Nitrogen in reactive forms is essential for life and use of nitrogen fertilizers is necessary to produce sufficient food for a growing human population. However, excessive levels of reactive nitrogen in the biosphere and atmosphere constitute a major threat to biodiversity in terrestrial, aquatic, and coastal ecosystems. Human activities have markedly increase the reactive nitrogen in the biosphere through fertilizer production, fossil fuel use, and widespread cultivation of legume crops, and crops like wetland rice that stimulate biological nitrogen fixation. More than 50% of all the synthetic nitrogen fertilizer ever used has been used since 1985.  Globally, anthropogenic sources of Nr now exceed natural terrestrial sources.

Nitrogen is the limiting factor in many ecosystems and many native species are adapted to function best under low-nitrogen conditions. Higher-than-natural levels of reactive nitrogen as a result of nitrogen deposition in natural terrestrial ecosystems, especially temperate grasslands, shrublands, and forests, leads directly to lower plant diversity. Slow-growing plant species are out-competed by a small number of faster-growing species. Excessive levels of reactive nitrogen in water bodies, including rivers, coastal zones, and other wetlands, results from run-off of nitrogenous compounds from agricultural lands and atmospheric deposition. This excess Nr frequently leads to algal blooms and eutrophication, including low oxygen conditions. Eutrophication can cause major decreases in biodiversity in seaweeds, seagrasses, corals, and planktonic organisms.

While there is good knowledge about wet deposition of nitrogen for many regions of the world (e.g., Europe, North America), there is poor knowledge for the rest of the world, both populated and remote.

Data on the wet deposition of inorganic nitrogen species (and to a lesser extent for dry deposition of inorganic N) are available at the local scale at hundreds of location across the globe.  For those regions where the local data are part of established networks (e.g., North America, Europe), the local data can be scaled up to national and multi-national deposition fluxes. In order to estimate nitrogen critical load, exceedance maps of both deposition and critical loads are necessary. Currently there are only critical load maps for Europe on an ecosystem scale. There is a much poorer understanding of the magnitude of dry deposition of nitrogen. Furthermore, knowledge on thresholds for ecosystem protection is poorly developed. These are all areas for development of a global-scale indicator of nitrogen deposition as a threat to biodiversity.