Soil recovery after removal of the N 2 -fixing invasive Acacia longifolia: consequences for ecosystem restoration

Abstract Invasion by Acacia longifolia alters soil characteristics and processes. The present study was conducted to determine if the changes in soil C and N pools and processes induced by A. longifolia persist after its removal, at the São Jacinto Dunes Nature Reserve (Portugal). Some areas had been invaded for a long time ([20 years) and others more recently (\10 years). For each type of invasion, (i.e., longinvaded and recently invaded), three treatments were used: (1) A. longifolia left intact; (2) A. longifolia was removed; and (3) both A. longifolia and litter layer were removed. Soil samples were collected once a year for four and half years and analysed for chemical and microbial properties. In general, microbial parameters responded faster than C and N pools. In long-invaded areas, two and half years after removal of plants and litter, basal respiration and microbial biomass had already decreased [30%, b-glucosaminidase activity (N mineralization index) [60% and potential nitrification [95%. Removal of plants and litter resulted in a[35% decrease in C and N content after four and half years. In recently invaded areas, b-glucosaminidase activity and potential nitrification showed a marked decrease ([54% and [95%, respectively) after removal of both A. longifolia and litter. Our results suggest that after removal of an N 2 -fixing invasive tree that changes ecosystem-level processes, it takes several years before soil nutrients and processes return to preinvasion levels, but this legacy slowly diminish, suggesting that the susceptibility of native areas to (re)invasion is a function of the time elapsed since removal. Removal of the N-rich litter layer facilitates ecosystem recovery