The flow of nitrogen through the environment
| Although carbon is important in the formation of organic compounds - and is essential for life on Earth - the element nitrogen is also vital. Nitrogen forms simple chemicals called amino acids, the essential building blocks of all proteins, enzymes, and especially DNA. Although three-fourths of our atmosphere is in the form of diatomic nitrogen gas, the gas itself is very unreactive. Plants and animals simply cannot absorb the gas directly from the atmosphere. Instead, lightning strikes split the nitrogen molecule into free nitrogen, which immediately reacts with oxygen in the air to form nitrogen oxides. Some of these nitrogen oxide gases dissolve in rainwater and eventually percolate into the soil. Certain forms of bacteria that cling to roots within the soil convert (or "fix") this inorganic nitrogen into organic forms (ammonia and nitrate ions) that plants can absorb. The nutrients needed for plant growth are drawn from the soil from the roots to the leaves. Therefore, any organism (including humans) consuming the nuts, leaves, seeds, roots, tubercules, or fruits of plants can digest this organic form of nitrogen. Any organic waste or dead creatures are decomposed by bacteria that return nitrogen back to the atmosphere, or re-fix the nitrogen to the soil again. In agriculture, soils are generally not rich enough in fixed nitrogen to sustain repetitive crop yields year after year; as a result, farmers use compost heaps or add industrially mass-produced fertilizers such as ammonium nitrate (containing high amounts of organic nitrogen), to enhance the soil. |
Credit: NASA/GSFC
For more info: http://ltpwww.gsfc.nasa.gov/globe/NFTG/nitrocyc.htm