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Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying±rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. #

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Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay‐associated organic matter. Soil samples with a C content of 4 to 53 g kg−1 were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3‐ to 5‐mm aggregates. The <2‐μm fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content This change in clay wettability could partly explain the higher water stability of soils rich in C.

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Recent initiatives, such as the United Nations declaring 2015 as the International Year of Soils and the French « 4 per 1000 » initiative call attention on soils and on the importance of maintaining and increasing soil organic matter stocks for soil fertility and food security, and for climate change adaptation and mitigation. We stress that soil organic carbon storage (i.e. an increase of soil organic carbon stocks) should be clearly differentiated from soil organic carbon sequestration, as the latter assumes a net removal of atmospheric CO 2 . Implementing management options that allow increasing soil organic carbon stocks at the local scale raises several questions, which are discussed in this article: how can we increase SOC stocks, at which rate and for how long; where do we prioritize SOC storage; how do we estimate the potential gain in C and which agricultural practices should we implement? We show that knowledge and tools are available to answer many of these questions, while further research remains necessary for others. A range of agricultural practices would require a re-assessment of their potential to store C and a better understanding of the underlying processes, such as no tillage and conservation agriculture, irrigation, practices increasing below ground inputs, organic amendments, and N fertilization. The vision emerging from the literature, showing the prominent role of soil microorganisms in the stabilization of soil organic matter, draw the attention to more exploratory potential levers, through changes in microbial physiology or soil biodiversity induced by agricultural practices, that require in-depth research.

It is estimated that in excess of 50% of the soil carbon stock is found in the subsoil (below 20-30 cm). Despite this very few studies have paid attention to the subsoil. Although surface and subsurface horizons differ in pedological, environmental and physicochemical features, which are all likely to affect the mechanisms and biological actors involved, models of carbon dynamics tend to assume that the underlying processes are identical in all horizons, but with lower gross fluxes in the subsurface. The aim of this study was to test this assumption by analysing factors governing organic matter decomposition in topsoil (from depths of 5-10 cm) and subsoil (from depths of 80-100 cm). To this end, we established incubations that lasted 51 days, in which factors that were thought to control organic matter mineralization were altered: oxygen concentration, soil structure and the energetic and nutritional status. At the end of the incubation period, the microbial biomass was measured and the community level physiological profiles established. The mineralization per unit organic carbon proved to be as important in the subsoil as it was in surface samples, in spite of lower carbon contents and different catabolic profiles. Differences in the treatment effects indicated that the controls on C dynamics were different in topsoil and subsoil: disrupting the structure of the subsoil caused a 75% increase in mineralization while the surface samples remained unaffected. On the other hand, a significant priming affect was found in the topsoil but not in the subsoil samples. Spatial heterogeneity in carbon content, respiration and microbial communities was greater in subsoil than in topsoil at the field scale. These data suggest greater attention should be paid to the subsoil if global C dynamics is to be fully understood.

Primary organo-mineral complexes are defined as organic matter (OM) bound to primary mineral particles, isolated after complete dispersion of soil. Organic matter present in < 2 mm particle-size fractions of soils has slow turnover times and it is assumed to be stabilized mainly by interaction with minerals. We aimed to quantify how much of the organic matter in < 2 mm particle-size fractions was free versus bound to minerals and to describe the nature of the association. Furthermore, we hypothesized that this bound organic matter was more resistant to biodegradation than free organic particles. We tested this by using a cultivation chronosequence on temperate silty soils and quantified free and claybound organic matter using density fractionation coupled with elemental analyses, as well as transmission electron microscopy (TEM) coupled with image analyses. Both methods showed that free organic matter was a minor fraction and that it was more depleted by cultivation than clay-bound organic matter. We deduced that clay-bound organic matter was more resistant to biodegradation. TEM showed that the distribution of organic matter in clay-sized fractions was patchy and that many of the so-called < 2 mm 'particles' were in fact nanometre-to micrometre-sized microaggregates in which OM was encrusted by minerals or coated minerals. We conclude that true primary organo-mineral complexes do not correspond to reality and must be regarded as conceptual entities. We suggest that the very small microaggregates, which were evidenced here, are major sites of OM stabilization, both by adsorption and by entrapment of organic matter.Complexes organo-mine´raux < 2mm au sein d'une chronose´quence de mise en culture de sols: une re´e´valuation du concept de ''complexe organo-mine´ral primare'' Re´sumeĹ es complexes organo-mine´raux primaires sont de´finis comme l'association entre des matie`res organiques (MO) et des particules primaires mine´rales, que l'on se´pare apre`s une dispersion comple`te du sol. On attribue ge´ne´ralement le renouvellement lent des MO des fractions granulome´triques texturales < 2 mm al eur interaction avec les mine´raux. Notre objectif e´tait de quantifier la proportion de MO de ces fractions lie´e aux mine´raux et de de´crire ces associations. De plus, nous avons fait l'hypothe`se que les MO lie´es e´taient plus re´sistantes a`la biode´gradation que les MO libres < 2 mm. Pour le tester, nous avons utiliseú ne chronose´quence de mise en culture de sols limoneux tempe´re´s et avons quantifie´et de´crit les MO libres et lie´es de la fraction < 2 mm par fractionnement densime´trique ainsi que par microscopie e´lectro-nique a`transmission et analyse d'images. Les deux me´thodes ont montre´que les MO libres e´taient tre`s peu abondantes et plus affecte´es par la mise en culture que les MO lie´es. Nous en avons de´duit que les MO lie´es se biode´gradent plus lentement. Les observations en microscopie e´lectronique ont montre´que la Correspondence: Claire Chenu. distribution des MO dans la fraction < 2 mm e´tait t...