ABSTRACT
Nitrogen, an essential constituent in many of the compounds in living cells, is found in amino acids, proteins and nucleic acids. Molecular nitrogen (N2) forms about 78% of the atmosphere by volume; however, this reservoir of nitrogen is not available to plants directly. The molecular nitrogen has a strong triple covalent bond, and higher plants do not have the ability to break this bond directly. Plants, instead, assimilate nitrogen in the form of nitrate (N 03) or ammonia (NH4). The availability of nitrogen to plants can be a limiting factor in agricultural production. As a result, farmers often add chemical fertilizers (formed through an industrial process known as nitro gen fixation where N2 is changed into ammonia or nitrate) to the soils in order to provide a source of nitrogen to meet the requirements for crops. Lack of mineral nitrogen in the soil often limits plant growth and, as a result, modem agriculture uses approximately 80 million tonnes of nitrogen fertil izer annually to maintain the world's food supply (Trevaskis et al., 2002). However, this can be expensive, and over-application can lead to waste and environmental pollution. Natural processes, such as lightning, which result in nitrogen fixation, contribute about 10% of the nitrogen fixed on earth. Certain microorganisms can also fix nitrogen. In fact, approximately 90% of nitrogen fixed through natural processes is carried out by free-living bac teria (including cyanobacteria) and by bacteria, that live in symbiotic asso ciation with plants. These microorganisms contain enzyme systems that catalyze certain chemical reactions to break the N2 triple bond.
