Soil recovery after removal of the N2-fixing invasive Acacia longifolia: consequences for ecosystem restoration

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 SA o pound 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., long-invaded 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%, beta-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, beta-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 pre-invasion 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.