Nitrogen Cycle

The movement of nitrogen through the atmosphere, soil and living organisms, converting it between forms that life can and cannot use directly. Although nitrogen gas makes up most of Earth's atmosphere, most organisms cannot use it in that form; nitrogen-fixing bacteria, some living freely in soil and some in a mutualistic partnership with the roots of legumes and other plants, convert it into compounds plants can take up, and other bacteria later convert waste and decayed matter back toward atmospheric nitrogen through nitrification and denitrification. Large grazing herds accelerate a local version of this cycle: on the Serengeti, the dung and urine of enormous wildebeest and zebra populations returns nitrogen compounds to the grassland soil far faster than leaf litter alone would, measurably shaping which grasses grow where.