Vincent Chaplot
email: vincent.chaplot@ird.fr

Khatab Abdalla

Marie Alexis

Hocine Bourennane

Frédéric Darboux

Phesheya Dlamini

Colin Everson

Charmaine Mchunu

D Muller-Nedebock

Macdex Mutema

Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils

Keywords: No-tillage; Carbon Dioxide, Climate change, Corn, Small holders, Africa

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with measurements of CO 2 fluxes twice a day during the first three days and once a day thereafter. Soil organic C (SOC) content, soil bulk density (ρb), aggregate stability, soil organic matter quality, and microbial biomass and its activity were evaluated at the onset of the incubation. CO 2 emissions were 22% lower under NT compared with T with CO 2 emissions of 0.9±0.10 vs 1.1±0.10 mg C-CO 2 gC -1 day -1 under NT and T, respectively, suggesting greater SOC protection under NT. However, there were greater total CO 2 emissions per unit of surface by 9% under NT (from 1.05±0.04 to 1.15±0.03 g C-CO 2 m -2 day -1 ). SOC protection significantly increased with the increase in soil bulk density (r = 0.89) and aggregate stability (from 1.7±0.25 mm to 2.3±0.31, r=0.50), and to the decrease in microbial biomass and its activity (r=-0.59 and -0.57, respectively). In contrast, the greater NT CO 2 emissions per m² were explained by top-soil enrichment in SOC by 48% (from 12.4±0.2 to 19.1±0.4 g kg -1 , r=0.59).

These results on the soil controls of tillage impact on CO 2 emissions are expected to inform on the required shifts in agricultural practices for enhancing C sequestration in soils. In the context of the study, any mechanism favoring aggregate stability and promoting SOC alloca-

Introduction

The soil C pool, which is the largest terrestrial pool, storing 2344 Pg C (1 Pg = 10 15 g = 1 billion tons) in the top three meters of the soil [START_REF] Jobbagy | The vertical distribution of soil organic carbon and its relation to climate and vegetation[END_REF] exhibits direct and dynamic exchanges with the atmosphere through photosynthesis and organic matter decomposition. Because the soil C pool has been drastically reduced by human activities, by approximately 66% [START_REF] Lal | Soil erosion and the global carbon budget[END_REF], C sequestration in soils is thought to potentially mitigate the rising atmospheric CO 2 concentration [START_REF] Batjes | Total carbon and nitrogen in the soils of the world[END_REF][START_REF] Lal | Soil erosion and the global carbon budget[END_REF]. Improving current land use and land management has the potential to reduce the rate of anthropogenic carbon loss in agricultural soils.

Among the land management practices commonly cited that could influence soil organic C (SOC) stocks are soil tillage, chemical and organic fertilizations, crop rotation, crop associations and planting density [START_REF] Lal | Soil erosion and the global carbon budget[END_REF].

No-tillage (NT), where mechanical soil disturbance is minimized and permanent organic soil cover consisting of a growing crop or mulch of crop residues is maintained, is currently being promoted under both commercial and smallholder agriculture as it contributes to reduced labor and energy inputs [START_REF] Huggins | No-till: the quiet revolution[END_REF]. In addition, by keeping crop residues on the soil surface longer than under conventional tillage (T), NT has been demonstrated to prevent soil surface sealing [START_REF] Bradford | Interrill soil erosion as affected by tillage and residue cover[END_REF][START_REF] Rao | Rainfall infiltration and runoff from an Alifisol in semi-arid India. I. No-till systems[END_REF], reduce splash erosion [START_REF] Bradford | Interrill soil erosion as affected by tillage and residue cover[END_REF][START_REF] Choudhary | Long-term tillage effects on runoff and soil erosion under simulated rainfall for a central Ohio soil[END_REF], and to enhance soil porosity and soil infiltration through improved biological activity [START_REF] Doran | Soil microbial and biochemical changes associated with reduced tillage[END_REF][START_REF] Granatstein | Long-term tillage and rotation effects on soil microbial biomass, carbon and nitrogen[END_REF][START_REF] Meek | Infltration rate of a sandy loam soil: Effects of traffic, tillage, and plant roots[END_REF][START_REF] Edwards | Tillage and crop rotation effects on fertility status of a Hapludult[END_REF][START_REF] Pierce | Periodic plowing effects on soil properties in a no-till farming system[END_REF]Feng et al., 2003;[START_REF] Gosai | Tillage effects on soil microbial biomass in a rainfed agricultural system of northeast India[END_REF]. In addition, the absence of soil disturbance in the NT system fosters SOC protection within aggregates, an important factor of C sequestration [START_REF] Bolliger | Taking stock of the Brazilian "Zero-Till Revolution": A review of landmark research and farmer practice[END_REF][START_REF] Triplett | No-tillage crop production: A revolution in agriculture[END_REF].

However, despite numerous investigations, the impacts of tillage on soil C losses and especially CO 2 emissions have been largely inconsistent. Several studies have reported lower CO 2 emissions under NT [START_REF] Fang | The dependence of soil CO 2 efflux on temperature[END_REF][START_REF] Ussiri | Long-term tillage effects on soil carbon storage and carbon dioxide emissions in continuous corn cropping system from an Alfisol in Ohio[END_REF][START_REF] Chaplot | Water erosion-induced CO 2 emissions from tilled and no-tilled soils and sediments[END_REF]. For instance [START_REF] Reicosky | Moldboard plow tillage depth and short-term carbon dioxide release[END_REF] reported cumulative 20 days emissions of 237 g CO 2 m -2 for NT versus 891 g CO 2 m -2 for T in Minnesota, USA. Similarly, on a clayey tropical soil from the Amazonian Basin, [START_REF] Carmo | Short-term changes in nitrogen availability, gas fluxes (CO 2 , NO, N 2 O) and microbial biomass after tillage during pasture re-establishment in Rondônia, Brazil[END_REF] reported that tillage cessation lowered CO 2 emissions from 235 to 181 mg CO 2 -C m -2 h -1 . However, several authors reported greater soil CO 2 emissions from NT soils [START_REF] Aslam | Influence of land-use management on CO 2 emissions from a silt loam soil in New Zealand[END_REF][START_REF] Carmo | Short-term changes in nitrogen availability, gas fluxes (CO 2 , NO, N 2 O) and microbial biomass after tillage during pasture re-establishment in Rondônia, Brazil[END_REF][START_REF] Barreto | The impact of soil management on aggregation, carbon stabilization and carbon loss as CO 2 in the surface layer of a Rhodic Ferralsol in Southern Brazil[END_REF][START_REF] Lee | Tillage and seasonal emissions of CO 2 , N 2 O and NO across a seed bed and at the field scale in a Mediterranean climate[END_REF][START_REF] Sainju | Land use and management practices impact on plant biomass carbon and soil carbon dioxide emission[END_REF][START_REF] Smith | Fertilizer and tillage management impacts on non-carbon-dioxide greenhouse gas emissions[END_REF], among which [START_REF] Barreto | The impact of soil management on aggregation, carbon stabilization and carbon loss as CO 2 in the surface layer of a Rhodic Ferralsol in Southern Brazil[END_REF] reported in Brazil an increase of CO 2 emissions by 700%.

The lower gaseous emissions from NT soils are assumed to come from the greater organic matter protection from microbial decomposition within undisturbed soil aggregates (e.g. [START_REF] Six | Stabilization mechanisms of soil organic matter: Implications for C-saturation[END_REF]. Other factors would however have to be considered to explain for instance the greater NT C losses such as soil compaction, soil surface crusting, organic matter quantity and quality, biological activity among others [START_REF] Six | Stabilization mechanisms of soil organic matter: Implications for C-saturation[END_REF][START_REF] Lützow | Stabilization of organic matter in temperate soils: Mechanisms and their relevance under different soil conditions. A review[END_REF][START_REF] Elder | Tillage effects on physical properties of agricultural organic soils of north central Ohio[END_REF]Kogel-Knabner et al., 2008) and these need further investigation.

The relative contribution of these physical, biological or chemical factors of control to explain the changes in CO 2 emissions following tillage abandonment need further appraisal. What are the main soil factors that control changes in tillage-induced CO 2 emissions and their relative contribution? This constitutes an important research question that comes as a prerequisite to better understand the effects of land management on the global C cycle as well as for designing agricultural practices aiming at sequestrating more C into soils while improving their overall functioning.

Consequently, the main objective of the present study was to investigate the underlying controls of the changes in soil CO 2 emissions consecutive to tillage abandonment. A research trial was established in an agricultural maize field of South Africa, where crop residues were exported to feed livestock, to investigate the short-term (two years) consequences of tillage abandonment on CO 2 emissions and soil properties. The emissions from undisturbed 0 to 0.05 m T and NT soil samples (N=33) were compared to their SOC content, soil bulk density, SOC stocks, dissolved organic C quality, microbial biomass and activity, aggregate stability, and soil surface crusting. Additionally, undisturbed soil samples were compacted by 5 to 10% and some had their crust removed in an attempt to further the understanding of the controls of CO 2 outputs from soils.

Materials and methods

Study Area

The study was conducted in a smallholder farming community of Potshini located 10 km south of the town of Bergville (Long: 29.38°; Lat:-28.81°), within the sloping lands of the KwaZulu-Natal province of South Africa.

Following the classification of Köppen, the climate of the area is temperate with cold dry winters and rainy summers [START_REF] Peel | Updated world map of the Köppen-Geiger climate classification[END_REF]). The mean annual precipitation is 684 mm, the potential evaporation is 1600 mm and the mean annual temperature is 13°C. In the landscape, steep slopes with shallow soils are used for cattle grazing, whereas the lowland areas, characterized by gentle slopes (0-10%) and deep soils (>1.5 m), are used predominantly for rain-fed cropping with maize (Zea mays) production as the main crop in KwaZulu-Natal.

Maize is generally planted around mid-November on lands prepared with draft oxen, but mechanization is becoming more common. Planting, weeding and harvesting are done manually. Little fertilizer is used due to the limited access to markets and lack of funds.

Soils are acidic Acrisols (W.R.B., 1998), which are generally deep (average soil depth of 1.2 m). The A-horizon is a brown sandy loam (7.5YR4/4, 55 to 68% sand) and has a low clay percentage (17-19%) and a high content of fine sand (45-50%). This top-soil layer is 0.35-m thick, acidic (pH 4.9-5.2) and has a fine granular structure and is characterized by a low cation exchange capacity (2-4 cmol + kg -1 ) and SOC content (from 9-12 g C kg -1 ). A sub-surface organo-mineral AB W horizon (0.35-0.85 m) has similar texture but is lower in C and nitrogen (N) and has a lower cation exchange capacity and base saturation. Beneath these horizons lie two clayey mineral sub-surface (B w ) horizons, which are reddish in color (5YR4/6), have an apedal structure, and are significantly clay-enriched (211-224 g kg -1 for Bw1 and 297-326 g kg -1 for Bw2).

In the lowland areas the soils have been cultivated for several decades with the same farming system, marked by tillage and post-harvest residue grazing by cattle. No-tillage was introduced from 2010 as an experimental trial following its recent progression in Africa in both large-scale and small-scale farms [START_REF] Huggins | No-till: the quiet revolution[END_REF]. The fundamentals of a No-Till system are that a narrow furrow is made through the organic layer into the soil, the seed and fertilizer are placed into the furrow and then refilled, thereby reducing soil disturbance to a minimum.

The experimental design

A 450 m² (15m×30m) area was divided into three till (T) and three no-till (NT) 5m×15m

plots. An intensive random soil sampling of the top 0.05m was performed prior to the implementation of the trial. It showed no significant difference among the six plots for soil texture, pH, SOC content (SOCc), soil organic N content (SONc) and stocks (SOC S , SON S ), soil bulk density (b), Ca, P and K (data not presented here).

On three plots, tillage was performed manually to a 0.2 m depth, using a hand hoe, as commonly practiced in the area. In preparation for planting for the present study, the NT plots were weeded on the 20 th September 2012 with Glyphosate herbicide, at a rate of 4 L ha -1 . The T plots were conventionally tilled as per normal practice in the area, on the October 9, 2012.

Planting was carried out on all experimental plots on the October 9, 2012. At planting, 40 kg N ha -1 was applied on all treatments and 20kg N ha -1 were top-dressed at six weeks after planting. A cocktail of 2-4D, collisto, metagen and atrazine herbicides was applied twice to control weeds during the growing season, at a rate of 4 L ha -1 and using backpack-mounted sprayers. The maize was harvested by hand on June 20, 2013.

Evaluation of Soil Bulk Density and Soil Organic Carbon Stocks

The crusts of three T and three NT cores were removed using a scalpel. Other core replicates were compacted by 5 and 10% (N=3 for T; N=3 for NT). The method used for compaction consisted in the gentle pushing of a metallic plain tube of the same diameter of the soil core into the undisturbed soil and to a depth corresponding to the required compaction percentage, i.e. from 2.5 mm for the compaction rate of 5% to 5 mm for 10%.

Finally, soil aggregates were separated from the bulk soils through dry sieving. For both T and NT treatments, ring replicates were filled with aggregates of 0 to 2 mm diameter and three others with 2-8 mm aggregates. Due to the lack of soil material, two treatments did not have triplicates: NT-2mm (N=1) and NT-8mm (N=2). Overall, this led to a total of 33 cylinders for incubation.

Soil incubations

The soil samples (N=33) from both T and NT soils were incubated in 1000 ml open-top jars for 20 days at 20±2 °C and in a 100% humidity environment, following [START_REF] Fang | The dependence of soil CO 2 efflux on temperature[END_REF]. Carbon dioxide fluxes were assessed on a daily basis, using a LI-6400XT portable photosynthesis analyzer (Li-cor Inc., Lincoln, NE) using the 6400-09 soil chamber based on non-dispersive infrared (NDIR) gas analyzer over a two minute time interval. Measurements were performed once a day during the first week, every two days thereafter.

Samples were checked for water content by weighing the jars every day after each gas sampling and samples had, when required, water added to maintain a constant moisture content [START_REF] Wienhold | Comparison of laboratory methods and an in situ method for estimating nitrogen mineralization in an irrigated silt-loam soil[END_REF].

Soil aggregate stability

The stability of soil aggregates was evaluated following the Le [START_REF] Bissonnais | Aggregate Stability and assessment of soil crustability and erodibility: Theory and methodology[END_REF] laboratory procedure (ISO 10930:2012). Aggregates 2-5 mm in size were obtained by hand breaking and ASI-5000 autosampler and Balston 78-30 high purity TOC gas generator (Shimadzu, Tokyo, Japan) and using standard solutions of 0, 10, 50 and 100 ppm C made up using 0, 1, 5 and 10 ml of TOC stock solution prepared by dissolving 2.125 g of oven dry reagent grade potassium hydrogen phthalate (C 8 H 5 KO 4 ) in 1000 ml of distilled water.

In addition, soil microbial activity of the soil samples was estimated by assessing the hydrolysis of Fluorescein Diacetate (FDA) by soil enzymes, following [START_REF] Alef | Soil respiration[END_REF].

Quantity and quality of soil dissolved organic C

Bulk top-soil samples from the T and NT plots were sifted through a 2-mm sieve and visible roots were removed. The amount of extractible dissolved organic C (DOC) in soil samples was considered to access the quantity and proportion of the most easily decomposable soil organic matter [START_REF] Gregorich | Carbon distribution and losses: erosion and deposition effects[END_REF].

Production of DOC solutions was done by adding cold deionised water to soil (solid/solution ratio=0.1; [START_REF] Kalbitz | Biodegradation of soil-derived dissolved organic matter as related to its properties[END_REF]. Samples suspensions were thoroughly mixed for 2 hours, centrifuged (8000 rpm for 30 minutes at 10 o C) and filtered through 0.45µm filter (Millipore). After filtering, an aliquot of each sample was allowed to reach room temperature and analyzed for UV-visible absorbance. A single quartz cuvette (0.01 m, rinsed three times) was used to measure the absorbance spectra via UV-VIS spectrophotometer with Milli-Q water as reference.

The remaining filtered samples were stored in the dark (+4 o C) for DOM analysis. Dissolved organic C concentration was measured on acidified samples (pH 2 with 2N HCL) with Shimadzu TOC-V CSH analyzer via a non-purgeable organic C method. All DOC data are the mean of 3 replicate injections for which the coefficient of variance was always less than 1%.

In addition, Specific UV-absorbance (SUVA 254 ) values were used to estimate DOC aromaticity by dividing UV absorbance at a wavelength of 254 nm (A 254 ) by DOC concentration [START_REF] Weishaar | Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon[END_REF]. The spectral slope parameter for the 275-295 nm (S 275-295 ) and 350-400 nm (S 350-400 ) wavelength ranges, which are linked to DOM aromatic [START_REF] Helms | Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter[END_REF][START_REF] Spencer | Utilizing chromophoric dissolved organic matter measurements to derive export and reactivity of dissolved organic carbon exported to the Arctic Ocean: A case study of the Yukon River, Alaska[END_REF] were calculated by applying a non-linear exponential function regression to the absorbance spectrum over these ranges [START_REF] Stedmon | Optical properties and signatures of chromophoric dissolved organic matter (CDOM) in Danish coastal waters[END_REF][START_REF] Helms | Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter[END_REF][START_REF] Yamashita | Photobleaching as a factor controlling spectral characteristics of chromophoric dissolved organic matter in Open Ocean[END_REF].

Statistical analysis

Basic descriptive statistics were reported for soil C-CO 2 emissions and soil characteristics of T and NT soils (Tables 123). Comparisons of mean values were carried out using t tests. A significance threshold of 5% was used throughout the study. Exploratory multivariate analyses were applied to the data through multivariate correlations (Table 5). Comparisons of C-CO 2 emissions were based on a regression analysis. First, each set of data was fitted separately. Then a regression was carried out using all the data together. The null hypothesis was that the regression that used all the data performed as well as the ones taking the datasets separately. If the hull hypothesis of a single regression was rejected, we concluded that the sets of data were different.

Results

Tillage impact on selected soil properties

Selected physical parameters

The general statistics of bulk density ( b ), SOC C , SON C , C:N ratio, SOCs and mean weight diameter (MWD) for tilled (T) and no-tilled (NT) soils are shown in Table 1. NT soils were 18% denser than the T ones ( b =1.03 vs 1.24 g cm -3 ). The SOC C increased by 54% from 12.4 g kg -1 under T to 19.1 g kg -1 under NT, and the SOC S were 32% higher under NT, all these differences being significant. SON C was 26% greater under NT compared to T (1.85 vs 1.47 g kg -1 ) and the resultant C:N ratio was the greatest under NT (10.2 vs 8.4). Finally, NT soil aggregates were significantly more stable than T ones as expressed by a MWD of 2.27 mm for NT compared to 1.67 mm for T (Table 1), with most of the differences between the two tillage treatments occurring for fast wetting (MWD of 0.8 for T and 1.7 for NT), followed by mechanical breakdown (1.33 mm for T and 1.83 mm for NT) and slow wetting (2.86 and 3.23 mm) (Figure 1).

Dissolved organic C and microbial biomass C

The content of water extractable DOC increased significantly with the cessation of tillage from 95±21 to 112±24 mg C L -1 , which corresponded to a 18% difference (Table 2). Water extractable DOC was biochemically less stable under NT compared to T as expressed by a lower spectral slope ratio (S R ) (0.69±0.15 vs 0.60±0.13), but higher SUVA 254 values (1.48±0.32 vs 1.36±0.30 mg C -1 m -1 ). However, tillage impact on S 275-295 was not statistically significant.

Finally, both microbial biomass C and microbial activity significantly decreased with the abandonment of tillage. As shown by figure 2, microbial biomass C decreased significantly from 522.0±31.3 mg C kg -1 under T to 213.1±10.9 mg C kg -1 under NT, which corresponded to a 59% difference. Concomitantly, the cessation of tillage induced a significant and sharp fall in microbial activity with FDA hydrolysis values decreasing from 12.8 to 8.0 µg g -1 h -1 (Figure 3), a 37% difference.

Impact of tillage on CO 2 emissions

Table 3 shows the summary statistics of CO 2 -C emission for both undisturbed T and NT soils.

The average daily CO 2 -C emission per gram of C in the soil was significantly greater by 22% on T compared to NT (1.10 vs 0.90 mg C-CO 2 g C -1 day -1 ). After 20 days, T soil had emitted 0.0169±0.00076 g CO 2 -C g -1 C, while the NT soils had released 0.0134±0.00091g CO 2 -C g -1 C

(Figure 4). However, expressed per area basis, the total CO 2 -C emission appeared significantly greater from NT soils (1.15±0.03 g CO 2 -C m -2 day -1 ) compared to T soils (1.05±0.04 g CO 2 -C m -2 day -1 ), a 9% difference.

Impact of selected soil properties on CO 2 emissions

Impact of soil compaction

Results on the impact of soil compaction on soil C-CO 2 emissions are presented in Figures 5A and6A. Soil compaction decreased significantly soil CO 2 emissions from 0.85 mg CO 2 -C g -1 C day -1 for the undisturbed T bulk soil to 0.79 mg CO 2 -C g -1 C day -1 for the 5% compaction (T-c5) and to 0.72 mg CO 2 -C g -1 C day -1 for the 10% treatment (T-c10), which in the latter case corresponded to an 18% difference.

In contrast, compaction of the NT soils resulted in an opposite effect on CO 2 emissions: the compaction decreased significantly the emission level.

Impact of the size of soil aggregates

Aggregates with a diameter from 2 to 8 mm represented 80% of the bulk soil mass under NT and 55% under T, the remaining 20 and 45% corresponding to aggregates with a diameter lower than 2 mm.

Aggregates with a 0 to 2 mm diameter for T soil (T-s2) were characterized by the highest CO 2 emissions (cumulative 20 days of 0.0256 g CO 2 -C g -1 C), 51% significantly greater than soil aggregates with a 2 to 8 mm diameter (T-s8, 0.0143 gC-CO 2 gC -1 ), while the bulk T soil showed intermediate gaseous emissions (0.0169 g CO 2 -C g -1 C) (Fig. 5B). All the NT soil aggregates sizes under study also showed higher emissions than the bulk soil, (0.0066 g CO 2 -C g -1 C for NT-s8 vs 0.0116 g CO 2 -C g -1 C for NT-s2; Figure 6B). Interestingly, the greatest CO 2 emissions from NT aggregates occurred for NT-s8 during the first day of the incubation, i.e. immediately after aggregate sieving.

Impact of soil crusting

Both tillage treatments exhibited significantly greater CO 2 emissions when a soil surface crust was present on the soil surface than when the crust was removed (Fig. 7A-B; Table 4). After 20 days of incubation, the cumulative CO 2 emissions from NT soils were 0.0134 g CO 2 -C g -1

C with surface crust compared to 0.0119 g CO 2 -C g -1 C when surface crust was removed, a 11% difference. Similarly, crust removal decreased significantly CO 2 emissions from T soils from 0.0169 g CO 2 -C g -1 C to 0.0154 g CO 2 -C g -1 C.

The main controls of CO 2 emissions

Soil CO 2 emissions per unit of surface appeared to significantly increase with the increase in SOC C (r=0.59), followed by SON C (r=0.52), SOC S (r=0.48) but decreased with increasing MA (r=-0.37) and MB (r=-0.34), which corresponded in all cases to significant correlations (Table 4). In the mean time, emissions expressed in gC-CO 2 gC sol -1 were shown to decrease the 4).

Table 5 presents the coefficients of determination (r) between CO 2 emissions per square meter and per gram of C in the soil, and multiple environmental factors. Variations in CO 2 emission per square meter appeared to be explained the most by SOC C ,  b and MWD with 63% of the data variance explained by these variables. In contrast, SOC C and  b explained 91% of the variance of CO 2 emissions per gram of OC in the soil (Table 5).

Discussion

Bulk soil C-CO 2 emissions

There is a general consensus that the suppression of tillage decreases soil CO 2 emissions per unit of surface since, as suggested by [START_REF] Six | Soil macroaggregate turnover and microaggregate formation : a mechanism for C sequestration under no-tillage agriculture[END_REF], the aggregates' disruption due to tillage renders the initially protected organic matter (OM) accessible to decomposers. This was supported by various studies worldwide and pointed to reductions of gaseous emissions per m² as high as 73% by La [START_REF] Scala | Short-term soil CO2 emission after conventional and reduced tillage of a no-till sugar cane area in southern Brazil[END_REF] in Brazil, 66% in the study of [START_REF] Carbonell-Bojollo | Soil management systems and short term CO2 emissions in a clayey soil in southern Spain[END_REF] in Spain, 35% by [START_REF] Jabro | Carbon dioxide flux as affected by tillage and irrigation in soil converted from perennial forages to annual crops[END_REF] in USA. However, several studies pointed to much lower reduction rates, as for instance 2% by [START_REF] Brye | Impact of tillage and residue burning on carbon dioxide flux in a wheat-soybean production system[END_REF] and [START_REF] Sainju | Land use and management practices impact on plant biomass carbon and soil carbon dioxide emission[END_REF] in the USA. In contrast, greater CO 2 emissions from NT soils were reported. [START_REF] Barreto | The impact of soil management on aggregation, carbon stabilization and carbon loss as CO 2 in the surface layer of a Rhodic Ferralsol in Southern Brazil[END_REF] showed for instance a 700% increase in C-CO 2 emissions by using NT. This rate was as much as 29% in a study by [START_REF] Aslam | Influence of land-use management on CO 2 emissions from a silt loam soil in New Zealand[END_REF] in New Zealand, 15% in Brazil [START_REF] Carmo | Short-term changes in nitrogen availability, gas fluxes (CO 2 , NO, N 2 O) and microbial biomass after tillage during pasture re-establishment in Rondônia, Brazil[END_REF], 9% by [START_REF] Smith | Fertilizer and tillage management impacts on non-carbon-dioxide greenhouse gas emissions[END_REF] in the USA, 5% by [START_REF] Sainju | Land use and management practices impact on plant biomass carbon and soil carbon dioxide emission[END_REF] in the USA and 4% by [START_REF] Lee | Tillage and seasonal emissions of CO 2 , N 2 O and NO across a seed bed and at the field scale in a Mediterranean climate[END_REF] in the USA. This is in line with the 17% observed in the present study,

The greater CO 2 emissions per m² from NT soils are often attributed to the presence of fresh organic material or mulch on the soil surface, which are easily accessible to decomposers [START_REF] Oorts | Determinants of annual fluxes of CO 2 and N 2 O in long-term no-tillage and conventional tillage systems in northern France[END_REF]. Since, in the present study no mulch was present on the soil surface, what can explain the greater C-CO 2 emissions from NT soils?

Among the possible explanations of the greater soil C-CO 2 emissions from NT soils is their greater levels of microbial biomass and biomass activity as cited by [START_REF] Helgason | Fungal and bacterial abundance in long term no-till and intensive-till soils of the northern great plains[END_REF], [START_REF] Spedding | Soil microbial dynamics in maize-growing soils under different tillage and residue management system systems[END_REF], [START_REF] Drigber | Changers in soil microbial community structure with tillage under long long-term wheat-fallow management[END_REF], [START_REF] Shi | An assessment of the soil microbial status after 17 years of tillage and mineral P fertilization management[END_REF]), Zhang et al. (2005), Zhang et al. ( 2012), [START_REF] Nakamoto | Effect of reduced tillage on weeds and soil organisms in winter wheat and summer maize cropping on humic andosols in centralJapan[END_REF], [START_REF] Follett | Effects of tillage on microbial biomass dynamics[END_REF].

However, the present study pointed to a significant decrease in top-soil microbial biomass and biomass activity associated with the abandonment of tillage, a result similar to the one of [START_REF] Govaerts | Influence of tillage, residue management and crop rotation on soil microbial biomass and catabolic diversity[END_REF]. Another possible explanation of the lower microbial activity under NT soils is the lower soil porosity and associated reduced oxygen inputs from the atmosphere, which limits the development and activity of living organisms [START_REF] Pandey | Effects of conventional tillage and no tillage permutations on extracellular enzyme activities and microbial biomass under rice cultivation[END_REF][START_REF] Nsabimana | Size, activity and catabolic diversity of the soil microbial biomass as affected by land use[END_REF]. The presence of more recalcitrant OM, as found in this study could be another cause of lower biological activity and soil C-CO 2 emissions.

Here we show that the most significant control of the enhanced gaseous emissions per m² from NT soils lies into the presence of a surface crust a the top-soil enrichment in SOC, which increases the proximity of OM to decomposers and to the atmosphere, thus potentiating OM decomposition and CO 2 release. The top millimeter of the soil appeared to play an important role in the release of greenhouse gases as greater emissions characterized crusted soils compared to soils for which the surface crusts had been removed. While [START_REF] Mchunu | No-Till impact on soil and Soil organic carbon erosion under crop residues scarcity in South Africa[END_REF] pointed to the presence of biological features on soil crusts such as algae, which might favor CO 2 emissions, more is to be done on soil crusting impact on soil C exports.

Organic matter protection

Despite exhibiting greater C-CO 2 emissions per m² to the atmosphere, the NT system in this study yielded lower C exports per gram of soil carbon and thus greater OM protection from decomposers. This was likely to come from the lower biota activity, a direct consequence of the higher soil aggregate stability and soil bulk density, both which protect OM from decomposers and gaseous exchanges to the atmosphere. While the present study pointed to the primary impact of soil compaction on lowering soil CO 2 emissions, C-CO 2 emissions surprisingly increased at the 10% compaction level of the NT treatment. This result could be explained by the breaking-down of soil aggregates and the associated release and decomposition of the encapsulated OM.

Conclusion

In this study of tillage impact on the global Carbon cycle, our main objective was to assess the changes in soil C-CO 2 emissions following the suppression of tillage and to investigate some of the underlying physical, chemical and biological factors of control. Two main conclusions can be drawn from laboratory measurements of undisturbed soil samples and microcosms.

The first one is that tillage abandonment significantly decreased soil CO 2 emissions per unit of soil carbon by an average of 22%. Such an improved OM protection correlated with the increase in soil organic carbon content, aggregate stability and soil bulk density, and the decrease in both microbial biomass and microbial activity. The second conclusion was that despite tillage cessation promotes greater OM protection, the increase of SOC stocks in the top-soil by about 50%, resulted in increased total soil C-CO 2 emissions by an average of 9.5%.

These results are expected to improve our understanding of the role of agricultural practices on the global Carbon cycle. Furthermore, they are expected to inform on best management practices for improved C sequestration in soils. Under the conditions of the study, i.e. acidic and sandy conditions, a greater carbon sequestration could be probably achieved by increasing soil bulk density and allocating more organic carbon deep in the soil profile, rather in the topsoil. This might be achieved by planting for instance crops with high carbon allocation to deep roots and by providing a favorable environment for the soil engineers such as epigenic earthworms, to move OM deep in soil profiles.

These results need however to be confirmed in long-term no-till trials, as well as under different climates and environmental conditions before incorporation could be made in carbon models. More research is also required to better understand the reasons and controlling factors of the observed physical, biological and chemical changes following shifts in land use or land gC-CO2 m -2 0,59* 0,52* 0,48* 0,20 0,16 -0,17 -0,14 0,05 0,30 -0,34* -0,37* gC-CO2 gC sol -1 -0,26 -0,39* -0,81* -0,89* -0,50* -0,06 0,14 0,16 -0,47* 0,59* 0,57* * Statistically significant determinants at 95% confidence level

Figure 1 .

 1 Figure 1. Soil tillage impact on soil mean weight diameter (MWD) as a proxy of soil disaggregation potential by the three involved mechanisms. N=9.

  Version définitive du manuscrit publié dans / Final version of the manuscript published in :

	Agriculture, Ecosystems & Environment, 2015, 203, 110-118	http://dx.doi.org/10.1016/j.agee.2015.02.001
	Manuscrit d'auteur / Author Manuscript
	Manuscrit d'auteur / Author Manuscript
	Manuscrit d'auteur / Author Manuscript

Comment citer ce document : Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C.,

Mchunu

, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P. (2015). Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems and Environment, 203, 110-118. DOI : 10.1016/j.agee.2015.02.001

  Version définitive du manuscrit publié dans / Final version of the manuscript published in :

		Agriculture, Ecosystems & Environment, 2015, 203, 110-118	http://dx.doi.org/10.1016/j.agee.2015.02.001
	Manuscrit d'auteur / Author Manuscript	most with increasing  b (r=-0.89) and SOC S (r=-0.81), far in front of MWD (r=0.50), but decreased with increasing MB and MA (Table
	Manuscrit d'auteur / Author Manuscript
	Manuscrit d'auteur / Author Manuscript

Comment citer ce document : Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C.,

Mchunu

, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P. (2015). Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems and Environment, 203, 110-118. DOI : 10.1016/j.agee.2015.02.001

Table 1 . Summary statistics for bulk density, carbon content, nitrogen content, soil carbon stocks and mean weight diameter (MWD) from the 0-0.05m soil layer.

 1 Comment citer ce document : Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C., Mchunu, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P. (2015). Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems and Environment, 203, 110-118. DOI : 10.1016/j.agee.2015.02.001 Version définitive du manuscrit publié dans / Final version of the manuscript published in : Rui, W., Tu, C., Diab, H.G., Louws, F.J., Mueller, J.P., Creamer, N., Bell, M., Wagger,M.G., Hu, S. 2005. Responces of soil microbial community structure and diversity to agricultural deintensification. Pedosphere 15, 440-447. Version définitive du manuscrit publié dans / Final version of the manuscript published in :

	Agriculture, Ecosystems & Environment, 2015, 203, 110-118	http://dx.doi.org/10.1016/j.agee.2015.02.001

Comment citer ce document : Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C.,

Mchunu

, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P. (2015). Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems and Environment, 203, 110-118. DOI : 10.1016/j.agee.2015.02.001

Table 2 . Summary statistics for spectral slope ratio (SR), average absorbance between 275 and 295 nm -1 (S275-295), specific UV-absorbance (SUVA254) and concentration in water soluble DOC (WSDOC). All T and NT samples were considered, n=18.

 2 Comment citer ce document : Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C., Mchunu, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P. (2015). Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems and Environment, 203, 110-118. DOI : 10.1016/j.agee.2015.02.001 Version définitive du manuscrit publié dans / Final version of the manuscript published in : Agriculture, Ecosystems & Environment, 2015, 203, 110-118 http://dx.doi.org/10.1016/j.agee.2015.02.001

		WSDOC	Sr		S275-295	SUVA254
		mg C l -1			nm -1 (x10 -3 )	L mg C -1 m -1
		T	NT	T	NT	T	NT	T	NT
	Mean	95	112 0.69 0.60 0.0158 0.0154 1.36 1.48
	SD	12	20	0.20 0.13 0.0019 0.0011 0.31 0.24
	Var.	142 405 0.04 0.02 0.0000 0.0000 0.10 0.06
	Min	79	81	0.33 0.39 0.0133 0.0135 0.96 1.00
	Q1	89	94	0.62 0.51 0.0151 0.0146 1.14 1.36
	Median 93	112 0.74 0.64 0.0156 0.0152 1.38 1.53
	Q3	98	126 0.82 0.69 0.0158 0.0163 1.61 1.56
	Max	115 144 0.90 0.80 0.0192 0.0168 1.72 2.01
	Skew 0.71 0.18 -1.24 -0.03 0.9045 -0.2193 -0.14 0.07
	Kurt	1.41 -1.05 1.45 -1.15 2.1663 -1.4241 -2.38 1.44
	SE	21	24	0.15 0.13 0.0004 0.0002 0.30 0.32
	CV	13	18	30	22	12.2	7.4	23	16

Table 3 .

 3 Summary statistics for CO 2 emission for undisturbed tilled (T) and no-tilled (NT) soils. N=6.

				CO2-C
		mgC-CO2 g -1 day -1 mgC-CO2 gC -1 day -1 gCm -2 day -1
		T	NT	T	NT	T	NT
	Mean	0.24	0.21	1.10	0.90	1.05 1.15
	SD	0.15	0.03	0.19	0.10	0.19 0.15
	Var.	0.02	0.00	0.00	0.00	0.04 0.02
	Min	0.16	0.16	0.94	0.75	0.88 0.94
	Q1	0.19	0.19	0.95	0.80	0.92 1.03
	Median	0.21	0.21	1.03	0.94	1.03 1.13
	Q3	0.33	0.23	1.24	0.97	1.08 1.26
	Max	0.44	0.25	1.40	1.07	1.41 1.42
	Skew	1.11	-0.18	0.84	4.42	1.44 0.44
	Kurt	0.10	-0.93	-0.78	-1.46	2.61 -0.78
	SE	0.02	0.05	0.24	0.25	0.04 0.03
	CV	7.0	12.4	17.2	12.8	18.5 12.8

Table 4 .

 4 Coefficients of determination (r) between soil CO 2 emission per gram of carbon in the soil and per square meter and multiple environmental factors: top-soil organic carbon and organic nitrogen content (SOC C ; SON C ), SOC stocks (SOC S ), soil bulk density ( b ); mean weighted diameter (MWD); spectral slope ratio (S R ) of S 275-295 and S 350-400 ; Specific UV-absorbance (SUVA 254 ); water soluble dissolved organic carbon (WSDOC), microbial biomass carbon (MB); microbial activity (MA).

	SOCc SONc SOCs	 b	MWD SR S275-295 SUVA254 WSDOC MB	MA

Table 5 .

 5 Coefficients of determination (r) between soil CO 2 emission per gram of carbon in the soil and per square meter and multiple environmental factors.

			CO2 emissions
		gC-CO2 m -2	gC-CO2 gC sol -1
	SOCc	0.59	-0.26
	SOCc + b	0.60	-0.90
	SOCc + b + MWD	0.63	-0.91
	SOCc + b + MA	0.62	-0.91
	SOCc + b + MB	0.63	-0.91
	SOCc + b + SR	0.60	-0.90
	SOCc + b + S275-295	0.58	-0.90
	SOCc + b + SUVA254	0.58	-0.90