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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.

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Summary In southwest France, much of the forest lands on sandy Spodosols has been converted to continuous maize cropping in the last few decades. To evaluate the impacts of such change on the content and properties of the soil organic matter, we compared the amount of organic carbon and 13C natural abundance in soil and particle‐size separates at three locations, selected on the basis of different contents of 0–50 μm particles (clay + silt). After three decades of cultivation, the amount of carbon from the forest pools (Cf) decreased by about 60%, attributable mainly to easily degradable material in sand‐sized fractions (−70%). However, a recalcitrant residue remained in soil at a constant proportion, showing that organic matter in these fractions is heterogeneous. Organic matter in the clay + silt fraction was relatively resistant, decreasing by only 20% after 30 years of cultivation. Intensive agricultural management has homogenized the characteristics of the soil and the mineralization of the organic matter, which has resulted in a long‐term convergence of organic carbon from the three locations. However, small natural variations in fine particle content were associated with significant differences in the accumulation of carbon in soil. The protective capacity of the soil depended on the proportion of clay + silt fraction, which stabilized the organic matter. Furthermore, the degree of saturation of this fraction with original carbon from forest and its rate of decomposition determined the soil's capacity to accumulate newly added carbon derived from maize.

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International audienceThe intensive use for over 100 yr of copper sulfate (Bordeaux mixt.) to fight mildew in vineyards has led to a substantial accumulation of copper (Cu) in surface soils. To assess the effects of such large concns., the surface soils of 10 Burgundy vineyards were sampled and analyzed for total org. matter (carbon and nitrogen) and metal (copper and iron) contents. Phys. (i.e. size fractionation) and chem. (sequential extn.) methods were used to det. the distribution of these elements. The most Cu-​contaminated plots showed the largest accumulation of org. carbon and Cu in the coarse sand and fine sand fractions. Copper was strongly correlated with org. carbon and org. nitrogen in the coarse sand fraction and with org. nitrogen in the fine sand fraction. Copper was also highly correlated with both Fe and org. nitrogen in the clay fraction but not significantly with org. carbon. The sequential extn. showed that Cu was bound mainly to the Fe oxides. However, in the most Cu-​contaminated plots, a part of added Cu was bound to org. matter. Apparently, Cu protected indirectly the org. matter present in the coarse fractions against biodegrdn., and therefore modified the distribution of org. carbon among the particle-​size fractions. Iron appeared as the main factor responsible for Cu accumulation in the clay fraction, mainly through inclusion of Cu in Fe oxyhydroxides and possibly in clay-​humus complexes