The Nitrogen Cycle is a natural process by which nitrogen circulates through the atmosphere, soil, water, and living organisms, mainly through the activities of bacteria, plants, and animals. This cycle comprises of three main steps, which are nitrogen fixation, nitrification, and denitrification.
1. Nitrogen fixation: Nitrogen fixation is the process of converting nitrogen gas (N2) from the atmosphere into ammonia (NH3) or related nitrogenous compounds that can be utilized by plants. This is mainly achieved by nitrogen-fixing bacteria that live in the soil or in symbiotic relationships with plants, such as Rhizobium species found in legume root nodules. Additionally, nitrogen fixation can occur due to lightning, which generates enough energy to break the strong triple bond in nitrogen gas, allowing it to react with oxygen to form nitrogen oxides (NOx) that dissolve in rainwater and form nitrate (NO3-).
2. Nitrification: This is the process of converting ammonia (NH3) or ammonium (NH4+) into nitrite (NO2-) and then into nitrate (NO3-). This occurs through two primary stages: first, ammonia-oxidizing bacteria, such as Nitrosomonas species, convert ammonia into nitrite. Then, nitrite-oxidizing bacteria, such as Nitrobacter species, convert nitrite into nitrate. Nitrates can be directly absorbed and used by plants as a nutrient for growth and reproduction.
3. Denitrification: This process is the conversion of nitrate (NO3-) back into nitrogen gas (N2) and is carried out by denitrifying bacteria, such as Pseudomonas species. These bacteria use nitrate as a terminal electron acceptor in their respiration process under low oxygen conditions, commonly found in water-logged soils or other oxygen-depleted environments. When bacteria convert nitrate into nitrogen gas, it is released back into the atmosphere, completing the nitrogen cycle.
Describe the three steps of the Nitrogen Cycle.
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