Nitrogen Cycle

The Nitrogen cycle is the biogeochemical cycle that describes the transformation and translocation of nitrogen (N) in soil, water, and living and dead organic material. The biogeochemical cycling of N is highly dependent on the activities of microorganisms. Nitrogen fixation generally refers to the process in which atmospheric nitrogen (N2) is converted into ammonia (NH3). Conversion of atmospheric N to ammonia requires the enzyme nitrogenase. The conversion of organic nitrogen to NH3 is called ammonification. In the presence of water, NH3 becomes ionized and forms ammonium (NH4+). Ammonium in the soil is the starting point for a series of processes and reac­tions including: uptake by plants; fixation by clay minerals and organic matter; immobilization by microorganisms; transformation into ammonia gas and exported to the atmosphere by volitalization; and nitrification. Nitrification is a two-step process. In the first step of this process, nitrite (NO2-) is formed when NH3 or ammonium (NH4+) is oxidized by nitrifying bacteria. Nitrifying bacteria are also responsible for the second step of the nitrification process which involves the oxidation of NO2- to nitrate (NO3-). Plants readily take up NO3- ions through their roots and assimilate them into organic compounds. Denitrification is the microbially mediated process of NO3- reduction. In the absence of oxygen, NO3- ions can act as terminal electron acceptors and can result in the production of molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products including nitrous oxide (N2O). Immobilization occurs when inorganic N, as NH4+ and NO3-, are assimilated by microorganisms. As microorganisms decompose carbonaceous organic residues they may require more N than is contained in the residue. Subsequently they assimilate inorganic N into their cellular components which becomes unavailable for plant uptake. The conversion of organic N compounds to inorganic N ions is known as mineralization. In nitrogen mineralization, organic nitrogen from decaying plant and animal residues (proteins, nucleic acids, amino sugars, urea) is converted back to NH4+ and NO3-. Leaching refers to the export of N as NO3- from the soil which makes it unavailable for plant uptake. In contrast to NH4+ ions, which are attracted to negatively charged soil particles, the net negative charge of NO3- ions means that they are repelled by negatively charges soil particles. Consequently, under wet conditions, NO3- ions move downward with drainage water and are readily leached from the soil.

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