Dynamics of soil organic matter pools after agricultural abandonment

Changes of land use from croplands to natural vegetation usually increase Carbon (C) stocks in soil. However, thecontribution of old and new C to various pools still is not clearly analyzed.We measured the 13C signature of soilorganic carbon (SOC) pools after vegetation change from vineyard (C3) to grassland (C4) under Mediterraneanclimate to assess the changes of old and new C in total SOC, microbial biomass (MB), dissolved organic C (DOC),and CO2 efflux from soil.Development of the perennial grass Hyparrhenia hirta (C4) on vineyard abandoned for 15 or 35 years ago increasedC stocks for 13% and 16%, respectively (in the upper 15 cm). This increase was linked to the incorporation of newC in SOC and with exchange of 25% of old C by new C after 35 years. The maximal incorporation of new Cwas observed in MB, thus reflecting the maximal turnover and availability of this pool. The DOC was producedmainly from old C of soil organic matter (SOM), showing that under Mediterranean climate DOC will be mainlyproduced not from fresh litter but from old SOM sources. Decomposition of SOM during a 51 days laboratoryincubation was higher in cultivated vineyard than H: hirta soils. Based on changes in 13C values of SOM, MB,DOC and CO2 in C3 soil and in soils after 15 and 35 years of C4 plant colonization, we separated 13C fractionationin soil from changes of isotopic composition by preferential utilization of substrates with different availability. Theutilization pattern in this soil under Mediterranean climate was different from that in temperate ecosystems.