Manure has become a minor source of nitrogen in all major agricultural countries.

在主要的农业国家动物粪便已经不是氮肥的主要来源。

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Another source of nitrogen pollution comes from fossil fuels, which produce nitrogen oxides when they are burnt.

另一种氮污染的来源为矿物燃料，矿物燃料燃烧时，会产生氮氧化物。

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The leaf was the major storage organ for nitrogen and the source of nitrogen for grain filling.

叶片是前期组织氮素的重要贮存器官和籽粒灌浆中氮素的主要供给源。

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