Dynamics of soil organic carbon pools after agricultural abandonment

article i nfo 13 Cs ignature Microbial biomass C Dissolved organic C 13 C fractionation Substrate preferential utilization Abandonmentof agricultural land and the subsequent recolonization by natural vegetation isknown to cause in- creases in C contents, contributing to reduction in atmospheric CO2 concentrations. Assessment of the possible mitigation of CO2 excess requires understanding the SOC dynamics, the origin of C pools and the pathways of their transformation. The aims of this work were to assess, by using the δ 13 C signature, the changes of old and new organic C in total (soil organic carbon, SOC) and labile (microbial biomass C, MBC, dissolved organic C, DOC, CO2 efflux from soil) pools after vegetation change from vineyard (C3) to grassland (C4) under semiarid Mediterranean climate. Colonization of abandoned vineyard by the perennial C4-grass Hyparrhenia hirta after 15 or 35 years increased topsoil C stocks by 13% and 16%, respectively. Such an increase was attributed to new above- and below-ground biomass C input from H. hirta. The maximal incorporation of new C was observed in MBC, whereas the DOC derived mainly from old SOC. Based on δ 13 C values of SOC, MBC, DOC and CO2 in C3 soil and in soils after 15 and 35 years of C4 grass colonization, 13 C fractionation per se from changes in isotopic com- position by preferential utilization of substrates with different availability was separated. MBC in C3-C4 soil used more recent ( 13 C-enriched) versus old C (relatively 13 C-depleted) sources. The Δ 13 C by decomposition of SOC to CO2 (δ 13 Co f CO2 minus δ 13 C of SOC) was higher than Δ 13 C by microbial respiration (CO2 minus MBC), demon- stratingthatundersemiaridclimate,soilmicroorganismsdonotalwayspreferentiallydecomposethemostavail-