固定氮(nitrogen fixation)

使空气中富含的不活泼气体--游离氮与其他元素化合而生成较活泼化合物的自然或工业过程。这些化合物通常是氨、硝酸盐或亚硝酸盐。土壤微生物（例如生活在豆科植物根瘤中的根瘤菌）能完成90%以上的氮的固化。虽然氮是所有蛋白质的组成部分，在植物和动物的代谢过程中是非常重要的，但动、植物不能利用元素状态的氮，比如占大气80%的氮气。共生的固氮菌侵袭宿主植物的根毛，并在此繁殖和促进根瘤的形成、植物细胞和亲密共生细菌的扩大。根瘤内的固氮菌将游离氮转化为硝酸盐，被宿主植物在发育中利用。根瘤中细菌的固氮作用在农业中有头等重要的意义。在工业化国家使用合成肥料之前，由粪肥和轮作（包括一种带根瘤的作物）来提供可以利用的氮。

English version：

nitrogen fixation

Any natural or industrial process that causes free nitrogen in the air to combine chemically with other elements to form more reactive nitrogen compounds such as ammonia, nitrates, or nitrites. Soil microorganisms (e.g., Rhizobium bacteria living in root nodules of legumes) are responsible for more than 90% of all nitrogen fixation. Though nitrogen is part of all proteins and essential in both plant and animal metabolism, plants and animals cannot use elemental nitrogen such as the nitrogen gas (N2) that forms 80% of the atmosphere. Symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate the formation of root nodules, enlargements of plant cells and bacteria in close association. Within the nodules the bacteria convert free nitrogen to nitrates, which the host plant uses for its development. Nitrogen fixation by bacteria associated with legumes is of prime importance in agriculture. Before the use of synthetic fertilizers in the industrial countries, usable nitrogen was supplied as manure and by crop rotation that included a legume crop.