Cumulative nitrogen input drives species loss in terrestrial ecosystems

Aim Elevated inputs of biologically reactive nitrogen (N) are considered to be one of the most substantial threats to biodiversity in terrestrial ecosystems. Several attempts have been made to scrutinize the factors driving species loss following excess N input, but generalizations across sites or vegetation types cannot yet be made. Here we focus on the relative importance of the vegetation type, the local environment (climate, soil pH, wet deposition load) and the experimentally applied (cumulative) N dose on the response of the vegetation to N addition. Location Mainly North America and Europe. Methods We conducted a large-scale meta-analysis of in situ N addition experiments in different vegetation types, focusing on the response of biomass and species richness. Results Whereas the biomass of grasslands and salt marshes significantly increased with N fertilization, forest understorey vegetation, heathlands, freshwater wetlands and bogs did not show any significant response. Graminoids significantly increased in biomass following N addition, whereas bryophytes significantly lost biomass; shrubs, forbs and lichens did not significantly respond. The yearly N fertilization dose significantly influenced the biomass response of grassland and salt marshes, while for the other vegetation types none of the collected predictor variables were of significant influence. Species richness significantly decreased with N addition in grasslands and heathlands [Correction added on 23 March 2011, after first online publication: 'across all vegetation types' changed to 'in grasslands and heathlands']. The relative change in species richness following N addition was significantly driven by the cumulative N dose. Main conclusions The decline in species richness with cumulative N input follows a negative exponential pathway. Species loss occurs faster at low levels of cumulative N input or at the beginning of the addition, followed by an increasingly slower species loss at higher cumulative N inputs. These findings lead us to stress the importance of including the cumulative effect of N additions in calculations of critical load values.