Nitrogen for plant nutrition is mainly obtained from three sources: nitrogen-fixingplant, rain water and manure.
氮素是植物必要的营养元素之一，其主要来源有：生物固氮作用、降雨和灌溉水以及人工施肥。
2
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.