The causes are man made and have come to dominate Nature's global nitrogen cycle.

BBC: Leaving our mark: Fossils of the future

Eating meat is the dominant driver of the nitrogen cycle in Europe.

BBC: Nitrogen pollution 'costs EU up to ?280bn a year'

And according to the Planetary Boundaries concept, the nitrogen cycle is already one of the places where humanity has already broken through the "safe limit".

BBC: Food: Organic growth?

It points out, though, that there is little evidence things like the behaviour of the nitrogen cycle or the phosphate cycle in the Holocene were particularly well-suited to humans.

ECONOMIST: The global environment

The fact that people have used industrial chemistry to short-circuit the nitrogen cycle, by making fertilisers out of nitrogen in the air at a rate which greatly exceeds what natural systems can manage, has real environmental effects.

ECONOMIST: The global environment

If you're going to be an effective and profitable farmer, nitrogen fertilizers must be applied at every crop cycle.

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