Atmospheric Nitrogen Deposition in Spain: Emission and Deposition Trends, Critical Load Exceedances and Effects on Terrestrial Ecosystems

The Mediterranean Basin presents an extraordinary biological richness, but very little information is available on the threat posed by air pollution, in particular reactive nitrogen (Nr). Atmospheric N deposition in Spain (up to 30 kg N ha−1 year−1) is lower than loads recorded in central Europe, but some evidences indicate that N enrichment is already occurring in some ecosystems. Total N deposition exceeds empirical N critical loads in some protected areas of the Spanish Natura 2000 network. The habitats showing the highest risk are the Pyrenean grasslands, mountain forests of Pinus uncinata or Abies pinsapo, Mediterranean sclerophyllous forests in Catalonia and the oro-Mediterranean heathlands of the Cantabrian Range. Nitrogen deposition effects have already been reported in some of these ecosystems. Further research is needed to define Mediterranean-specific empirical N critical loads and to understand the possible interactions between N and other air pollutants and climate.