Présentation donnée par Pierre Barré à l'occasion du symposium "International symposium on soil organic matter dynamics: Land use, management and global change" (6 au 9 juillet 2009, Etats-Unis).

Résumé : Understanding and modelling the dynamics of Soil Organic Carbon (SOC) with high turnover time is crucial to quantify and predict the ability of soil to sequester carbon. Long term bare fallow experiments, where organic C inputs are essentially zero for an extended period of time, are particularly relevant to investigate SOC with long turnover time. We have gathered a unique dataset combining soil, plant and climatic data monitored on six sites presenting different pedo-climatic conditions: Askov (Denmark), Kursk (Russia), Rothamsted (UK), Ultuna (Sweden), Grignon (France) and Versailles (France). We observed that SOC content is still significantly decreasing in all of these bare fallow experiments, even after a long period of time (50 to 80 years). The initial C content and land use strongly affected C dynamics. We then modelled SOC dynamics in bare fallow and adjacent control (cropped or grassland) plots using the ecosystem model ORCHIDEE, whose soil carbon module is derived from CENTURY. ORCHIDEE successfully reproduced carbon stock dynamics under both control and bare fallow plots for the six sites. The decomposition rates of SOC pools in control and bare fallow plots across sites allowed us to discuss the relative impact of land-use history and climate and textural parameters on labile and more stable soil carbon pools.