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 ammonia 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 compensate 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:
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 cultivation 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 symbiosis 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 widespread 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 artificial 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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