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RESEARCH PRODUCT

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

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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. Resume Depuis les annees 50, la deforestation et la mise en culture des podzols sableux des Landes de Gascogne representent un changement majeur d'occupation du sol. L'impact de la monoculture de mais sur les stocks et la qualite des matieres organiques des sols a ete analyse en comparant les teneurs en carbone organique et l'abondance naturelle en 13C dans le sol et les fractions granulometriques de trois chronosequences selectionnees pour leur differentes teneurs en fraction minerale 0–50 μm (argile + limon). Apres trois decennies de monoculture, les stocks de carbone d'origine forestiere (Cf) ont diminue d'environ 60%. Cette decroissance est essentiellement liee aux matieres organiques labiles des fractions grossieres (−70%). Une partie de ce carbone forestier resiste a la mise en culture, temoignant de la composition heterogene des matieres organiques dans cette fraction. Le carbone de la fraction 0–50 μm est relativement resistant puisque ce compartiment ne diminue que de 20% par rapport aux teneurs initiales. La mise en culture se traduit par une homogeneisation du sol et des conditions de mineralisation des matieres organiques, conduisant a long-terme a une convergence des stocks de carbone des trois chronosequences. Cependant, de faibles variations naturelles de la taille de la fraction minerale 0–50 μm suffisent pour modifier l'accumulation de carbone dans le sol. Le potentiel de stockage de carbone des podzols landais depend donc de leur fraction minerale fine qui constitue un compartiment de stabilisation des matieres organiques. De meme, la capacite de stockage de carbone maisicole depend de la quantite de carbone forestier accumulee dans cette fraction (degre de saturation) et de sa vitesse de decomposition.