The nitrogen cycle is based on natural events in which atmospheric nitrogen is fixed into compounds, largely as ammonium and nitrate, which are essential mineral nutrients for the growth, repair, and reproduction of plants and algae. Nitrogen enters the food chain through plants, where it appears in a variety of compounds like amino acids. Nitrogen is fixed in a number of natural processes. Some algae and bacteria convert atmospheric nitrogen into plant-useable nitrogen. The best known of these live in a symbiotic relationship with legume roots.
Lightning is a much smaller source of fixed nitrogen.
Since nitrogen is often the mineral nutrient limiting crop plant growth, humans have expended great energy increasing its production in plant-usable form. One method is the widespread cultivation of legume crops and forage, like soybeans, clover, and alfalfa. More recently, the manufacture of inorganic nitrogen fertilizer has skyrocketed to double the amount fixed by nitrogen-fixing crops. These 2 sources add approximately 120 Tg of nitrogen per year. Burning forests, grasslands, and fossil fuels releases nitric oxides and other gases.