-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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Soil carbon (C) distribution, natural 13C abundances and their changes as a consequence of cropping were studied in three neighboring areas on an Oxisol from Brazil. One site (T0) was under forest, while the two other sites (T12 and T50) had been deforested, then cultivated with sugar cane for 12 and 50 yr, respectively. Soil morphological, chemical and mineralogical characteristics in all three sites were very similar. Total C content of the 0.06‐m layer of T0 was twice that of T12 and T50, then decreased sharply with depth, to values similar to the other profiles. Delta 13C had practically constant values of −25.1, −22.8, and −20.4‰, throughout the 0 to 0.30‐m layer of T0, T12, and T50 respectively. These values increased in deeper layers, to about −17‰, due to increased humification and possibly to deposition of organic matter from a former 13C‐rich vegetation. The 0.10‐ to 0.20‐m layer was separated into particle‐size fractions and alkaline extract. Carbon contents decreased from T0 to T50 in the sand‐size fractions and alkaline extracts, but did not change in the clay‐size fractions. Delta 13C values were used to estimate the proportions of C derived from forest (Cdff) and from sugar cane (Cdfc). Carbon derived from sugar cane represented 17.3 ± 3.2% and 40.5 ± 2.2% of total C in T12 and T50, respectively. It reached its maximum value (67 ± 3.7%) in the coarse sand fraction of T12 and T50 and decreased with decreasing fraction size, to 13.8 ± 9.4% and 30.5 ± 6.5% in the fine clay fractions of T12 and T50, respectively. Thus, Cdff persisted mainly in the clay‐size fraction.