM and macrofauna-SOM interactions are needed, and guidelines for future experimentation and best regenerative practices to exploit soil multifunctionality have to be developed, tested and validated.

Soils and crops are vulnerable to climate change and environmental stress, and will be more and more in the near future. Many crops are endangered by increasing water shortage, often due to changes in rainfall frequency, and rise of soil aridity and desertification, eventually resulting in deteriorated soil structure and critically low levels of SOM, macro-and micronutrients, all of which essential for water provision and plant growth [START_REF] Matson | Agricultural intensification and ecosystem properties[END_REF]Sofo et al., 2019a). The frequent and strong soil tillage, typical of intensive agriculture, significantly affects the stability of soil microaggregates, that have a key role in SOM stabilization and support long term carbon sequestration, being more stable than macroaggregates [START_REF] Filser | Soil fauna: key to new carbon models[END_REF]. This triggers a detrimental vicious circle which ultimately leads to an increase in the use of mineral fertilizers and pesticides, and needs continuous and strong soil tillage to replace the burrowing and aggregating activity of soil animals, again increasing SOM loss [START_REF] Silva | Pesticide residues in European agricultural soils -A hidden reality unfolded[END_REF]. In brief, such soils become mere "containers" for plant roots and function as a carbon source, worsening the greenhouse effect even further.

Healthy, fertile soils are rich in SOM built of carbon that living plants remove from the atmosphere through photosynthesis. SOM fuels the soil organisms which improve soil structure and recycle mineral elements that plants take up as nutrients (FAO and ITPS, 2015;[START_REF] Mechri | Productivity and Efficiency Measurement in Agriculture[END_REF]. But soils release carbon, too. The frequent use of tillage and fertilizers, characteristic of modern conventional agriculture, has accelerated SOM degradation, releasing more carbon into the atmosphere. The last IPCC report [START_REF] Shukla | IPCC -Summary for Policymakers[END_REF] concludes that, globally, cropland soils have lost 20-60% of their original SOM content. On top of those losses, modern agriculture consumes a lot of fossil fuels to pull plows and manufacture the synthetic nitrogen fertilizers that farmers rely on to coax large harvests from degraded soils.

Additionally, restoring soil health would help mitigate the effects of climate change.

Increasing the amount of SOM enhances its ability to hold water. Improving soil structure would reduce erosion and retain more rainfall, where it can better sustain crops -especially during drought-stressed years (Sofo et al., 2019b). In addition to benefiting the climate, less fertilizer use will reduce off-farm water pollution [START_REF] Silva | Pesticide residues in European agricultural soils -A hidden reality unfolded[END_REF]. Land management choices also affect the amount of carbon stored in trees, plants and soil (FAO, 2020). The last IPCC report [START_REF] Shukla | IPCC -Summary for Policymakers[END_REF] estimates that serious changes in forestry and agriculture to curtail deforestation and improve soil management could reduce global emissions by 5 to 20 percent. While this won't solve the climate problem, it would represent a significant down payment on a global solution.

What is it really important to monitor in a soil? Certainly, a comprehensive and in-depth analysis of soil characteristics is required. To do this, correct soil sampling (e.g., by analyzing composite soil samples taken at different soil depths in specific soil horizons) and/or an appropriate pedological excavation are essential. This can allow the definition of physical soil properties (accumulation of salts, soil compaction, reduction of macropores, soil hydraulic conductivity -vertical and horizontal water infiltration), soil macroporosity (with macropores analysis and their shape and size, both relevant for water infiltration), soil moisture, preferably measured by sensors in different soil horizons during the year. Other important parameters for defining soil fertility are the root status, evaluation of the healthy status of the roots and of root morphology (root density, root diameter, amount of white, suberized and dead roots, etc.), microscopic analysis of the roots to observe eventual physical damage of the roots (e.g., necrotic parts, increased lignification, etc.). Among chemical parameters, a key role for the fertility of agricultural soils is played by soil nutrients (particularly nitrogen and phosphorus) and the presence/absence of soil contaminants, such as heavy metals, pesticides and hydrocarbons. Soil gas analyses are also relevant, as the determination of carbon dioxide, nitrogen oxides and methane (and oxygen too, even if it cannot be easily detected) by means of gas chromatographic techniques or portable laser-based trace gas analyzers will allow us to distinguish between anaerobic and aerobic micro-environments, with the latter able to promote several plant diseases due to pathogenic microbial attacks.

Soil biological quality/fertility

Soils host a quarter of our planet's biodiversity, but most of it remains unknown [START_REF] Sofo | Effects of Trichoderma harzianum strain T-22 on the growth of two Prunus rootstocks during the rooting phase[END_REF][START_REF] Wall | Biodiversity in the dark[END_REF][START_REF] Fierer | Embracing the unknown: Disentangling the complexities of the soil microbiome[END_REF]Sofo et al., 2020a). One gram of soil may contain up to one billion bacterial cells, tens of thousands of taxa, up to 200 million of fungal hyphae, and a wide range of invertebrates like earthworms, springtails and nematodes [START_REF] Wagg | Soil biodiversity and soil community composition determine ecosystem multifunctionality[END_REF], that are all part of a complex and interconnected food web [START_REF] Lavelle | Biodiversity: Conserving Biodiversity in Agroecosystems[END_REF][START_REF] Wall | Soil biodiversity and human health[END_REF][START_REF] Williamson | Viruses in Soil Ecosystems: An Unknown Quantity Within an Unexplored Territory[END_REF]. The health of all multicellular organisms (including plants, animals, and humans) and their surrounding ecosystems are interconnected through a subset of microbial organisms found in the plant and soil compartments, particularly in the rhizosphere. Plants nurture an entire world of soil organisms that feed and protect the plants according to aboveground-belowground, plant-soil feedbacks (PSFs) that have different spatio-temporal scales and are greatly affected by climate-related factors [START_REF] Ponge | Plant-soil feedbacks mediated by humus forms: A review[END_REF][START_REF] Veen | Applying the aboveground-belowground interaction concept in agriculture: Spatio-temporal scales matter[END_REF].

The diverse communities of telluric bacteria and fungi keep the soil healthy and fertile and determine the main biogeochemical processes that make life possible on Earth [START_REF] Dastgerdi | Toward An Innovative Strategic Approach For Sustainable Management Of Natural Protected Areas In Italy[END_REF][START_REF] Sweeney | Root traits explain rhizosphere fungal community composition among temperate grassland plant species[END_REF][START_REF] Wilpiszeski | Soil Aggregate Microbial Communities: Towards Understanding Microbiome Interactions at Biologically Relevant Scales[END_REF]. They play fundamental roles in driving many ecosystem processes on which the functioning of terrestrial ecosystems depends, including soil formation, nutrient and water cycling, climate regulation, carbon storage, production of food, medicine and fibre, disease and pest control, and greater resilience to global change [START_REF] Bardgett | Belowground biodiversity and ecosystem functioning[END_REF][START_REF] Lavelle | Biodiversity: Conserving Biodiversity in Agroecosystems[END_REF]. While soil biodiversity represents an important biological and genetic resource for biotechnological innovation with benefits to society, it is increasingly threatened by different forms of land degradation (FAO, 2020;FAO and ITPS, 2015). Soil biodiversity is vulnerable to many human disturbances, including intensive and non-sustainable agricultural practices, land use, climate change, nitrogen-enrichment, soil pollution, invasive species, and sealing of soil [START_REF] Orgiazzi | Soil biodiversity and DNA barcodes: opportunities and challenges[END_REF]. Soil microorganisms' dynamics (e.g., mobility, growth, nutrient absorption and respiration), mainly responsible for soil fertility and quality [START_REF] Bünemann | Soil quality -A critical review[END_REF], are strongly affected by the soil management (Enwall et al., 2007;[START_REF] Jeanbille | Soil Parameters Drive the Structure, Diversity and Metabolic Potentials of the Bacterial Communities Across Temperate Beech Forest Soil Sequences[END_REF]Sofo et al., 2020a[START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF]. When the soil microbial biocenosis is significantly altered, cultivated plants are more susceptible to diseases and display stunted growth. In this respect, correct agronomic techniques (fertilisation, irrigation, soil tillage, etc.) become instruments to recover the disrupted equilibrium. However, the functionality and metabolism of soil microorganisms are related to soil quality and fertility, as they influence and, at the same time, are influenced by the soil carbon and nitrogen contents, bacteria being an essential part of carbon and nitrogen cycling processes [START_REF] De Vries | Controls on soil microbial community stability under climate change[END_REF][START_REF] Mooshammer | Adjustment of microbial nitrogen use efficiency to carbon:nitrogen imbalances regulates soil nitrogen cycling[END_REF][START_REF] De Vries | Below-ground connections underlying above-ground food production: a framework for optimising ecological connections in the rhizosphere[END_REF][START_REF] Li | Contributions of residue-C and -N to plant growth and soil organic matter pools under planted and unplanted conditions[END_REF]. Microbial interactions play a critical role not only in regulating ecological functions and processes but ultimately in determining the health of plants, animals and humans as components of terrestrial ecosystems [START_REF] Fierer | Embracing the unknown: Disentangling the complexities of the soil microbiome[END_REF][START_REF] Sofo | Comparing the effects of soil fauna on litter decomposition and organic matter turnover in sustainably and conventionally managed olive orchards[END_REF][START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF]. Having co-evolved with a plethora of microorganisms, plants benefit from microbial symbiosis, while simultaneously facing challenges from pathogens.

Microbial communities are regulated by the activity of soil animals, some of which play an essential role as ecosystem engineers [START_REF] Lavelle | Functional domains in soils[END_REF]. Earthworms, ants and termites (in arid tropical countries) contribute significantly to the creation of an interconnected pore network [START_REF] Pagenkemper | The effect of earthworm activity on soil bioporosity -Investigated with X-ray computed tomography and endoscopy[END_REF] into which air and water are circulating [START_REF] Pla | Role of soil pore structure in water infiltration and CO2 exchange between the atmosphere and underground air in the vadose zone: A combined laboratory and field approach[END_REF], and which are hot spots of microbial activity [START_REF] Hoang | Hotspots of microbial activity induced by earthworm burrows, old root channels, and their combination in subsoil[END_REF]. They also contribute to the creation of soil aggregates [START_REF] Lavelle | Soil aggregation, ecosystem engineers and the C cycle[END_REF], offering a dynamic habitat to microbial colonies [START_REF] Gupta | Soil aggregation: Influence on microbial biomass and implications for biological processes[END_REF] and preventing loss of carbon, water and nutrients [START_REF] Pulleman | Physical protection of mineralizable C in aggregates from long-term pasture and arable soil[END_REF]. Soil engineers and their associated microbiome produce metabolites which exert a hormonal effect on plants [START_REF] Muscolo | Effect of molecular complexity and acidity of earthworm faeces humic fractions on glutamate dehydrogenase, glutamine synthetase, and phosphoenolpyruvate carboxylase in Daucus carota α II cells[END_REF] and act as signals which stimulate the defence metabolism of plants [START_REF] Blouin | Belowground organism activities affect plant aboveground phenotype, inducing plant tolerance to parasites[END_REF]. Soil animals of smaller size, although not directly involved in the physical transformation of the soil, are the main regulating agents of the microbial compartment. By feeding on fungi and bacteria they contribute to maintaining the microbial biomass in an active state [START_REF] Kaneda | Collembolans feeding on soil affect carbon and nitrogen mineralization by their influence on microbial and nematode activities[END_REF], and decrease competition among microbial strains by feeding preferentially on those growing faster [START_REF] Newell | Interaction between two decomposer basidiomycetes and a collembolan under Sitka spruce: Distribution, abundance and selective grazing[END_REF]. Although still in need of research, it can be expected that selective grazing of microbial colonies by tiny soil animals (nematodes, protozoa, microarthropods, enchytraeids) increases soil microbial biodiversity locally, as this has been shown to occur with grassland vegetation under moderate herbivore grazing. It has even been suggested that some soil animals would consume preferentially microbial pathogens and thus could contribute to decrease soil pathogenicity [START_REF] Friberg | Influence of soil fauna on fungal plant pathogens in agricultural and horticultural systems[END_REF]. Soil food webs, from microbes to top predators, include trophic chains [START_REF] Pollierer | Compound-specific isotope analysis of amino acids as a new tool to uncover trophic chains in soil food webs[END_REF] by which fresh organic matter is transformed in humus [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF] at a rate increasing with the number and complementarity of functional niches of soil animals [START_REF] Heemsbergen | Biodiversity effects on soil processes explained by interspecific functional dissimilarity[END_REF].

It is essential to link biodiversity measures with specific soil functions and plant status under particular environmental contexts, particularly in agrosystems [START_REF] Ramirez | Toward a global platform for linking soil biodiversity data[END_REF].

For instance, while some soil functions are driven by a diverse set of organisms that contribute to functional resilience (e.g., decomposition), other soil functions involve a more specific set of organisms (e.g., nitrifiers, bio-control agents) which make them more vulnerable to biodiversity loss. [START_REF] Wagg | Soil biodiversity and soil community composition determine ecosystem multifunctionality[END_REF] showed that soil biodiversity loss or simplification of soil community composition can impair multiple ecosystem functions, including plant diversity, decomposition, nutrient retention, and nutrient cycling. A better understanding of the pivotal roles of soil organisms in mediating soil-based ecosystem services, as affected by ecosystem management approaches and practices adapted to socioecological contexts, is also central to guiding biodiversity-friendly agricultural intensification [START_REF] Lal | Restoring Soil Quality to Mitigate Soil Degradation[END_REF]FAO, 2020).

Soil classification

Crucial historical legacies

In this section we summarize the historical legacies of two masters of soil classification, Dukuchaev Vasily Vasilyevich (1846-1903), considered the founder of the soil classification in Europe, and Hans Jenny (1899Jenny ( -1992)), one of the greatest American pedologists, a Swiss who ended up working in California at Berkeley University. 1) From "Zones verticales des sols, zones agricoles, sols du Caucase" (Dokuchaev, 1900), cited in "A l'aube de la Science du sol" (Legros, 2019 c) At the field scale, climatic variability does not have to be considered, while topography and variability of the geological substrate can still modify soils. d) Dokuchaev and his students also observed that there were exceptions to climate zonality. Local environmental factors play a predominant role and mask the role of climate. It will take many years to conceptually resolve this problem, as evidenced by the changes in vocabulary introduced on the subject over time. Currently we distinguish between "zonal" soils, that are part of the climatic zonality, "azonal" soils, whenever the rock outcrops directly, and "intra-zonal" soils, whose characteristics are linked to special conditions, such as excess water or salt.

2) In "Factors of soil formation -A System of Quantitative Pedology" [START_REF] Jenny | Factors of Soil Formation : A System of Quantitative Pedology[END_REF]:

a) The soil system is an open system; substances may be added to or removed from it.

Every system is characterized by properties that we may designate by symbols, such as s1, s2, s3, s4, s5, etc. For example, s1 may indicate nitrogen content, s2 acidity, s3 apparent density, s4 amount of calcium, s5 pressure of carbon dioxide, etc. Any system is defined when its properties are stated.

b) The initial state of the soil system has been designated as parent material. Climate (cl), Organisms (o), Topography (r), Parent material (p), and Time (t) completely describe the soil system. The total change of any soil property depends on all the changes of the soil-forming factors following a fundamental equation: ,o,r,p,t,• • • ), where: s stands for "soil property", f for "function of," or "dependent on", dots show that, besides the variables listed, additional soil formers may have to be included. In a more precise differential mathematical formula, the equation (1) becomes:

(1) s = f (cl
(2) c) In selecting cl, o, r, p, and t as the independent variables of the soil system, we do not assert that these factors never enter functional relationships among themselves.

We emphasize the fact that the soil formers may vary independently and may be present in a great variety of combinations, either in nature or under experimental conditions. To find out the role played by each soil-forming factor, it is necessary that all the remaining factors be kept constant. A serious practical difficulty in solving Equation (1) in the field arises from the requirement of keeping the soil formers constant. In laboratory experiments on soil formation, we can exercise rigid control of the conditioning variables (e.g., temperature, moisture, etc.) and thus obtain sets of data that leave no doubt as to the functional relationship between them. Under field conditions, considerable variation in the magnitude of the variables cannot be avoided, in consequence of which we arrive at scatter diagrams rather than perfect functions. Statistical considerations must be introduced, and the resulting equations possess the character of general trends only. Even so, the gain in scientific knowledge fully justifies the mode of approach.

Soil and vegetation

A major problem with the current system of soil classification is that it is difficult for non-specialists to use and understand. Perhaps it is possible to take an example from botanists and the synthetic methods they have prepared. There are several aspects that vegetation and soil coverings have in common: a) both are "covers", both correspond to a continuous layer that shows changes inside and outside perceptible to the human eye. If there is more water in the system, for example, the vegetation changes and also the underlying soil; external change can also be seen from satellite; the internal change (structure) is noticed by the naked eye with some training. Of course, the more the water (or other main ecological factors) balance differs, the more the change is visible; b) in both, the whole cover can be broken down into sub-layers. For example, for vegetation we speak of arboreal, shrubby and herbaceous layers; for the soil of organic, organic-mineral and mineral horizons; c) in both cases, circumscribing spatial sub-units (horizontal or vertical) is not so simple, because the transition from one to the other unit is rarely abrupt; very often it is gradual and nuanced. It is normal for this to happen, because in the two cases the factors involved in the distribution of the plant and animal species that inhabit these two coverages are interdependent and evolving.

We know that using the characteristic species of the phytosociological units to map the forest vegetation, the part of the forest occupied by undefined vegetation types becomes larger than that occupied by known vegetation types (Zanella and Géhu (Directeur), 1994). In fact, to define vegetation units, lists of particular species are needed; these characteristic species are not the most common in each vegetation unit (if they were the most common, they would also be present elsewhere). Thus, by definition, a large part of an area covered by phytosociological units lacks the characteristic species. This prevents phytosociology from best expressing its operationality. A map of the distribution of vegetation units is always ambiguous and nuanced in many transitional places (Bartoli, 1966;[START_REF] Susmel | Normalizzazione delle foreste Alpine: basi ecosistemiche, equilibrio, modelli colturali, produttivit : con applicazione alle foreste del Trentino[END_REF][START_REF] Susmel | Saggio critico-sperimentale sulla applicabilita del metodo fitosociologico in selvicoltura[END_REF]. The founding principle [START_REF] Braun-Blanquet | Pflanzensoziologie[END_REF][START_REF] Clements | Nature and Structure of the Climax[END_REF] says that the species lists are repetitive in the space, and therefore it is possible to put a name on each list and produce vegetation maps. We know, however, that for ecological reasons, plant lists can only be partially repetitive. The species respond to the environment and form an unlimited number of combinations that are constantly evolving in space and time [START_REF] Tansley | The use and abuse of vegetational terms and concepts[END_REF]. To make an ecosystem survey, in which a given group of plants and animals coevolve, people have to place themselves in the center of an ecologically homogeneous environment. Unfortunately, detecting such ecological homogeneity remains a subjective matter, also because the success of this coexistence can only be discovered in the future, during the succession of an ecosystem.

Classical soil classification

Foresters considered mainly the organic surface part of the soil, calling it the "humus form", P.E. Müller in [START_REF] Jabiol | Avant et après P.E. Müller: évolution des conceptions sur la description et la typologie des humus[END_REF], whilst agronomists concentrated mostly on the mineral part of the soil (which also contained the organic part buried with the cultivation) which they called "soil". While foresters selected morpho-functional traits, agronomists focused on climate (USDA) and physical-chemical composition, texture, structure, thickness of various diagnostic horizons (USDA and other National and International classifications), with the scope to provide mineral elements and nutrients for crops. Foresters tried to link humus forms to the floristic composition or soil animal lists [START_REF] Hartmann | Waldhumusdiagnose auf Biomorphologischer Grundlage[END_REF][START_REF] Hartmann | Diagnosi degli humus forestali su basi biomorfologiche[END_REF][START_REF] Klinka | Taxonomic classification of humus forms in ecosystems of British Columbia. First approximation[END_REF] and the ability to regenerate the forest [START_REF] Toutain | Les humus forestiers[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Camaret | Distribution spatiale et évolution temporelle de la végétation et de sa diversité : relations avec l'hétérogénéité des structures des peuplements en pessi re d'altitude[END_REF][START_REF] Ponge | Effets des amendements sur le fonctionnement biologique des sols forestiers: mieux comprendre le rôle de la méso-et de la macrofaune dans l'évolution des humus (in French, with English summary)[END_REF][START_REF] Bernier | Hotspots of biodiversity in the underground: A matter of humus form?[END_REF]. Agronomists, on the other hand, linked soil types to the specific climate and needs of crops [START_REF] Jenny | Factors of Soil Formation : A System of Quantitative Pedology[END_REF][START_REF] Birkeland | Soils and geomorphology[END_REF][START_REF] Beaudette | Algorithms for quantitative pedology: A toolkit for soil scientists[END_REF][START_REF] Brenna | Guidelines for Conservation Agriculture application and dissemination[END_REF][START_REF] Berdugo | Global ecosystem thresholds driven by aridity[END_REF].

Each country built its own classifications.

For the soil, the need for unification was necessary when international organizations wanted to map soils at planet level. A summary of the history of soil classification can be found in the FAO-website portal. The FAO-Unesco Soil Map of the World (1974) is structured in 10 Volumes composed of a common Legend (Volume 1) and nine sections corresponding to different areas of our planet (http://www.fao.org/soils-portal/data-hub/soilmaps-and-databases/faounesco-soil-map-of-the-world/en/). The authors of the cartography tried to respect the principles mentioned above (5.1. Crucial historical legacies). In FAO- UNESCO (1974) we can read: "The number of soil units which compose the legend of the Soil Map of the World is 106. The legend sheets present these soil units in an order which reflects the general processes of soil formation. The basic principles which underlie the separation of these soil units and their definitions are discussed in Chapter 3. Areas of "non soil" are shown on the map as miscellaneous land units." It sounded very promising. Unfortunately, the agreement was only apparent. It hid strong differences of thought that later revealed themselves in the construction of different national operational classification systems. The USA built their own classification (Soil Survey Staff, 1975[START_REF] Birkeland | Soils and geomorphology[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: A framework to biodiversity[END_REF], 2010, 2014, 2015;Soil Science Division Staff, 2017), FAO and IUSS another [START_REF] Jahn | Guidelines for Soil Description -Fourth Edition[END_REF][START_REF] Wrb | World Reference Base for Soil Resources[END_REF] IUSS Working Group WRB, 2007; IUSS Working Group WRB, 2010; IUSS Working Group WRB, 2015; [START_REF] Charzyński | Classification of urban soils[END_REF], and many states operated independently in their countries, often agronomists on one side and foresters on the other, for example in France [START_REF] Afes | Référentiel Pédologique 2008[END_REF]. In Italy, for historical reasons, the classification of soils depends on the region in which it is practiced: in the North West, the French system prevails, even for humus forms, in the North East, the German or USDA systems are adopted but an ancient Austrian system [START_REF] Hartmann | Diagnosi degli humus forestali su basi biomorfologiche[END_REF] was until recently used for humus forms, and in the Central-South often the USDA is preferred sometimes that of IUSS Working Group, while for humus it is not uncommon to refer to a Canadian manual [START_REF] Klinka | Taxonomic classification of humus forms in ecosystems of British Columbia. First approximation[END_REF].

Even worse, when there are more than three variables interacting, and in the soil there are dozens, a natural system ends up in chaotic and unpredictable movement [START_REF] Mayr | Systematics and the origin of species from the viewpoint of a zoologist[END_REF][START_REF] Lorenz | Deterministic Nonperiodic Flow[END_REF]. Soil profiles, as other natural systems, are all different and impossible to classify through a list of characters subdivided in categories as foreseen by [START_REF] Jenny | Factors of Soil Formation : A System of Quantitative Pedology[END_REF]. It is thus impossible to give a name to the soil by fixing the position of a diagnostic horizon in the profile, or the color of this horizon using a panel of reference colors (Munsell), or the content of clay of this horizon. Each variable depends of so many other variables that, in a given point in the field, the combination of them is very large and unpredictable (Equation 1 or 2).

However, soil scientists decided to separately improve existing classifications. The result was that the national classifications diverged over time, making it impossible to merge them into a single reference.

For some years now, the specialists of the different classification schools have been organizing joint outings, such as soil classification IUSS activities. The best attempt to make the classification easier to understand to other natural science specialists is that of the Soil Survey Staff (Staff Soil Survey, 2015), which proposed a beautiful Illustrated Guide to Soil Taxonomy (version 2) based on the American system. The corresponding model of the World Reference Base Working Group was published in 2018 [START_REF] Blum | Essentials of Soil Science Soil formation, functions, use and classification[END_REF]. What is most surprising in these manuals is the almost absence of information on soil biology, zero pages in the Staff Soil Survey (2015), four pages in the [START_REF] Blum | Essentials of Soil Science Soil formation, functions, use and classification[END_REF], even if the "fathers" of soil science put soil organisms in second position, after the climate. And we know how much the climate itself is connected to the living beings of our planet, which have modified it to make it suitable for their development [START_REF] Lenton | Earliest land plants created modern levels of atmospheric oxygen[END_REF].

Is there a way to classify soil that is useful and also accessible to non-specialists?

To classify the soil in a simple way, it is necessary to define from the outset the purpose of the classification and the "limits of the boxes" in which it is useful to put the soils. The Unified Soil Classification System (USCS), for example, is a standardized method used in engineering and geology to describe the texture and grain size of a soil. It can be applied to most unconsolidated materials and is represented by a two-letter symbol. A similar "logical" model is already used for agricultural soils. The owner of a crop sends the soil sample to the laboratory which does the analysis and classifies that soil for that specific crop, and reports how much nutrients are missing to optimize its production. Although you may disagree on the principle that knowing what plants are made of makes possible to calculate what needs to be put into the soil [START_REF] Fusaro | Evaluation, maintenance and improvement of biodiversity for environmental protection and crop[END_REF][START_REF] Lowenfels | Teaming with Fungi: The Organic Grower's Guide to ycorrhizae[END_REF], with adjustments of doses and nutrients distributed over time, and crop rotation, the method works. There is now an important amount of research results on the biological quality of the soil. Some of the thousands of articles on the subject include historical context [START_REF] Magdoff | Soil Organic Matter in Sustainable Agriculture[END_REF][START_REF] Wander | Developments in Agricultural Soil Quality and Health: Reflections by the Research Committee on Soil Organic Matter Management[END_REF]; European references [START_REF] Parisi | Biologie e ecologia del suolo -Tecniche di ricerca[END_REF][START_REF] Parisi | La qualità biologica del suolo. Un metodo basato sui microartropodi[END_REF]Chaussod, 1996;[START_REF] Balbo | Bio-Bio Project[END_REF][START_REF] Ponge | The impact of agricultural practices on soil biota: A regional study[END_REF] and climate change implications [START_REF] Brussaard | Soil biodiversity for agricultural sustainability[END_REF][START_REF] Bispo | Carrefour de l'innovation agronomique -18 octobre 2018 Conseil Régional Bourgogn-Franche Comté -Dijon -INRA[END_REF][START_REF] Yin | Soil functional biodiversity and biological quality under threat: Intensive land use outweighs climate change[END_REF].

In few words, a QBS index (Biological Quality of the Soil) is calculated by sampling soil animals, especially arthropods and worms. These animals are sorted into functional groups and studied in different types of soils. It can be seen if one soil is richer in functional biodiversity (which guarantees the functioning of the soil, with the right proportion between mites and springtails, for example) than another [START_REF] Angelini | Tecniche di biomonitoraggio della qualit del suolo[END_REF][START_REF] Ruiz-Camacho | INDICE BIOLOGIQUE DE LA QUALITE DES SOLS (IBQS) Bio-indicateur de la qualité des sols basés sur l'étude des peuplements de macroinvertébrés[END_REF][START_REF] Nuria | IBQS: A synthetic index of soil quality based on soil macro-invertebrate communities[END_REF]. Similarly, soil microorganisms are classified into functional groups designed to reflect soil management [START_REF] Torsvik | Microbial diversity and function in soil: from genes to ecosystems[END_REF][START_REF] Nesme | Back to the Future of Soil Metagenomics[END_REF].

In summary, there are many useful ways to classify the soil, but none which satisfy what the founders of the soil science wished, that is to understand the soil on a planetary level and to appreciate how this system was distributed/developed on the planet in harmony with life.

There is perhaps a way to get to this; not to attempt to circumscribe the infinite final soil types, but to identify a small number of soil-forming processes to which more soil types are arbitrarily assigned.

Soil and humus

When an artificial crop system is compared to a forest, things change. We think of a whole system composed of species that have historically come together to collaborate and optimize energy resources. Forest managers call the attempt to mathematically imitate the achievement of a balanced forest ecosystem in terms of age, number and size of trees as "normalization" [START_REF] Susmel | Normalizzazione delle foreste Alpine: basi ecosistemiche, equilibrio, modelli colturali, produttivit : con applicazione alle foreste del Trentino[END_REF][START_REF] Phillips | Pattern and process in Amazon tree turnover, 1976-2001[END_REF][START_REF] Pan | A Large and Persistent Carbon Sink in the World's Forests[END_REF][START_REF] Oldeman | Forests: Elements of Silvology[END_REF][START_REF] Reinmann | Edge effects enhance carbon uptake and its vulnerability to climate change in temperate broadleaf forests[END_REF][START_REF] Hasan | Optimization of selected parameters of the forest growth model Biome-BGC (version ZALF) using HOPSPACK[END_REF][START_REF] Mahdavi | A hidden harvest from semiarid forests: landscape-level livelihood contributions in Zagros, Iran. For[END_REF]. The cycle concerns a part of the forest surface, a mosaic piece that regenerates when an old tree die and leads to the stability of the forest mosaic as a whole. The number of trees on the surface under renewal must decrease exponentially following a known curve with parameters related to the species and called the "norm". Soil plays a fundamental role in the cycle, and it is the superficial part of the soil profile composed of organic (O or H) and organo-mineral (A) horizons that changes most rapidly, the whole profile following within a much longer time [START_REF] Achat | Forest soil carbon is threatened by intensive biomass harvesting[END_REF][START_REF] Baldrian | Forest microbiome: diversity, complexity and dynamics[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Osman | Nutrient Dynamics in Forest Soil[END_REF][START_REF] Poeplau | Depletion of soil carbon and aggregation after strong warming of a subarctic Andosol under forest and grassland cover[END_REF][START_REF] Takahashi | Atypical Pattern of Soil Carbon Stocks along the Slope Position in a Seasonally Dry Tropical Forest in Thailand[END_REF][START_REF] Zanella | Humus Forestali -anuale di ecologia per il riconoscimento e l'interpretazione -Applicazione alle faggete, CEA[END_REF]Zanella et al., , 2018a)).

The need to have one international system for classification of all humus forms is quite recent. Unification began in July 2003, with a meeting in Trento (Italy) of 26 European specialists with the specific aim of arriving at a European proposal for the classification of humus forms. The need for unification arose from the necessity to accurately calculate the carbon cycle at the level of several European countries. The profile of a natural soil is very complex, and it is not enough to take samples at predefined depths to know the amount of carbon that it stores. It is better to subdivide the soil into horizons and then take a sample in each of them. With ½ or more of the carbon being concentrated in the organic and organomineral horizons, which correspond to the humus forms, it was decided to try to classify them in a standardized way. In 2018, the summary of the work (Fig. 3) was collected in threes Applied Soil Ecology special issues (Zanella and Ascher-Jenull, 2018a, 2018b, 2018c).

[Figure 3. HERE]

The soil develops and changes with the system that contains it, and depending on the space-time scale with which one looks at it (Zanella et al., 2018e, 2018b. It seems like a trivial matter; in reality it changes the perspective with which one has to look at the soil when wanting to classify it. One way to classify soil taking into account its dynamics is to break it down into three layers which are evolving and influencing each other but which remain and can be classified independently (Fig. 4).

[Figure 4. HERE]

Living organisms organize the superficial part of the Earth's crust into layers that are visible to the naked eye. If we consider the soil as a living ecosystem, it begins with the colonization process of the rock by microorganisms (time 1 on Fig. 4). In time 2, we can see an organic matter that begins to bond with the mineral one and form aggregates (OA horizon), and the rock fragments generating an initial Lithopedon (C). In time 3, the organo-mineral aggregates become well amalgamated and form a structured A horizon. In time 4, a new layer called Copedon begins to form between well-developed Humipedon and Lithopedon; it arises from the interaction of them. In time 5, Copedon (horizon B) increases in volume and takes on a mature form. In time 6, we arrive at a complete, mature soil profile, where the Copedon can also present an E horizon.

It is possible to break down the soil profile into three main sections: Humipedon (organic O and/or organic-mineral A horizons); Copedon (mineral horizons E and/or B); Lithopedon (mineral horizons C and R). The advantage is that of being able to study these parts of the soil as if they were relatively independent (Zanella et al., 2018d). In the natural environment, the formation of the soil profile can stop at the Humipedon stage (in films of microorganisms on bare rock, or in soil crust), or also comprise a Lithopedon (sometimes very thick in soils at high altitude, such as Rendzina or Rakers, for example), or form an additional Copedon (as in the most common mature soils) which may be very deep in tropical soils (Fig. 4, time 5 and 6); eroded soils often lost their whole Humipedon. Moving on to classification, morpho-functional diagnostic horizons were chosen for each Humipedon. As an example, the Mull system contains those humus forms that are able to integrate into an organic-mineral horizon all the litter annually produced by the ecosystem in which they develop. As consequence, all the humus forms belonging to a Mull system are characterized by 1. the absence of the last stage of litter transformation (corresponding to a diagnostic horizon called OH, made of organic thin fragments, mixed with small animal droppings) and 2. the presence of an organic-mineral A horizons made of zoogenic large aggregates. In the same way, other morpho-functional, organic and organo-mineral, zoogenic and non-zoogenic horizons were defined to classify each Humipedon in five Terrestrial humus systems (Mull, Moder, Mor, Amphi and Tangel). In each humus system the relative thickness of the diagnostic horizons allows to individuate 3-4 finer humus forms.

TerrHum, a free available Android app, can help to remember the diagnostic characters and horizons (https://play.google.com/store/apps/details?id=fr.inra.terrhum) corresponding to each humus system and forms. The same reasoning was done for submerged (Histic) tidalic seaside (Aqueous) and initial, or man-modified, or artificial (Para) humus systems. The classification method is very similar to that described in the FAO-UNICEF map legend: to circumscribe a soil ecosystem at the top of the Earth crust, that grows over time and evolves following a pedogenetic process, dependent on those same soil forming-factors reported in the preceding Crucial historical legacies. The recognition of morpho-functional elements then makes it possible to identify humus systems and forms.

To obtain a classification of the whole soil profile, it would be enough to do the same proceeding for the Copedon and Lithopedon, and to identify some morpho-functional diagnostic horizons corresponding to given processes of pedogenesis. This work is already present and explicitly reported in the FAO-UNICEF first world soil maps. As if the soil could not correspond to a real ecosystem, recent classifications have given up looking for a purpose in the evolution of the soil.. We think it is a serious fundamental error, which generates confusion and misunderstanding. We suggest assigning the classic pedogenesis processes to

Copedon, in order to recover much of the classification work that is also found in recent soil classifications (examples: Eluviation/Illuviation, with the formation of an E horizon in missing the finished in Bt clay and genesis of a Luvisol profile; Podzolization, with formation of Bhs or/and Bs horizons and genesis of a profile of Podzol; Brunification/weathering, with formation of Bw horizons and genesis of a profile of Cambisols, and so on). It is a question of returning to the origin of Pedology, where pedogenesis processes were established, and which implies that the soil is a real evolving ecosystem.

A classification of the Lithopedon would help to distinguish parent material from substrate. In addition, Lithopedon physical and chemical qualities certainly influence the cycles of nutrients, carbon, air and water in the soil (Cremeens et al., 1994;[START_REF] Gupta | Encyclopedia of Soil Science, Encyclopedia of Earth Sciences Series[END_REF]. [START_REF] Juilleret | What about the regolith, the saprolite and the bedrock? Proposals for classifying the subsolum in WRB[END_REF][START_REF] Moragues-Quiroga | Genesis and evolution of regoliths: Evidence from trace and major elements and Sr-Nd-Pb-U isotopes[END_REF][START_REF] Juilleret | What about the regolith, the saprolite and the bedrock? Proposals for classifying the subsolum in WRB[END_REF] suggest to implement four "subsolum materials" in the WRB as qualifiers; Moragues-Quiroga et al.

(2017) suggest to survey potential geochemical tracers of exchange processes at the watermineral interface, suggest to differentiate surface organic compartment, upper horizons with presence of atmosphere-derived particles, lower horizons characterized by a mineralogical composition pointing to ancient events.

In Figure 5, the subdivisions (which are rarely abrupt) within the three soil layers (Humipedon, Copedon and Lithopedon) are expressed with colors. The arrows correspond to the interrelationships between the subsystems (Humi-Co-and Litho-pedon) of the soil. These have their own dynamics which are partly independent of the whole, but which can be detected with the naked eye by looking for signs of co-evolution (pedological processes), which ultimately generate each particular soil profile.

[Figure 5. HERE]

The forms of humus are directly linked to particular groups of soil animals [START_REF] Ponge | Humus forms in terrestrial ecosystems: A framework to biodiversity[END_REF][START_REF] Ponge | Plant-soil feedbacks mediated by humus forms: A review[END_REF]Zanella et al., 2018b) and agents of biodegradation which are in turn linked to soil microorganisms [START_REF] Sofo | Soil microbial diversity and activity in a Mediterranean olive orchard using sustainable agricultural practices[END_REF](Sofo et al., , 2019;;[START_REF] Bispo | Carrefour de l'innovation agronomique -18 octobre 2018 Conseil Régional Bourgogn-Franche Comté -Dijon -INRA[END_REF][START_REF] Bayat | Calcic soils as indicators of profound Quaternary climate change in eastern Isfahan, Iran[END_REF][START_REF] Karimi | Biogeography of soil bacteria and archaea across France[END_REF][START_REF] Karimi | Biogeography of Soil Bacterial Networks along a Gradient of Cropping Intensity[END_REF][START_REF] Liang | Quantitative assessment of microbial necromass contribution to soil organic matter[END_REF]. This connection could have very important consequences on the management of forest and agricultural soils to stop the climate from warming. The ultimate goal could be to integrate in the new classification all those data relating to biological functioning that are now collected by people who already practice a non-specialized soil classification.

From another point of view, which is the one concerning agricultural soil, soil that produces food for humans and domesticated animals, it was thought that to transform/modify/till such type of soil was an economically very profitable action. At the time of maximum expansion of the technological revolution of agriculture, Hans Jenny became aware of the possible misunderstanding, and in 1980 defined soil as follows: "Soil is more than farmer's dirt, or a pile of good topsoil, or engineering material; it is a body of nature that has its own internal organization and history of genesis" [START_REF] Jenny | The Soil Resource, Origin and Behaviour[END_REF]. In a letter to Science of the same year, he even wrote: "Because of a possible climatic warm-up, we do not wish accelerate humus oxidation and the concomitant flux of carbon dioxide from soil into the atmosphere… The humus capital, which is substantial, deserves to be maintained because good soils are a national asset".

6. Which agricultural practices can be defined sustainable?

Conventional or sustainable agriculture?

Nowadays, agricultural production is at risk due to many adverse abiotic and biotic factors. Furthermore, climate change can potentially decrease the effectiveness of plant defence mechanisms and increase the risk of diseases through excess growth and physiological alteration of cultivated plants [START_REF] Vitti | Trichoderma harzianum T-22 Induces Systemic Resistance in Tomato Infected by Cucumber mosaic virus[END_REF]. In terms of increased temperature and extreme precipitation regimes, whether aridity or flooding, climate change will have detrimental agricultural consequences due to the interrelations between climate, land and water use, soil degradation and landscape changes [START_REF] Dale | The relationship between land-use change and climate change[END_REF][START_REF] Tsiafouli | Intensive agriculture reduces soil biodiversity across Europe[END_REF]. Nowadays, food security is an increasing concern in a growing number of countries.

This situation calls for a relevant appraisal of factors that could affect crop production. One of the factors promoting a sustainable food production system is soil biodiversity [START_REF] Sofo | Comparing the effects of soil fauna on litter decomposition and organic matter turnover in sustainably and conventionally managed olive orchards[END_REF]. Unfortunately, despite the promotion of sustainable soil management by the Global Soil Partnership since 2012, in many cases soil management is still focused on directly managing soil fertility, rather than on protecting soil biodiversity as a whole or single species individually, although soil biodiversity is known to be a main agent of soil fertility [START_REF] Altieri | Agroecology and the design of climate change-resilient farming systems[END_REF].

The application of pesticides, herbicides and mineral fertilizers, typical of conventional agriculture, cannot be considered an eco-friendly approach for crop production and defence, as their overuse can provoke water contamination, air and water pollution, and release of harmful residues and by-products into the soils [START_REF] Blouin | Chemical communication: An evidence for co-evolution between plants and soil organisms[END_REF][START_REF] Korkina | Humus Index as an indicator of the topsoil response to the impacts of industrial pollution[END_REF][START_REF] Van Groenigen | How fertile are earthworm casts? A meta-analysis[END_REF]. A decrease in soil quality due to conventional soil management negatively influences important ecosystem processes, like nutrient cycling and carbon sequestration [START_REF] Blouin | Belowground organism activities affect plant aboveground phenotype, inducing plant tolerance to parasites[END_REF][START_REF] Bampa | Harvesting European knowledge on soil functions and land management using multi-criteria decision analysis[END_REF]. On the other side, sustainable agriculture offers new chances to mitigate the effects of climate change. In sustainable agroforestry systems, management practices are able to increase carbon inputs into the soil and possibly reduce greenhouse gases (GHGs) emissions due to some revised field operations, e.g. by irrigation, use of recycled water, pest and disease management, fertilization, and soil and plant farming systems [START_REF] Mutuo | Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics[END_REF]. In turn, carbon enrichment increases biological activity by improving soil structure, as well as soil moisture and nutrient content that are beneficial to plant growth and production [START_REF] Marinari | Influence of organic and mineral fertilisers on soil biological and physical properties[END_REF].

As [START_REF] Lago | Legacy Effects of Agricultural Practices Override Earthworm Control on C Dynamics in Kiwifruit Orchards[END_REF] recently indicated, there is clear evidence that more environmentally friendly land management represents a promising strategy to increase soil carbon sequestration.

Soil and vegetation coevolve

Soil functions as a living system. Climate, organisms, topography, parent material and time completely describe the soil system. Soil and vegetation co-evolve implementing other living beings and creating the well-known zonation of biomes of planet Earth. In recent years, soil quality has been recognized to play a double role in the entire agroecosystem: it is important for a good production as well as for a healthy environment (Doran and Zeiss, 2000).

In conventional agriculture, still adopted by most farmers, frequent soil tillage strongly reduces the complexity and diversity of soil microbiota [START_REF] Adl | Slow recovery of soil biodiversity in sandy loam soils of Georgia after 25 years of no-tillage management[END_REF]. For this reason, conventional, non-sustainable agronomic practices should evolve into a more sustainable management addressed to ameliorate the ecological networks and nutrient cycles, in which soil microorganisms are involved. The adoption of sustainable soil management practices and organic agriculture can be eco-friendly and include safe methods to ameliorate plant physiological status, reduce plant disease incidence, and increase yield and quality without side damages to the environment and human health. For instance, it would be possible to adopt a sustainable approach that enriches the soil with biocontrol microorganisms with action against pests (El-Tarabily and Sivasithamparam, 2006), plant-growth-promoting microorganisms able to promote plant growth and development [START_REF] Abhilash | Plant Growth-Promoting Microorganisms for Environmental Sustainability[END_REF], and microorganisms able to increase the availability and uptake of essential nutrients in plants, e.g. mycorrhiza for phosphorus and nitrogen-fixing bacteria for nitrogen [START_REF] Bolan | A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants[END_REF].

In this scenario, the advantages of the adoption of a sustainable management (or conservation and regeneration agriculture), that includes no/minimum tillage, cover crop application, incorporation of grass and crop residues into the soil, and endogenous and exogenous soil carbon inputs [START_REF] Palm | Conservation agriculture and ecosystem services: An overview[END_REF] can be a key factor to enhance soil quality/fertility and production in a sustainable way, preserving natural resources and avoiding detrimental effects on the environment. Such benefits include a high level of soil microbial genetic/functional diversity and complexity both in the soil (bulk soil and rhizosphere) and in the plant (phyllosphere, carposphere, and endosphere), a faster carbon and nitrogen turnover, higher levels of SOM and soil water content, and better soil physical and chemical characteristics (Fausto et al., 2018). Inappropriate or exploitative crop agroecosystems represent a key threat for soil degradation through erosion, nutrient depletion or structural collapse (Didden et al., 1994). Increasing our knowledge on biochemical processes of soil microorganisms and animals involved in carbon and nitrogen dynamics, that influence in turn their availability for plants [START_REF] Shaffer | Modeling Carbon and Nitrogen Dynamics for Soil Management[END_REF], will lead to optimized management strategies for a multifunctional concept of agriculture.

Sustainable agriculture application and monitoring

Addressing knowledge gaps of sustainable practices is of fundamental importance as an entry point to improve growing techniques and for understanding wider soil processes, such as consequences of land use or climatic change on both biodiversity and soil ecosystem services. The research should be focused on developing an ecological and holistic approach that combines traditional soil health assessment with sensitive indicators of the effects of the soil environment on soil microbial and faunal communities and cultivated plants, such as community dynamics (taxonomic, genetic, functional and metabolic) and plant stress physiology, mostly assessed by growth/yield, hormone levels and photosynthetic capacity (e.g., photosystem II activity). The sustainable approach will lead to a better understanding of the effects of management practices on soil organisms and plants. In the long-term, soil health should be included as a factor when management decisions are made by farmers, land managers and crop advisers. The studies should not be designed for a systematic monitoring of the differential effects of each component of sustainable practices such as soil cover, manuring, reduced pesticide use or reduced tillage [START_REF] Hobbs | The role of conservation agriculture in sustainable agriculture[END_REF], but with these practices in combination, in order to have an overall vision of the agrosystem, and how to manage it sustainably (Fig. 6). In this view, the metagenomics (that is the microbial identities and functional gene information) and the metaphenomics (that is the product of the combined genetic potential of the microbiome and resources) of bacteria or fungi [START_REF] Fierer | Embracing the unknown: Disentangling the complexities of the soil microbiome[END_REF][START_REF] Jansson | The soil microbiomefrom metagenomics to metaphenomics[END_REF] would allow to define the microbial communities living in the different soil layers. The DNA/RNA-based identification of specific alive and active bacterial/fungal taxa, according to their functional distinction (aerobic/anaerobic, saprophytic/parasitic/pathogenic, autotrophic/heterotrophic) can be of key importance for defining the microbiological fertility of a soil and its response to agricultural practices [START_REF] Crecchio | Functional and molecular responses of soil microbial communities under differing soil management practices[END_REF][START_REF] Badagliacca | Use of solid anaerobic digestate and no-tillage practice for restoring the fertility status of two Mediterranean orchard soils with contrasting properties[END_REF]. Identification by DNA fingerprinting, originally developed for the identification of poorly culturable microbial strains, has become a rapid and cost-effective method of current use in agricultural soils. It has been extended to the current assessment of nematode communities [START_REF] Wang | PCR-DGGE Analysis of Nematode Diversity in Cu-Contaminated Soil Project supported by the National Natural Science Foundation of China (No. 30600087) and the Knowledge Innovation Program of Institute of Applied Ecology[END_REF] and its application to other soil fauna is promising, although still not in current use [START_REF] Orgiazzi | Soil biodiversity and DNA barcodes: opportunities and challenges[END_REF]. Finally, eventual pathogenic organisms and mycorrhization index should be monitored, identifying probable microbial pathogens and/or anaerobic microorganisms by a culture based-approach (Phytophthora, Clostridium, Bacillus, etc.) and presence and types of mycorrhiza, including DNA analysis of eventual pathogenic microorganisms during the growing season.

The application of endogenous and exogenous carbon inputs would be necessary for improving soil status. Using specific commercial products containing biostimulants and biocontrol agents to improve physicochemical and microbiological properties of the soils, could increase resistance against eventual pathogens and enrich soil microbial communities.

The application of compost, manure, soil management techniques for facilitating water horizontal movement in the soil, use of decompacting plants (e.g., Raphanus spp. or other Brassicaceae) would facilitate water vertical movement in the soil, both directly [START_REF] Whelan | Influence of compost amendments on the hydraulic functioning of brownfield soils[END_REF] and through a favorable influence on earthworms [START_REF] Pérès | Earthworm activity and soil structure changes due to organic enrichments in vineyard systems[END_REF]. On this basis, sustainable management is a key factor for increasing the functionality and diversity of soil biota that enhance soil biological fertility. This amelioration leads to a higher soil quality, stability and multifunctionality, positively affecting plant physiological status and crop productivity.

[Figure 6. HERE]

Over the last decades, intensive agricultural practices (e.g., continuous soil tillage, high inputs of mineral fertilizers, application of low-quality irrigation water, removal of pruning residues) have determined the loss of soil fertility and the depletion of soil organic matter (SOM), with negative effects on both productivity and soil conservation [START_REF] Bonanomi | Assessing soil quality under intensive cultivation and tree orchards in Southern Italy[END_REF]. Many conventional agronomic practices have a negative impact on SOM [START_REF] Arrouays | Changes in carbon storage in temperate humic loamy soils after forest clearing and continuous corn cropping in France[END_REF], the soil microbiome [START_REF] Lupatini | Soil Microbiome Is More Heterogeneous in Organic Than in Conventional Farming System[END_REF] and soil trophic networks [START_REF] Tsiafouli | Intensive agriculture reduces soil biodiversity across Europe[END_REF], causing a decline in fertility: soil leveling (with consequent elimination of the organic horizon in many areas of the plant), deep tillage with a surface carryover of mineral horizons (non-organic), soil sterilization with destruction of microbiological diversity, the continuous and massive use of herbicides, mineral fertilizers and pesticides with biocidal action, and so on. Sustainable agronomic practices foresee a reworking of agronomic management which includes the reduction of the use of mineral fertilizers and pesticides (including cupric products), the rationalization of irrigation, the management -and not the elimination -of spontaneous vegetation and the contribution of different types of organic matter to restore the ecosystem complexity and heterogeneity, without leaving aside a careful analysis of the fields and the state of the plants, of the environmental conditions (pedological, microbiological, orographic and microclimatic), and, above all, of the agronomic and ecological history of the fields. The correct use of formulations of materials of natural origin, specially designed and processed according to the different cultivation and business conditions, can quickly re-establish the trophic balance and soil-plant compatibility, reactivating the nourishment of the crops and increasing their resistance to pathogens and parasites. Thus, soils rich in organic matter, or regularly fertilized organically with compost and vegetable residues (possibly not coming from the same crop), have a greater microbiological biodiversity. Crop rotation with phylogenetically distant species, the practice of fallow and crop associations make it possible to prevent or eliminate the decrease in soil fertility, and the use of graft carriers with tree crops can be helpful to mitigate problems of replanting of the same species. The crop succession, the inter-cropping of different species, the use of rootstocks phylogenetically distant from the cultivated variety make it possible to overcome the problems related to the decline in soil fertility. A very effective sustainable technique appears to be the use of compost tea, infused or more commonly fermented with compost, which has already been the subject of growing scientific and applicative interest for several years [START_REF] Scheuerell | Compost Tea: Principles and Prospects For Plant Disease Control[END_REF][START_REF] St | Compost and Compost Tea icrobiology: The "-Omics[END_REF][START_REF] Villecco | Enhancing Sustainability of Tomato, Pepper and Melon Nursery Production Systems by Using Compost Tea Spray Applications[END_REF]. The technique is based on the use of different compost, specially selected (and sometimes, according to the needs of the plant, combined with non-composted organic substance), placed in infusion under aerobic conditions.

Soil functions as a living system

Nowadays, there is evidence that sustainable management practices (e.g. no-tillage, supply of organic fertilizers, mulching of pruning residuals and cover crops, reduction or even suppression of pesticide use) can contribute to re-carbonize soils and reduce soil CO 2 emissions, recover soil fertility, and increase yield. In sustainable agrosystems, because of the composition of the recycled biomass (pruning residuals, leaf fall, cover crops) and of newly supplied (e.g. compost, manure), a huge amount of nutrients is released if external supply of mineral fertilizers could successfully be replaced. However, considering processes determining nitrogen availability (organic matter mineralization, leaching, cover crops uptake, etc.), interactions among nutrients (e.g., antagonistic effects), variability of soil moisture, and mineral nutrition, particular attention is required. Sustainable agriculture can give benefits to plant growth, such as increased biomass and productivity, enhanced photosynthesis and carbohydrate allocation, better regulation of root respiration and higher defenses against pests and diseases, with more soil microorganisms and fauna, and thus more efficient trophic networks. Mycorrhizal fungi, being involved in many ecosystem services [START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF], are important in many types of soil. Besides well-known negative effects of conventional practices [START_REF] Verbruggen | Positive effects of organic farming on below-ground mutualists: Large-scale comparison of mycorrhizal fungal communities in agricultural soils[END_REF], in over-exploited, conventionally managed agricultural landscapes, habitat loss and fragmentation prevent dispersal and are major threats to mycorrhizas [START_REF] Longo | Taxonomic and Functional Response of Arbuscular Mycorrhizal Fungi to Land Use Change in Central Argentina[END_REF].

Human societies benefit from a multitude of ecosystem services from both natural and managed ecosystems, to which soil organisms make a crucial and distinctive [START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF]. Unfortunately, it is well recognized that humans are changing the global environment at an unprecedented rate. An increasing proportion of the world's population is urban or suburban. For this reason, the demand to extend cultivated areas in cities is increasing, prompting to establish, restore and sustain urban ecosystems. In these new anthropogenic environments, the application of organic inputs and bio-products, included in sustainable agricultural practices, can be crucial for plants' survival. Sustainable agriculture can cause recreational, human health, economic and environmental benefits. The latter also includes lower GHGs emissions because of reduced use of synthetic fertilizers and pesticides, lower leaching losses to groundwater (e.g., nitrates and nitrites), and no eutrophication of ponds and streams because of excess phosphorus and nitrogen. Comparisons of soil biota across wild, rural and urban habitats have revealed dramatic differences between sustainably managed and conventionally managed areas, with the lowest biodiversity in the latter [START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF].

Regardless of practical challenges, there is untapped potential for sustainable agricultural practices to influence environmental outcomes, citizens' consciousness and market's trends soon [START_REF] Scotti | Organic amendments as sustainable tool to recovery fertility in intensive agricultural systems[END_REF][START_REF] Stevenson | The state of the world's urban ecosystems: What can we learn from trees, fungi, and bees? PLANTS, PEOPLE[END_REF]. The results of many recent studies encourage the use of sustainable agricultural practices able to enhance soil fertility [START_REF] Diacono | Long-term effects of organic amendments on soil fertility[END_REF][START_REF] Kassie | Adoption of interrelated sustainable agricultural practices in smallholder systems: Evidence from rural Tanzania[END_REF][START_REF] Scotti | Organic amendments as sustainable tool to recovery fertility in intensive agricultural systems[END_REF]. The ultimate goal is to convince farmers to adopt a sustainable farming system as a whole, and not just as individual elements/practices, in order to promote good-quality production without negative effects on the environment. For achieving this, the approaches should not be 'top down', but they must be 'bottom up', where farmers and citizens help to co-design and co-deliver soil management and food production systems [START_REF] Ajayi | User acceptability of sustainable soil fertility technologies: Lessons from farmers' knowledge, attitude and practice in Southern Africa[END_REF][START_REF] Kassie | Adoption of interrelated sustainable agricultural practices in smallholder systems: Evidence from rural Tanzania[END_REF]. It is time to switch to a modern and multifunctional concept of agriculture based not only on the production but above all on product quality, environmental protection, resource saving and promotion of human health.

Soil is organized into layers

The complexity is such that the time distinguishes at least three groups of layers in the soil: Humipedon, Copedon and Lithopedon. Microorganisms and plants condition the Humipedon evolution by generating a known series of systems that starts on mineral substrate and divides in two series, in submerged environment (Archeo, Anaero, Histo and Aqueous systems) and out of water in aerated sites (Crusto, Bryo, Rhizo, main Terrestrial systems) (Zanella et al., 2018d). Animals interact with microrganisms and plants in each system. Each system is structured around particular organisms and which have coevolved for millions of years.

Microorganisms and plants condition the Humipedon. Mineralogical aspects and more related to soil physics are more decisive in Copedon. The geological history of the soil and the cycles of some minerals are much more important in Lithopedon. The time separates the studies of these three soil layers, handing them over to specialists who meet periodically to make the right synthesis. The soil is a single body that reacts as if it were composed of superimposed organs. Soil classification must take this crucial aspect into account.

Humipedon reacts over the years and up to a few decades; Copedon takes from decades to hundreds of years; the Lithopedon centuries to millennia.

We must no longer think we can move, manipulate, destroy, create soil as if it were an inanimate object. Soil has its own internal, historical and precious organization: what we need to do is discover it and use it well. It is necessary to understand these processes well before intervening with means that risk destroying the "superimposed organs" of the soil.

Agrosystems (crop soils) correspond to artificially simplified natural systems (in general Mull Humipedons). When artificialization is extreme, we talk about Techno systems (hydroponic or compost are examples of that). These phrases hide an underestimated truth of intrinsic value. Agro-systems are not alternative systems, but old systems reorganized by man (Zanella et al., 2018f). The soil system incessantly tries to return to its original organization, made of different overlapping layers, but the work of using its stored energy takes it back annually in time. At the end of a conventional exploitation process there is a soil poor in organic matter and life, an original mother rock of the soil. We must no longer think we can move, manipulate, destroy, create soil as if it were an object and then expect it to work as a living system to render service to us.

If the purpose of conventional agriculture is to build new soils, then we need to think about building Copedons between Humipedons and Lithopedons, not destroying this historical organization. Ask us what these layers really are (studying natural references) and how they can be built in compliance with natural and biological laws. It is perhaps possible to build new Humipedons. This is what we try to do with composts or hydroponic solutions. We can try to make them better, more like existing Humipedons. The other possibility is organic farming. The right method should be very similar to the one proposed by Masanobu Fukuoka in his famous book "Natural Way of Farming" [START_REF] Fukuoka | The naturam way of farming. The theory and practice of green philosophy[END_REF] to intervene as little as possible. Understand how it works and accompany the movement, changing the cards in the game as little as possible. It is difficult and, for this reason, it is the right way, and it needs a scientific preparation of high naturalistic/biological level.

Agriculture, human health and environment

We offer the propositions that: a) Intensive agriculture should be abandoned; b) agriculture should produce enough food for an increasing number of humans.

Is it possible to produce enough food and to stay in good health all over planet Earth? For knowing this, other two parameters should be considered: a) the need for a global management of Earth resources, like a United Nations Organization that would decide democratically; b) the regulation of birth rate might be achieved with a calculation of Earth's bearing capacity (shared ecological footprint) within the framework of a humanity that shares resources better than today.

Agroforestry

This is also an important point, not detached from the previous one. The agro-forestry use of the lands of our planet is often presented as an ecological and sustainable method of exploitation both internationally and nationally. The concept is acceptable. It is known how trees increase the volume of the ecosystem, with helpful implications in the air and soil parts of the ecosystem [START_REF] Jose | Agroforestry for ecosystem services and environmental benefits: an overview[END_REF][START_REF] Altieri | Agroecology and the design of climate change-resilient farming systems[END_REF][START_REF] Marsden | How agroforestry systems influence soil fauna and their functions -a review[END_REF][START_REF] Wang | Impacts of forest conversion to plantations on the soil carbon and nitrogen dynamics, and microbial communities[END_REF].

What must be avoided is that it becomes a disguised method of stealing more land from the forest. Since the part already taken (1/2) does no longer satisfy a humanity in search of food, we operate on the part that remains, transforming it into an agricultural forest. If the process is conceivable for the ecological exploitation of those parts of the forest (equatorial and otherwise) which have already been converted into cultivated fields and then abandoned, it should be banned in the still forests, whether they are treated with tall trees or coppices.

Conclusions and future insights

In this commentary paper, we presented cross-disciplinary and holistic approaches applied to agricultural soils. Converting the conventional management systems of agricultural land into more sustainable and environment-preserving systems has become urgent.

Conventional vegetable and fruit production, because of the unavoidable lack of resources (particularly soil and water), is going to be economically and environmentally disadvantageous, while, on the other side, organic farming, whose benefits and costs are controversial, is not always self-sustaining and durable, and it cannot cover the enormous and increasing world demand. For avoiding this dilemma, the productive systems should be directed towards the principles of an innovative, sustainable, regeneration and conservative agriculture, which includes rationally the existing and innovative agro-technological practices, such as no-or minimum soil tillage, on-site nutrient input and recycling, adequate irrigation and rational management of crop residues. This innovative approach, aimed to keep production at a high level and cultivating lastingly, can render a wide range of benefits to farmers and the environment. In addition, better understanding the role of soil fauna in such systems has a key role to adapt management strategies and mitigating GHGs emissions.

Furthermore, the role of soil organisms (both microbes and animals) to ecosystem services and their close relationship with soil organic matter has been often overlooked, while it should be seriously taken into account in future land management strategies.

The world's soils are rapidly deteriorating because of soil erosion, nutrient depletion and other threats, but sustainable practices and technologies can reverse this trend. One key point from the new IPCC report [START_REF] Shukla | IPCC -Summary for Policymakers[END_REF] is that conventionally tilled soils erode over 100 times faster than they form and that land degradation represents "one of the biggest and most urgent challenges" that humanity faces. Humans have degraded roughly one-third of the world's topsoil, and about 3.2 billion people -more than a third of humanity -already suffer from the effects of degraded land. Continuing down this path does not bode well for feeding a growing world population. Barriers to adopting regenerative farming systems include force of habit, lack of knowledge about new practices and real and perceived economic risk during the transition. The benefits of rebuilding healthy, fertile soil are clear. Economic benefits of land restoration average 10 times the costs. Thus, sustainable agriculture appears to have a big economic, social and political impact [START_REF] Baggaley | Soil risk maps -Interpreting soils data for policy makers, agencies and industry[END_REF]. We trust that sustainable agriculture will contribute to understand how important the soil as a living matrix is for both climate regulation and plant production. A better grasp of how soil organisms interact with organic matter turnover and stabilization will open novel ways for the sustainable management of soils. It is time to take soil seriously in consideration and to rethink humanity's relationship to the environment and particularly to soil. In their article, [START_REF] Kopnina | Anthropocentrism: More than Just a Misunderstood Problem[END_REF] hope for change from an anthropocentric mentality based on human-centered values and things to a more comprehensive vision that includes non-human living beings and things, such as environmental life-support systems. Thus, it would be necessary to promote a general law of soil protection, as soil produces food and sustains all ecosystems, independently from human need to economically grow. People need to change agriculture and land use, and we all wish to have 'more sustainable' soils, the only basis for a healthier world.

Geurs, M., Brussaard, L., 1994. Soil meso-and macrofauna [START_REF] Jenny | The Soil Resource, Origin and Behaviour[END_REF] Because of a possible climatic warm-up, we do not wish accelerate humus oxidation and the concomitant flux of carbon dioxide from soil into the atmosphere… The humus capital, which is substantial, deserves to be maintained because good soils are a national asset.

Solution?

A biological classification of the soil on a DNA basis could probably clarify the living essence of the soil. Reconciliation of current conceptual models for the fate of organic debris into a consolidated view of organic matter cycles and ecosystem controls in soil [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF].

Appendices

Appendix 1. Quotation from Waksman, S. A. (1936). HUMUS. Origin, chemical composition and importance in nature. The Williams & Wilkins Company. The whole "outlook" of the authors can be found here: https://soilcarboncoalition.org/files/Waksman-Humus.pdf,

  ): a) Climate, biological agents, rock, topography and duration are the factors of soil differentiation, or the factors of pedogenesis. b) Climate and corresponding vegetation are the main factors responsible for the organization of soils on a global scale.

Figure 1 .

 1 Figure 1. The three key attributes of soil, considered as a living system.

Figure 2 .

 2 Figure 2. From "The contentious nature of soil organic matter".Figure entitled:

  Figure 2. From "The contentious nature of soil organic matter".Figure entitled:

Figure 3 .

 3 Figure3. The FAO-Unesco Soil Map of the World, a first immense effort to synthetize the distribution of the soil on our planet. On the right the cover of Volume I, which is identical in the other volumes, only changes the squared that frame the described section. Notice the design, which recalls the horizons of the soil, but also soil, vegetation and atmosphere, or an interlocking of concentric circles typical of a modern ecological vision. Just beautiful, with the maps scattered as if in a vacuum.

Figure 4 .

 4 Figure 4. The 20 humus systems of the classification published by the Humus Group are divided into: 6 young or very particular natural systems (Para); 2 systems closely linked to man, one semi-natural (Agro) and one completely built by man (Techno); 5 terrestrial systems containing 17 humus forms; 5 Histic systems containing 16 humus forms; 2 Aqueous systems containing 3 humus forms. In total, the forms of humus described are 36 (not reported in the figure).

Figure 5 .

 5 Figure5. On the left, the "layers" that can be identified in the soil profile. On the right, the phases of formation of the soil profile.

Figure 6 .

 6 Figure 6. Soil is more complex than we think. It is almost as if it were an underground forest, much more concentrated and with less air than that which lives above the ground. We can imagine it composed of three layers in which it evolves without being perceptible in its dynamic state. Perhaps the blue arrows can represent the real processes of soil forming generating the large groups of soils (corresponding to theoretical main pedogenetic processes) described by the modern IUSS Working Group.

Figure 7 .

 7 Figure 7. Major benefits of the adoption of sustainable agricultural practices. Examples of the sustainable agricultural practices explained in the text.

  in two agricultural systems: factors affecting population dynamics and evaluation of their role in carbon and nitrogen dynamics.Agric. Ecosyst. Environ. 51,[171][172][173][174][175][176][177][178][179][180][181][182][183][184][185][186]://doi.org/10.1016/0167-8809(94)90042-6 Dokuchaev, V. V., 1900. Zones verticales des sols, zones agricoles, sols du Caucase. Collection pédologique, Exposition Universelle, Ed. Ministère des Finances de St-Péterbourg. Doran, J.W., Zeiss, M.R., 2000. Soil health and sustainability: managing the biotic component of soil quality. Appl. Soil Ecol. 15, 3-11. https://doi.org/10.1016/S0929-1393(00)00067-6 Drosos, M., Nebbioso, A., Piccolo, A., 2018. Humeomics: A key to unravel the humusic pentagram. Appl. Soil Ecol. 123, 513-516. https://doi.org/10.1016/j.apsoil.2017.07.027 Duru, M., Therond, O., Martin, G., Martin-Clouaire, R., Magne, M.-A., Justes, E., Journet, E.-P., Aubertot, J.-N., Savary, S., Bergez, J.-E., Sarthou, J.P., 2015. How to implement biodiversity-based agriculture to enhance ecosystem services: a review. Agron. Sustain. Dev. 35, 1259-1281. https://doi.org/10.1007/s13593-015-0306-1 EASAC -European Academies Science Advisory Council, 2018. Opportunities for soil sustainability in Europe. EASAC policy report 36. German National Academy of Sciences Leopoldina 2018. El-Tarabily, K.A., Sivasithamparam, K., 2006. Non-streptomycete actinomycetes as biocontrol agents of soil-borne fungal plant pathogens and as plant growth promoters. Soil Biol. Biochem. 38, 1505-1520. https://doi.org/10.1016/j.soilbio.2005.12.017 Enwall, K., Nyberg, K., Bertilsson, S., Cederlund, H., Stenström, J., Hallin, S., 2007. Longterm impact of fertilization on activity and composition of bacterial communities and metabolic guilds in agricultural soil. Soil Biol. Biochem. 39, 106-115. https://doi.org/10.1016/j.soilbio.2006.06.015 FAO-UNESCO, 1974. Soil map of the world 1:5 000 000. Vol. 1. UNESCO, Paris. FAO, 2020. Technical specifications and country guidelines for Global Soil Organic Carbon Sequestration Potential Map GSOCseq. Rome, Italy.FAO and ITPS, 2015. Status of the World's Soil Resources (SWSR) -Technical Summary. Food and Agriculture Organization of the United Nations and Intergovernmental Technical Panel on Soils, FAO. ed. Rome, Italy.Fausto, C., Mininni, A.N.,Sofo, A., Crecchio, C., Scagliola, M., Dichio, B., Xiloyannis, C., Topsoils are directly linked to particular groups of soil animals. This connection could have very important consequences on the management of forest and agricultural soils to stop climate from warming.

Sofo, Zanella, Ponge 58 particularly at pages 397-398.

There were exceptions to the climate zonality. Locally, this or that environmental factor plays a preponderant role and masks the role of climate. "Zonal" soils, that are part of the climatic zonality; "azonal" soils, whenever the rock outcrops directly; and "intra-zonal" soils, whose characteristics are linked to special conditions, such as excess water or salt. [START_REF] Jenny | Factors of Soil Formation : A System of Quantitative Pedology[END_REF] The soil system is an open system. The initial state of the soil system has been designated as parent material. In selecting climate (cl), biological agents (o), rock (r), topography (p) and duration (t) as the independent variables of the soil system, we do not assert that these factors never enter functional relationships among themselves.

Soil and vegetation

Soil and vegetation correspond to a continuous layer that shows changes inside and outside perceptible to the human eye. Both soil and vegetation covers can be broken down into sub-layers. Circumscribing spatial sub-units (horizontal or vertical soil and vegetation sub-units) is not so simple, because the transition from one to the other unit is rarely abrupt; very often it is gradual and nuanced. We know that using the characteristic species of the phytosociological units to map the forest vegetation, the part of the forest occupied by undefined vegetation types becomes larger than that occupied by known vegetation types. It is crucial to choose carefully the environment in which you are taking the survey, it must be as homogeneous as possible considering the purposes of your work; work objectives define the size of your survey.

Need of unification

The need for unification is necessary when international organizations wanted to map soils at planet level. The number of soil units which compose the legend of the Soil Map of the World is 106. The legend sheets present these soil units in an order which reflects the general processes of soil formation. The basic principles which underlie the separation of these soil units and their definitions are discussed in Chapter 3. Areas of "non soil" are shown on the map as miscellaneous land units.

The disagreement among soil scientists produced a scientific impasse. When there are over three variables interacting, and in the soil there are dozens, a natural system end up in a chaotic and unpredictable movement. Illustrated Guide to Soil Taxonomy (2015). USDA attempt to approach a larger audience (ecologists, environmental scientists, etc.). Essentials of Soil Science -Soil formation, functions, use and classification (2018). WRB attempt to open to other disciplines.

Soil as living system

Living organisms organize the superficial part of the Earth's crust into layers that are visible to the naked eye. Each soil horizon gets its own relative independence: since it is built by living beings and develops by its-own, it can also be considered a subsystem contained in a larger complete soil system.

To classify the soil means to circumscribe a surface volume of the earth's crust that grows over time and evolves as a system (called pedogenetic processes by soil scientists), dependent on those same soil forming-factors.