How long do ecosystems take to recover from atmospheric nitrogen deposition?

Atmospheric nitrogen (N) deposition is a considerable threat to biodiversity and ecosystem function globally. Many experimental N additions and studies using gradients of ambient deposition have demonstrated impacts on plant species richness, diversity and composition in a broad range of habitats together with changes in soil biogeochemistry. In the last two decades levels of N deposition have begun to decline in some parts of Europe but it is currently difficult to assess the extent to which reductions in N deposition will result in recovery within semi-natural habitats. There have been a number of investigations using the cessation of N additions in long-term experiments, monitoring in areas where ambient deposition has declined, transplants to situations with lower N inputs and roof experiments where rain is collected and cleaned. This review collates evidence from experiments in grasslands, forests, heathlands and wetlands where N additions have ceased or where N inputs have been reduced to assess how likely it is that habitats will recover from N deposition. The results of the majority of studies suggest that vegetation species composition, below-ground communities and soil processes may be slow to recover whereas some soil variables, such as nitrate and ammonium concentrations, can respond relatively rapidly to reductions in N inputs. There are a number of barriers to recovery such as continued critical load exceedance and lack of seed bank or local seed source, and there is the potential for vegetation communities to reach an alternative stable state where species lost as a consequence of changes due to N deposition may not be able to recolonise. In these cases only active restoration efforts can restore damaged habitats.