Turnover and availability of soil organic carbon under different<scp>M</scp>editerranean land‐uses as estimated by<scp>¹³C</scp>natural abundance

Summary Soil organic matter ( SOM ) is an important factor in ecosystem stability and productivity. This is especially the case for Mediterranean soils suffering from the impact of human degradation as well as harsh climatic conditions. We used the carbon ( C ) exchange resulting from C 3 ‐C 4 and C 4 ‐C 3 vegetation change under field conditions combined with incubations under controlled conditions to evaluate the turnover and availability of soil organic C under different land‐uses. The 40‐year succession of Hyparrenia hirta L . ( C 4 photosynthesis) after more than 85 years of olive tree ( Olea europaea L .; C 3 photosynthesis) growth led to the exchange of 54% of soil organic C from C 3 to C 4 forms. In contrast, 21 years of vine ( Vitis vinifera L.) growing after H. hirta decreased the organic C content to 57%. Considering this exchange and decrease as well as the periods after the land‐use changes, we calculated the mean residence time ( MRT ) of soil C of different ages. The MRT of C under grassland dominated by H. hirta was about 19 years, but was 180 years under the vineyard. The rates of C accumulation under the H. hirta grassland were about 0.36 Mg C ha −1 year −1 . In contrast, the rates of C losses after conversion from natural grassland to a vineyard were 1.8 times greater and amounted to 0.65 Mg C ha −1 year −1 . We conclude that changes of land use from natural M editerranean grassland to a vineyard lead to very large C losses that cannot be compensated for over the same periods.