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植物生长和繁殖都需要氮素,缺乏氮素会降低作物生长量、产量和质量。栽培植物的有效氮素来自土壤的有机质、肥料和微生物的生物固氮作用。某些自生的土壤微生物和植物微生物共生,把大气中大量的氮气（近80％）转换成可供植物生长利用的化合物。用“生物方法”固定下来的氮素补充了土壤中氮素的贮备,并有助于增进土壤肥力。有一种称为根瘤菌的土壤细菌与豆科植物共生就能产生这种固氮系统。根瘤菌侵染豆科植物的根毛而产生肿块,称为根瘤。在根瘤里面,气态氮被转换成多种氨基酸,使植物能直接利用。植物反过来为假菌体提供光合碳水化合物。结果,大部份固定下来的氮素结合在土壤中而成为动植物残渣;随着微生物分解作用,被固定的氮就转换成含氮的物质供植物群丛使用。热带地区生物固氮每年每公顷为20—650公斤。而100公斤固氮量等于施用500公斤硫酸铵肥料,因此,对农业生产具有重要的经济意义。
Nitrogen is required for plant growth and reproduction, and the lack of nitrogen can reduce crop growth, yield and quality. The effective nitrogen from cultivated plants comes from the biological nitrogen fixation of organic matter, fertilizers and microorganisms in the soil. Some indigenous soil microbes and plant microbes symbiotic, the large amount of nitrogen in the atmosphere (nearly 80%) into compounds for plant growth and utilization. Nitrogen fixed with “biological methods” complements the storage of nitrogen in the soil and helps to improve soil fertility. There is a soil bacteria called Rhizobium symbiotic with legumes can produce this nitrogen fixation system. Rhizobium infection of legume root hair and produce mass, called nodules. In nodules, gaseous nitrogen is converted into a variety of amino acids, allowing plants to be used directly. Plants in turn provide photosynthetic carbohydrates to the pseudotrophs. As a result, most of the immobilized nitrogen binds to the soil and becomes animal and plant residues; as microbes decompose, immobilized nitrogen is converted into nitrogenous material for use by the plant community. Biological nitrogen fixation in the tropics is 20-650 kg per hectare per year. 100 kg of nitrogen fixation amount is equivalent to 500 kg of ammonium sulfate fertilizer, therefore, it has important economic significance for agricultural production.