Nitrogen Cycle
Nitrogen “fixed” as ammonia and nitrates is taken up directly by plants and incorporated in their tissues as plant proteins. The nitrogen then passes through the food chain from plants to herbivores to carnivores. When plants and animals die, the nitrogenous compounds are broken down by decomposing into ammonia, a process called ammonification. Some of this ammonia is taken up by plants; the rest is dissolved in water or held in the soil, where microorganisms convert it into nitrates and nitrites in a process called nitrification. Nitrates may be stored in decomposing humus or leached from the soil and carried to streams and lakes. They may also be converted to free nitrogen through denitrification and returned to the atmosphere.