In this article we will discuss about the biological nitrogen cycle, explained with the help of a diagram.
Ecosystems continually exchange energy and carbon with the environment; mineral nutrients, on the other hand, are mostly cycled back and forth between plants, animals, microbes and the soil. Most nitrogen enters into ecosystems through biological nitrogen fixation. This is deposited through precipitation, dust, gases or is applied as fertilizer. Since most terrestrial ecosystems are nitrogen-limited, nitrogen cycling is an important control on ecosystem production.
Nitrogen fixation is the major source of nitrogen for ecosystems. Nitrogen fixing bacteria either live symbiotically with plants, or live freely in the soil. Many members of the legume family support nitrogen fixing symbionts. Some cyanobacteria are also capable of nitrogen fixation.
These are phototrophs, which carry out photosynthesis. Like other nitrogen-fixing bacteria, they can either be free-living or have symbiotic relationships with plants. Other sources of nitrogen include acid deposition produced through the combustion of fossil fuels, ammonia gas which evaporates from agricultural fields which have had fertilizers applied to them.
When plant tissues (parts) are shed or are eaten, the nitrogen in those tissues becomes available to animals and microbes. Microbial decomposition releases nitrogen compounds from dead organic matter in the soil, where plants, fungi and bacteria compete for it. Some soil bacteria use organic nitrogen-containing compounds as a source of carbon, and release ammonium ions into the soil.
This process is known as nitrogen mineralization. Others convert ammonium to nitrite and nitrate ions, a process known as nitrification. Nitric oxide and nitrous oxide is also produced during nitrification. Under nitrogen rich and oxygen-poor conditions, nitrates and nitrites are converted to nitrogen gas, a process known as denitrification.