Turnover and availability of soil organic carbon under differentMediterranean land‐uses as estimated by13Cnatural abundance

Abstract

SummarySoil 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 fromC3‐C4andC4‐C3vegetation change under field conditions combined with incubations under controlled conditions to evaluate the turnover and availability of soil organicCunder different land‐uses. The 40‐year succession ofHyparrenia hirtaL. (C4photosynthesis) after more than 85 years of olive tree (Olea europaeaL.;C3photosynthesis) growth led to the exchange of 54% of soil organic C fromC3toC4forms. In contrast, 21 years of vine (Vitis viniferaL.) growing afterH. hirtadecreased 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. TheMRTof C under grassland dominated byH. hirtawas about 19 years, but was 180 years under the vineyard. The rates of C accumulation under theH. hirtagrassland were about 0.36 Mg C ha−1 year−1. In contrast, the rates ofClosses 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 naturalMediterranean grassland to a vineyard lead to very largeClosses that cannot be compensated for over the same periods.