Effects of nitrogen deposition on freshwater ecosystems

Most of the earth’s biodiversity is found in (semi-)natural ecosystems, both in aquatic and terrestrial habitats. Human activities pose a number of threats to the structure and functioning of these ecosystems, and thus to the natural variety of plant and animal species. One of the major threats is the increase in airborne nitrogen pollution (NOx and NHy) in recent decades. Nitrogen is the limiting nutrient for plant growth in many of the (semi-)natural ecosystems. Most of the plant species from these (semi-)natural habitats are adapted to nutrient-poor conditions, and can only compete successfully on soils with low nitrogen levels (1,2). Nitrogen is the only nutrient whose cycling through the ecosystem is almost exclusively regulated by biological processes. To establish reliable critical loads for nitrogen, it is essential to understand the effects of nitrogen on these ecosystem processes. The critical loads for nitrogen depend on (a) the type of ecosystem; (b) the land use and management in the past and present; and (c) the abiotic conditions, especially those that influence the nitrification potential and immobilization rate in the soil. Where possible, these points are taken into account in this section.