Nitrogen is an essential element in many key biological molecules such as amino acids, proteins & enzymes, nucleic acids, vitamins and cell structural components such as cell membranes and cell walls. Nitrogen may enter into the biological cycle at many points and through many forms; organic N such as urea, nitrates, nitrites, ammonia. Free nitrogen from the atmosphere (nitrogen fixation) is channelled into the environment via ammonia and then to organic compounds into the soil to be able to be utilised by plants (Van der Heijden et al., 2008). An introduction to the nitrogen cycle (shown opposite) can be found in Campbell (1983) and Killham (1994).

In agricultural systems where nitrogen is limited or in low access to fertilisers, biological nitrogen fixation is essential and contributes towards

• Harvested products directly such as grain, fuel wood
• Fodder for animal production
• Soil fertility

Although plants have a very specific requirement for nitrogen, the processing of nitrogen in its various forms (other than that supplied directly to the plant by fertilisers) is carried out by microorganisms which can either by symbiotically associated with plants or are free living. Cycling of non-organic forms of nitrogen was once believed to be was carried out solely by bacteria. However, both fungi and archaea are now known to be involved. Forest systems, dominated by fungal activity, and also grassland and semi arid soils are therefore potential sources of nitrous oxide derived from fungal activity (Hayatsu et al., 2008). The rate of nitrogen fixation is different depending on which organisms are responsible. Symbiotic (nodule inhabiting) N fixation by Rhizobium is the greatest at 400 kg N ha^-1 yr^-1 compared to the free living cyanobacterium Anabaena at 76 kg N ha^-1 yr^-1 in lowland rice systems (Barrios, 2007)and that deposited directly from the atmosphere at 3-30 kg N/ha/year.

The ability to fix nitrogen non-symbiotically appears to be very wide spread amongst both bacteria, actinomycetes, archaea and in cyanobacteria , although the amount of nitrogen fixed is lower here compared to symbiotic N-fixation  at 1-3 kg N/ha/year.  These organisms supply a significant amount of nitrogen to a variety of habitats such as rice paddies, deserts and boreal forests (Van der Heijden et al., 2008), the guts of some animals particularly termites but is thought to be greatest in tropical savannah and some grasslands, accounting for as much as 20% of all plant nitrogen (Van der Heijden et al., 2006). Both aerobic bacteria such as Azotobacter and the anaerobe Clostridium have been shown to fix nitrogen. Most nitrogen fixation occurs at soil with a pH between 5-9 but some strains of the acidophile Thiobacillus ferroxidans  can fix nitrogen at pH 2 (Ehrlich, 1996). 

In  nitrogen limited systems, fungi can increase the N-fixation carried out by symbiotic bacteria. For example, alfalfa plants colonised by mycorrhizal fungi, nitrogen fixation rates were greater than in non mycorrhizal plants (Toro et al., 1998). In this case the mycorrizal fungi was improving phosphorous uptake by the plant thereby providing energy to the N-fixing bacteria via increased production of photosynthesis products (Artursson et al., 2006).

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