Organic carbon dynamics in soil particle-size separates of sandy Spodosols when forest is cleared for maize cropping

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 C-13 natural abundance in soil and particle-size separates at three locations, selected on the basis of different contents of 0-50 mum particles (clay + silt). After three decades of cultivation, the amount of carbon from the forest pools (C-f) 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.