Soil carbon erosion and stock as affected by land use changes at the catchment scale in Mediterranean ecosystems

Catchments behave as sources or sinks of soil carbon, depending on the magnitude and type of land use changes within their drainage area, on the intensity of erosion processes and on the fate of eroded sediments. The effect of changing land uses on the organic soil carbon (C) stock and the soil C transported by water erosion and buried in depositional wedges behind check-dams was estimated in a Mediterranean catchment in SE Spain. Changes in land use patterns in the catchment between 1956 and 1997 (57% decrease in areas dedicated to agriculture and 1.5-fold increase of the total forest cover) induced an accumulation rate of total organic carbon (TOC) in the soil of 10.73 g m−2 year−1. Mineral-associated organic carbon (MOC) was the main soil carbon pool (70%). Particulate organic carbon (POC) was highest in the shrubland soils (33%). The average sediments/soil enrichment ratio at the subcatchment scale (8–125 ha) was 0.59 ± 0.43 g kg−1. Eroded soil C accounted for between 2% and 78% of the soil C stock in the first 5 cm of the soil in the subcatchments. The C erosion rate varied between 0.008 and 0.2 t ha−1 year−1. Observed changes in land use (decrease in agricultural areas) reduced soil C erosion, although sediments from non-agricultural sources are richer in organic C. At catchment scale from the 4% of the soil C stock mobilized by water erosion, 77% is buried in the sediment wedges behind check-dams. Soil C replacement due to increased vegetation cover between 1974 and 1997 represented a 36% of the original soil organic C stock. All together represent an erosion-induced sink of soil organic C of 40% compared to the original levels of 23 years before. This has caused the catchment to behave as a soil C sink within the soil erosion subsystem since the 1950s. The meaning of this erosion-induced C sink in a wider C balance which takes into account soil respiration remains uncertain.