虽然在地球表面每英亩空气柱中有34,500吨气态氮，但是如果这些气态氮不被固氮菌和藻类固定，是不能被植物利用的。

Although there is 34,500 tons of gaseous nitrogen in the air column above each acre of the earth's surface, this cannot be used by plants until it has been fixed by nitrogen-fixing bacteria and algae.

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氮的固定是由无氧环境的根瘤中的根瘤菌完成的。

Nitrogen fixation is carried out by the rhizobia in the anaerobic environment of the nodule.

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