What could the nitrogen cycle not exist without?

Microbes.

Explanation:

Microscopic organisms break down dead things and release the nitrogen back into the soil and atmosphere.

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explanation:

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the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.