Symbiotic Nitrogen Fixation

Nitrogen is one of the most abundant elements on earth and occurs in three main reservoirs: lithosphere, atmosphere and biosphere. The complex relationships between these reservoirs constitute the nitrogen cycle. The lithosphere contains the largest reserves of nitrogen, but only a small part is accessible to plants e.g. as NO₃^- and NH₄⁺. The N content of most surface mineral soils is about 0.02-0.5%. However, most of the soil N is in organic form associated with humus and silicate clays and only about 2-3% of this is mineralised each year (Brady 1990). Thus, the amount of readily available N in the form of NO₃^- and available NH₄⁺ compounds is generally only about 1-2% of the total soil N, with the exception to areas where large amounts of fertiliser have been added.

About 80% of the atmosphere is composed of N₂. The latter is the substrate of nitrogen fixation and the products of this process are the primary source of fixed N, in fact for all living organisms including humans. The quantity of nitrogen fixed in nature is about 230 x 10⁶ Tm annually. Of this, approximately 13% is fixed by electric storms (into nitric acid), whereas 87% is the result of biological fixation of nitrogen. Of the latter, 80% is fixed via symbiotic associations and 20% by free living organisms (Gutschick 1980, Vance 1988). The biological costs of fixing N2 is substantial, being equivalent to about 2.5% of the energy generated by primary photosynthesis on the earth (Gutschick 1980).

In agriculture, in addition to N sources created by biological nitrogen fixation, N-fertilisers are used. The current annual world industrial production of ammonia exceeds 130 x 10⁶ Tm per year (Bakemeier et al. 1997), mostly produced by the Haber-Bosch process. The production of artificial fertiliser is expensive due to high energy costs. Further, the excessive application of fertilisers, contributes to environmental hazard by leaching of nitrate into the ground. Thus biological fixation of N2 has considerable advantages from an ecological as well as an economical point of view (Galloway et al. 1995, Socolow 1999, Sainju et al. 2003).