The process by which ammonia compounds, including man-made fertilizer and the humus provided by organic matter or plant and animal origin, are converted into nitrites and then nitrates, which are then absorbed as a nutrient by crops. Excess nitrate can be leached into surface waters and groundwaters, causing pollution. Excess nitrate may also be converted by microbes back into gaseous nitrogen, which is an important greenhouse gas, and released back into the atmosphere. The ultimate source of nitrogen in the ecosystem is the molecular nitrogen in the atmosphere. To a very limited extent, some dissolves in water. However, none is found in rock.