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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.

The relative contribution of different soil organism groups to nutrient cycling has been quantified for a number of ecosystems. Some functions, particularly within the N‐cycle, are carried out by very specific organisms. Others, including those of decomposition and nutrient release from organic inputs are, however, mediated by a diverse group of bacteria, protozoa, fungi and invertebrate animals. Many authors have hypothesized that there is a high degree of equivalence and flexibility in function within this decomposer community and thence a substantial extent of redundancy in species richness and resilience in functional capacity. Three case studies are presented to examine the relationship between soil biodiversity and nitrogen cycling under global change in ecosystem types from three latitudes, i.e. tundra, temperate grassland and tropical rainforest. In all three ecosystems evidence exists for the potential impact of global change factors (temperature change, CO2 enrichment, land‐use‐change) on the composition and diversity of the soil community as well as on various aspects of the nitrogen and other cycles. There is, however, very little unequivocal evidence of direct causal linkage between species richness and nutrient cycling efficiency. Most of the changes detected are shifts in the influence of major functional groups of the soil biota (e.g. between microflora and fauna in decomposition). There seem to be few data, however, from which to judge the significance of changes in diversity within functional groups. Nonetheless the soil biota are hypothesized to be a sensitive link between plant detritus and the availability of nutrients to plant uptake. Any factors affecting the quantity or quality of plant detritus is likely to change this link. Rigorous experimentation on the relationships between soil species richness and the regulation or resilience of nutrient cycles under global change thus remains a high priority.

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