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Eutrophication, which is often caused by overabundance of reactive nitrogen, is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit.
Reactive nitrogen ("Nr"), also known as fixed nitrogen, refers to all forms of nitrogen present in the environment except for molecular nitrogen . While nitrogen is an essential element for life on Earth, molecular nitrogen is comparatively unreactive, and must be converted to other chemical forms via nitrogen fixation before it can be used for growth. Common Nr species include nitrogen oxides , ammonia , nitrous oxide , as well as the anion nitrate .
Biologically, nitrogen is "fixed" mainly by the microbes (eg., Bacteria and Archaea) of the soil that fix N2 into mainly NH3 but also other species. Legumes, a type of plant in the Fabacae family, are symbionts to some of these microbes that fix N2. NH3 is a building block to Amino acids and proteins amongst other things essential for life. However, just over half of all reactive nitrogen entering the biosphere is attributable to anthropogenic activity such as industrial fertilizer production. While reactive nitrogen is eventually converted back into molecular nitrogen via denitrification, an excess of reactive nitrogen can lead to problems such as eutrophication in marine ecosystems.
^Winiwarter, Wilfried; Erisman, Jan Willem; Galloway, James N.; Klimont, Zbigniew; Sutton, Mark A. (25 July 2013). "Estimating environmentally relevant fixed nitrogen demand in the 21st century". Climatic Change. 120 (4): 889-901. doi:10.1007/s10584-013-0834-0. eISSN1573-1480. ISSN0165-0009.