The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. This cycle includes gaseous components. The earth’s atmosphere contains approximately 78.08 per cent nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; it is crucial for any life on earth. Nitrogen is in all amino acids, is incorporated into proteins and is present in the bases that make up nucleic acids, such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules, which are essential for photosynthesis and further growth.
Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most of the fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenous enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds. Some nitrogen-fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutual relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. Nowadays, a very considerable portion of nitrogen is fixated in ammonia chemical plants. Other plants get nitrogen from the soil in the form of either nitrate ions or ammonium ions by absorption through their roots. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.
Due to their very high solubility, nitrates can enter groundwater. Elevated levels of nitrates in groundwater are a concern for drinking water use because nitrates can interfere with blood-oxygen levels in infants and cause blue-baby syndrome. Where groundwater recharges streamflow, nitrate-enriched groundwater can contribute to a process leading to high algal populations, especially blue-green algal populations, and the death of aquatic life due to excessive demand for oxygen. Although nitrates are not directly toxic to fish life like ammonia, they can have indirect effects on fish if they contribute to eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies and hence its application as fertilizer is being controlled. This is occurring along the same lines as control of phosphorus fertilizer, the restriction of which is normally considered essential for the recovery of eutrophied water bodies.
Ammonia is highly toxic to fish, and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored. To prevent loss of fish, nitrification prior to discharge is often desirable. Land application can be an attractive alternative to the mechanical aeration needed for nitrification.
During anaerobic or low-oxygen conditions, denitrification by bacteria occurs. This results in nitrates being converted to nitrogen gases (NO, N2O, N2) and returned to the atmosphere. Nitrate can also be reduced to nitrite and subsequently combine with ammonium in the anammox process, which also results in the production of dinitrogen gas.
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