Why nitrogen must cycle through an ecosystem

Nitrogen must be cycled through an ecosystem so that the nitrogen is available for organisms to make proteins. ... Bacteria in the soil combine nitrogen with hydrogen to form ammonia. 

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.