生物圈中无机氮通过生物固氮或者硝酸盐的同化转化为有机氮。每年由植物同化硝酸盐的速率大约是2×10~(13)公斤,比生物固氮的速率大100倍。因此,硝酸盐的同化作用在植物氮代谢中具有重要的生物学意义。植物中硝酸盐转化为氨:第一步是硝酸盐的两个电子还原,成为亚硝酸盐,由NR(硝酸还原 In the biosphere, inorganic nitrogen is converted to organic nitrogen by biological nitrogen fixation or nitrate assimilation. The annual rate of nitrate assimilation by plants is about 2 × 10 ~ (13) kg, which is 100 times greater than that of biological nitrogen fixation. Therefore, the assimilation of nitrate has important biological significance in plant nitrogen metabolism. Nitrates in plants are converted to ammonia: The first step is to reduce the two electrons of the nitrate to nitrite, produced by NR (nitric acid reduction