Nitrogen fixation enables plants to use the nitrogen of
the air for growth
In a
closed ecological system, the nitrate required for plant growth is derived from
the degradation of the biomass. In contrast to other plant nutrients (e.g.,
phosphate or sulfate), nitrate cannot be delivered by the weathering of rocks.
Smaller amounts of nitrate are generated by lightning and carried into the soil
by rain water (in temperate areas about 5 kg N/ha per year). Due to the effects
of civilization (e.g., car traffic, mass animal production, etc.), the amount
of nitrate, other nitrous oxides and ammo-nia carried into the soil by rain can
be in the range of 15 to 70 kg N/ha per year. Fertilizers are essential for
agricultural production to compen-sate for the nitrogen that is lost by the
withdrawal of harvest products. For the cultivation of maize, for instance,
about 200 kg N/ha per year have to be added as fertilizers in the form of
nitrate or ammonia. Ammonia, the primary product for the synthesis of nitrate
fertilizer, is produced from nitrogen and hydrogen by the Haber-Bosch process:
3 H2 + N2 → 2 N H3 (∆H - 92.6
kJ/mol)
Because
of the high bond energy of the N≡ N triple bond, this synthesis requires a high
activation energy and therefore has to be carried out at a pressure of several
hundred atmospheres and temperatures of 400–500°C. This involves very high
energy costs. The synthesis of nitrogen fertilizer amounts to about one-third
of the total energy expenditure for the culti-vation of maize. If it were not
for the production of nitrogen fertilizer by the Haber-Bosch synthesis, large
parts of the world’s population could no longer be fed. Using “organic cycle”
agriculture, one hectare of land can feed about 10 people, whereas with the use
of nitrogen fertilizer the amount is increased fourfold.
The
majority of cyanobacteria and some bacteria are able to synthesize ammonia from
atmospheric nitrogen. A number of plants live in symbio-sis with N2-fixing
bacteria, which supply the plant with organic nitrogen. In return, the plants
provide these bacteria with metabolites for their nutrition. The symbiosis of
legumes with nodule-inducing bacteria
(rhizobia) is wide-spread and
important for agriculture. Legumes,
which include soybean, lentil, pea, clover, and lupines, form a large family (Leguminosae)
with about 20,000 species. A very large part of the legumes have been shown to
form a symbiosis with rhizobia. In temperate climates, the cultivation of
legumes can lead to an N2 fixation of 100 to 400 kg N2/ha
per year. Therefore legumes are important as green manure; in crop rotation
they are an inexpensive alternative to artifi-cial fertilizers. The symbiosis
of the water fern Azolla with the
cyanobacterium Nostoc supplies rice fields with nitrogen. N2-fixing
actinomycetes of the genus Frankia form a symbiosis with woody
plants such as the alder or the Australian Casuarina.
The latter is a pioneer plant on nitrogen-deficient soils.
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