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The decomposition and transformation of above-and below-ground plant detritus (litter) is the main process by which soil organic matter (SOM) is formed. Yet, research on litter decay and SOM formation has been largely uncoupled, failing to provide an effective nexus between these two fundamental processes for carbon (C) and nitrogen (N) cycling and storage. We present the current understanding of the importance of microbial substrate use efficiency and C and N allocation in controlling the proportion of plant-derived C and N that is incorporated into SOM, and of soil matrix interactions in controlling SOM stabilization. We synthesize this understanding into the Microbial Efficiency-Matrix Stabilization (MEMS) framework. This framework leads to the hypothesis that labile plant constituents are the dominant source of microbial products, relative to input rates, because they are utilized more efficiently by microbes. These microbial products of decomposition would thus become the main precursors of stable SOM by promoting aggregation and through strong chemical bonding to the mineral soil matrix.

-The long-term stabilization of soil organic matter (SOM) in tropical and temperate regions is mediated by soil biota (e.g. fungi, bacteria, roots and earthworms), soil structure (e.g. aggregation) and their interactions. On average, soil C turnover was twice as fast in tropical compared with temperate regions, but no major differences were observed in SOM quality between the two regions. Probably due to the soil mineralogy dominated by 1:1 clay minerals and oxides in tropical regions, we found a higher aggregate stability, but a lower correlation between C contents and aggregate stability in tropical soils. In addition, a smaller amount of C associated with clay and silt particles was observed in tropical versus temperate soils. In both tropical and temperate soils, a general increase in C levels (≈ 325 ± 113 kg C·ha -1 ·yr -1 ) was observed under no-tillage compared with conventional tillage. On average, in temperate soils under no-tillage, compared with conventional tillage, CH 4 uptake (≈ 0.42 ± 0.10 kg C-CH 4 ·ha -1 ·yr -1 ) increased and N 2 O emissions increased (≈ 1.95 ± 0.45 kg N-N 2 O·ha -1 ·yr -1 ). These increased N 2 O emissions lead to a negative global warming potential when expressed on a CO 2 equivalent basis. La stabilisation à long terme de la matière organique du sol (MOS) dans les régions tempérées et intertropicales est sous la dépendance de l'activité biologique (champignons, bactéries, macrofaune et racines), de la structure du sol (agrégation) et de leurs interactions. En moyenne, si le turnover du carbone du sol (C) est environ deux fois plus rapide en régions intertropicales qu'en régions tempérées, peu de différences apparaissent toutefois quant à la qualité de la MOS sous ces climats différents. La stabilité de l'agrégation est plus élevée pour les sols des régions intertropicales, ceci étant probablement dû à leur minéralogie dominée par des argiles de type 1:1 associés à des oxihydroxides métalliques. Toutefois, pour les sols tropicaux, la corrélation entre teneur en C et stabilité de l'agrégation est plus faible et de moindres quantités de C sont associées avec les éléments fins (argile+limon). Aussi bien sous climats tempéré que tropical et subtropical, une augmentation générale des stocks de C du sol (≈ 325 ± 113 kg C·ha -1 ·an -1 ) est observée avec les pratiques de non labour. Pour les sols des régions tempérées, si une fixation de CH 4 (≈ 0.42 ± 0.10 kg C-CH 4 ·ha -1 ·an -1 ) est mesurée sous non-labour, parallèlement une émission de N 2 O est observée (≈ 1.95 ± 0.45 kg N-N 2 O·ha -1 ·an -1 ), conduisant finalement à un bilan négatif en terme de réchauffement global exprimé en équivalents de flux de C-CO 2 .carbone du sol / agrégation / émissions N 2 O / non-labour / régions tempérées et intertropicales INTRODUCTIONThe conservation of sufficient soil organic matter (SOM) levels is crucial for the biological, chemical and physical soil functioning in both temperate and tropical ecosystems. Appropriate levels of SOM ensure soil fertility and minimize agricultural impac...

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