Dynamics of soil organic carbon pools after agricultural abandonment

• Colonization of abandoned vineyard by Hyparrhenia hirta increases SOC in the topsoil
• Microbial biomass C incorporates the maximal amount of new C
• Dissolved organic C mainly originates from old SOC
• Soil microorganisms not always preferentially decompose the most available SOC pools
• δ13C signature and soil incubation are useful to assess dynamics of C pools

Abandonment of agricultural land and the subsequent recolonization by natural vegetation is known to cause increases 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 δ13C 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 δ13C values of SOC, MBC, DOC and CO2 in C3 soil and in soils after 15 and 35 years of C4 grass colonization, 13C fractionation per se from changes in isotopic composition by preferential utilization of substrates with different availability was separated. MBC in C3–C4 soil used more recent (13C-enriched) versus old C (relatively 13C-depleted) sources. The Δ13C by decomposition of SOC to CO2 (δ13C of CO2 minus δ13C of SOC) was higher than Δ13C by microbial respiration (CO2 minus MBC), demonstrating that under semiarid climate, soil microorganisms do not always preferentially decompose the most available SOC pools. The use of δ13C signature of SOC after C3–C4 vegetation change combined with soil incubation is a powerful tool to assess the exchange between old and new C in pools of various availability.