The circulation of nitrogen between living organisms and the environment. Atmospheric nitrogen is returned to the soil by nitrogen-fixing microorganisms (see nitrogen fixation
) and by electrical discharges in storms, which cause nitrogen and oxygen to combine. The oxides so formed dissolve in rain to form nitrous and nitric acids, which, in the soil, combine with mineral salts to form nitrites and nitrates. The nitrites and the ammonia in the soil derived from animal excretion and the decay of organic matter, are converted to nitrates by nitrification. Plants usually assimilate nitrogen as nitrates and the activities of nitrifying bacteria are thus essential to plant growth. The oxidation of ammonia to nitrite is carried out by Nitrosomonas species and the oxidation of nitrite to nitrate by Nitrobacter. The reverse process, *denitrification
, is mediated by different bacteria. Nitrates may also be lost by leaching. Man also affects the nitrogen cycle in various ways. He fixes nitrogen industrially by combining it with hydrogen in the Haber process and adds nitrogen to the soil in fertilizers. He removes nitrogen by overcultivation, by installing efficient sewage systems thus preventing urine and faeces from reaching the soil, and by factory farming, which often results in animal wastes not being returned to the land in manageable form.