3.5.1  Acquiring atmospheric nitrogen

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Plant growth is frequently limited by nitrogen. Plants generally obtain nitrogen from soil reserves of nitrate or ammonium (so-called mineral nitrogen) but these reserves are often scarce. 

Natural ecosystems can ‘run down’ with respect to nitrogen through soil leaching and fire. Relative abundance of nitrogen-fixing species will then increase. For example, a walk from east to west across Fraser Island, Queensland, will take you across progressively older and more nitrogen deficient sand dunes, and from rainforest to heathland.

In agriculture, harvest of saleable commodities (animal or plant) involves a removal of site nitrogen that might be replaced by further mineralisation of soil nitrogen, import of mineral nitrogen (fertiliser) or fixation of atmospheric dinitrogen (N2).

The earth’s atmosphere is rich in N2 (about 78% N2) but it is very unreactive. Hydrogen will react with N2 at high temperatures and pressures on a catalyst (Haber process, Equation 3.4). Large quantities of ammonia (NH3) are produced by this method for industrial and agricultural use. Amazingly, some prokaryotes have the ability to catalyse this reaction, with the enzyme nitrogenase donating at least four pairs of electrons to every N2 molecule to effect reduction to two NH4+ and at least one H2 (Equation 3.5). Efforts are continuing to duplicate biological N2 fixation in a ‘test tube’, as a cheaper alternative to the Haber process.

N2 + 3H2 → 2NH3 (3.4)

(reaction at 100–1000 atm, 400–550ºC, catalysed by Fe)

N2 + 16ATP + 8e + 10H+ → 2NH4+ + H2 + 16ADP + 6Pi (3.5)

(reaction at ambient conditions, catalysed by the Fe–Mo-containing enzyme, nitrogenase)

Biological N2 fixation is energetically expensive even though it occurs at ambient conditions — estimates fall between 3 and 7 g carbon respired g–1 nitrogen fixed (Layzell 1992). Photoassimilate consumed to support N2 fixation is unavailable for other processes such as growth. Consider a crop fertilised with 140 kg N ha–1. An N2 fixer could replace this fertiliser, but only at a cost of at least 420 kg C ha–1. As most plant dry matter contains 40% carbon, this is equivalent to a loss of one tonne of dry matter per hectare!