Soil organic carbon changes after deforestation and agrosystem establishment in Amazonia: An assessment by diachronic approach

International audienceDeforestation and agrosystem establishment can alter soil organic carbon (SOC) stocks, leading to greenhouse gases emissions and fertility losses, however SOC response has great variability. In the humid tropics like the Amazonia biome, carbon inputs from agrosystems are rarely quantified and described, despite their major contribution to the SOC dynamics after deforestation. We assessed SOC dynamics with repeated measurements in the layer 0–30 cm until five years after deforestation in a diachronic site in French Guiana, cleared with a fire-free method associated to large woody debris inputs. Three agrosystems were studied: one year maize/soybean rotation with disk tillage (DT) and without tillage (NT), and a mowed grassland (G). Aboveground carbon inputs from agrosystems were measured. In addition to SOC stocks assessment, we measured roots carbon stocks, and performed δ13C measurements in grassland soil. We found a transient SOC stock increase until 1.5 years after deforestation because of large woody debris inputs from deforestation method, but these C inputs were rapidly mineralized and poorly contributed to SOC stocks 5 years after deforestation. SOC stocks in grassland did not differ from forest despite large hay exports. Thanks to large root-derived carbon inputs, C4-SOC stock in grassland was 10.4 t ha−1 5 years after deforestation (18.7% of the SOC stock). In annual crops, 5 years after deforestation, SOC stocks decreased on average by 18.6% compared to forest. SOC stocks did not differ according to soil tillage since aboveground carbon inputs were similar in DT and NT systems. Lower SOC stocks in annual crops compared to grassland is explained by lower carbon restitutions and by higher mineralization rate of organic matter