Effect of cactus pear cultivation after Mediterranean maquis on soilcarbon stock, δ13C spatial distribution and root turnover

croplands and orchards leading to strong soil degradation. Organic carbon is usually accumulated in soils undermaquis leading to partial regeneration of fertility for future agricultural use. The aim of this work was to investigatethe effect of land use change from maquis to agriculture on soil organic carbon (SOC) stock and its spatialdistribution in a Mediterranean system. Three Mediterranean land use systems (seminatural vegetation, cactuspear crop and olive grove) were selected in Sicily and analysed for soil C stocks and their δ13C. Total SOC andδ13C were measured up to 75 cm soil depth within and between the rows of cactus pear and olive grove andalong a similar transect in maquis, in order to evaluate the distribution of new and old C derived from roots.The land use change from Mediterranean maquis (C3 plant) to cactus pear (CAM plant) lead to a SOC decreaseof 65% after 28 years of cultivation, and a further decrease for 14% after 7 years after the change from cactuspear to olive grove (C3 plant). Considering these SOC losses as well as the periods after the land use changeswe calculated the mean residence time (MRT) of soil organic matter. TheMRT of C under Mediterranean maquiswas about 142 years, but was just 10 years under cactus pear. Root biomass of cactus pear was used for a newapproach to estimate root turnover. The root turnover rate of cactus decreased along the soil profile from 7.1%per year in 0–15 cm to 3.7% in 60–75 cm soil depth. Along the transect, the average of root turnover valueswas highest in the middle of the intra-row. Root turnover and C inputwere correlated with SOC stocks to evaluateC sequestration potential of soils depending on land use and managements. Weconclude that the SOC under maquisis higher and has longer residence time compared to permanent agricultural crops like cactus pear andolives.