Nitrogen

Abstract

The atmosphere is a vast storehouse of nitrogen—it consists of 78 % by volume of this gas. Atmospheric nitrogen is converted by electrical activity to nitrate (NO₃ ⁻) that reaches the earth’s surface in rainfall. Atmospheric nitrogen also can be fixed as organic nitrogen by bacteria and blue-green algae, and it can be reduced to ammonia (NH₃) by industrial nitrogen fixation. Plants use ammonium (NH₄ ⁺) or nitrate as nutrients for making protein that is passed through the food web. Elevated concentrations of ammonium and nitrate contribute to eutrophication of water bodies. Because it has several valence states, nitrogen undergoes oxidations and reductions most of which are biologically mediated. Nitrogen in organic matter is converted to ammonia (and ammonium) by decomposition. Organic matter with high nitrogen content typically decomposes quickly with release of appreciable ammonia nitrogen (NH₃ + NH₄ ⁺). In aerobic zones, nitrifying bacteria oxidize ammonia nitrogen to nitrate, while in anaerobic zones, nitrate is reduced to nitrogen gas by denitrifying bacteria. Ammonia and ammonium exist in a temperature and pH dependent equilibrium—the proportion of NH₃ increases with greater temperature and pH. Elevated concentrations of un-ionized ammonia can be toxic to aquatic organisms. Nitrite sometimes reaches high concentrations even in aerobic water and is potentially toxic to aquatic animals.