Nitrogen gains

In a process called mineralisation, certain bacteria feed on organic matter, releasing ammonia (NH3) (which may be reduced to ammonium NH4+) and other nutrients. As long as the carbon to nitrogen ratio (C/N) in the soil is above 30:1, nitrogen will be in short supply and other bacteria will feed on the ammonium and incorporate its nitrogen into their cells. In that form the nitrogen is said to be immobilised. Later, when such bacteria die, they too are mineralised and some of the nitrogen is released as ammonium and nitrate. If the C/N is less than 15, ammonia is freed to the soil, where it may be used by bacteria which oxidise it to nitrate in a process called nitrification. Bacteria may on average add 25 pounds nitrogen per acre, and in an unfertilised field, this is the most important source of usable nitrogen. In a soil with 5 percent organic matter perhaps 2 to 5 percent of that is released to the soil by such decomposition. It occurs fastest in warm, moist, well aerated soil. The mineralisation of 3 percent of a soil that is 4 percent organic matter would release 120 pounds of nitrogen as ammonium per acre.
In symbiotic fixation, Rhizobium bacteria convert N2 to nitrate by way of nitrogen fixation. They have a symbiotic relationship with host plants, wherein they supply the host with nitrogen and the host provides the bacteria with nutrients and a safe environment. It is estimated that such symbiotic bacteria in the root nodules of legumes add 45 to 250 pounds of nitrogen per acre per year, which may be sufficient for the crop. Other, free-living nitrogen-fixing bacteria and blue-green algae live independently in the soil and release nitrate when their dead bodies are converted by way of mineralisation.
Some amount of usable nitrogen is fixed by lightning as nitric acid (HNO3). Ammonia, NH3, previously released from the soil or from combustion, may fall with precipitation as nitric acid at a rate of about five pounds nitrogen per acre per year.