INTRODUCTION Nitrogen is an element that is found in both the living portion of our planet and the inorganic parts of the Earth system. It is essential for many biological processes, and is crucial for any life on earth. The complex pathway that nitrogen follows in an ecosystem is called the nitrogen cycle. The nitrogen cycle is one of the biogeochemical cycles and is very important for ecosystems. Nitrogen moves slowly through the cycle and is stored in reservoirs such as the atmosphere, living organisms, soils, and oceans along its way.

All organisms need nitrogen to make DNA, RNA, and proteins, the building blocks of life. Although the Earth’s atmosphere is about 78% nitrogen, most of the nitrogen in the atmosphere occurs primarily in an inert form (N 2 ) that few organisms can use; therefore, in order for plants and animals to be able to use nitrogen, N 2 gas must first be converted into a more chemically available form. The movement of nitrogen between the atmosphere, biosphere, and geosphere in different forms is described by the nitrogen cycle.

Five main processes cycle nitrogen through the biosphere, atmosphere, and geosphere: nitrogen fixation, assimilation, ammonification, nitrification, and denitrification. Microorganisms, particularly bacteria, play major roles in all of the principal nitrogen transformations.

Nitrogen Fixation Most organisms rely on nitrogen-fixing bacteria to transform nitrogen gas into a usable form. Such bacteria, which live in the soil and inside swellings on the roots of legumes, such as beans, peas, clover, and alfalfa, have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia (NH 3 + ) in a process called nitrogen fixation. In aquatic environments, blue-green algae (a bacteria called cyanobacteria) is an important free-living nitrogen fixer. In addition to nitrogen-fixing bacteria, a small amount of nitrogen is ‘fixed’ through a process of high-energy fixation that occurs primarily as lighting strikes converting atmospheric nitrogen into ammonia and nitrates. Nitrogen can also be fixed through man-made processes, primarily industrial processes that create ammonia and nitrogen-rich fertilizers.

Assimilation The ammonia produced by nitrogen fixation is usually quickly incorporated into protein and other organic nitrogen compounds, either by a host plant, the bacteria itself, or another soil organism. The proteins made by plants enter and pass through food webs just as carbohydrates do.

Ammonification In most ecosystems nitrogen is primarily stored in living and dead organic matter. The bodies of dead organisms contain nitrogen, mainly in proteins and nucleic acids. Urine and animal waste also contain nitrogen. Decomposers (such as bacteria and fungi) convert the organic nitrogen within the remains back into ammonia. This process is called ammonification.

Nitrification While ammonia can be used by some plants, most of the nitrogen taken up by plants is converted from ammonia – which is highly toxic to many organisms – into nitrates. This process, called nitrification, is accomplished in two steps: Bacteria of the genus Nitrosomonas convert ammonia to nitrites (NO 2 - ). Bacteria of the genus Nitrobacteria convert nitrites to nitrates (NO 3 - ). These two groups of bacteria are called nitrifying bacteria.

Denitrification Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate is converted to N 2, and to a lesser extent, nitric oxide (NO) and nitrous oxide gas (NO 2 ). Nitric oxide and nitrous oxide are both environmentally important gases. Nitric oxide contributes to smog, and nitrous oxide is an important greenhouse gas, thereby contributing to global climate change. Nitrate is very soluble and it is easily lost from the soil system by leaching. Some of this leached nitrate flows through the hydrologic system until it reaches the oceans where it can be returned to the atmosphere by denitrification. Denitrification is an anaerobic process that is carried out by denitrifying bacteria. It occurs primarily in wet soils where the water makes it difficult for microorganisms to get oxygen.