Nitrogen Cycle

Plaboni Sharif

What is nitrogen?

Nitrogen is the chemical element of atomic number 7, a colorless, odorless unreactive gas that forms about 78 percent of the earth's atmosphere. Nitrogen is a crucial component of proteins, many vitamins, and nucleic acids such as DNA.
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N2 cannot cannot be absorbed and used directly as a nutrient by multicellular plants or animals. Natural processes fix N2 into compounds useful nutrients for plants and animals. One is electrical discharges and the other takes place in aquatic systems, soil, and the roots of some plants, where specialized bacteria, called nitrogen-fixing bacteria, complete this conversion.
In nitrogen fixation, specialized bacteria in soil and blue-green algae in aquatic environments combine gaseous N2 with hydrogen to make ammonia
(NH3). The bacteria use some of the ammonia they produce as a nutrient and excrete the rest of the soil or water. Some ammonia is converted to ammonium ions (NH4+) that can be used as a nutrient by plants.
Ammonia not taken up by plants may undergo nitrification. Specialized soil bacteria convert most of the NH3 and NH4+ in soil to nitrate ions (NO3-), which are easily taken up by the roots of plants. The plants then use these forms of nitrogen to produce various amino acids, proteins, nucleic acids, and vitamins. Animals that eat plants eventually consume these nitrogen-containing compounds, as do detritus feeders, or decomposers.
Plants and animals return nitrogen-rich organic compounds to the environment as waste, cast-off particles, and through their bodies when they die and are decomposed or eaten by detritus feeders. In ammonification, vast armies of specialized decomposer bacteria convert detritus into simpler nitrogen-containing inorganic compounds such as ammonia (NH3) and water-soluble salts containing ammonium ions (NH4+).
In denitrification, specialized bacteria in waterlogged soil and in the bottom sediments of lakes, oceans, swamps, and bogs convert NH3 and NH4+ back into nitrite and nitrate ions, and then into nitrogen gas (N2) and nitrous oxide gas (N2O). These gases are released to the atmosphere to begin the nitrogen cycle again.

Human Intervention

1. We add large amounts of nitric oxide (NO) into the atmosphere when N2 and O2 combine as we burn any fuel at high temperatures. This gas can be converted into nitrogen dioxide gas (NO2) and nitric acid vapor (HNO3), which can return to the earth's surface as acid rain.

2. We add nitrous oxide (N2O) to the atmosphere through the action of anaerobic bacteria on livestock wastes and commercial inorganic fertilizers applied to the soil. This greenhouse gas can warm the atmosphere and deplete the stratospheric ozone.

3. We release large quantities of nitrogen stored in soils and plants as gaseous compounds into the atmosphere through destruction of forests, grasslands, and wetlands.

4. We upset the nitrogen cycle in aquatic ecosystems by adding excess nitrates to bodies of water through agricultural runoff and discharges from municipal sewage systems.

5. We remove nitrogen from topsoil when we harvest nitrogen-rich crops, irrigate crops, and burn and clear grasslands and forests before planting crops.