Plants fixate nitrogen from the air from lightning or
precipitation fixation. Bacteria in the soil also fixate nitrogen
in the air. Animals eat the plants and defecate or urinate as they
eat. These wastes are incorporated into the soil through
decomposition as organic matter. Mineralization occurs, turning the
organic nitrate into ammonium. Nitrification then occurs turning
the ammonium into nitrites, and another nitrification step turns
nitrites to nitrates. These nitrates are absorbed by the plants and
the cycle continues.