Ecology: the Biology of Interactions. 2.07. The Biogeochemical Cycle of Nitrogen
The BGC cycle of nitrogen is more complex than that of carbon. It is also extremely important for living organisms. Although the atmosphere contains more nitrogen than other elements, incorporating it into living matter is a much more difficult task than fixing carbon during photosynthesis. Nitrogen is most available to plants in the form...
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2.06. Biogeochemical Cycle of Carbon
D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 2. Biospherology
2.08. Biogeochemical Cycle of Sulfur
2.07. Biogeochemical Nitrogen Cycle The biogeochemical cycle of nitrogen (Fig. 2.7.1) is more complex than that of carbon. It is also extremely important for living organisms. Although there is more nitrogen in the atmosphere than other gases, its incorporation into living matter is a much more complex task than carbon fixation during photosynthesis. The most accessible forms of nitrogen for plants are ammonia and nitrates, but ammonia in large quantities is toxic, while nitrates are not. The forms in which nitrogen is used in organic compounds are reduced, so the assimilation of ammonia requires fewer transformations. Both forms are easily leached from soils, especially nitrates, because in neutral and alkaline conditions, ammonium binds to some clay substances. During the decomposition of detritus, reduced nitrogen is released. Urea is also hydrolyzed to ammonia by soil bacteria. Nitrification is carried out by bacteria such as Nitrosomonas, which convert ammonium to nitrite; conversely, bacteria such as Nitrobacter convert nitrites to nitrates. Fig. 2.7.1. Global Nitrogen Cycle Nitrite is a common intermediate stage in transitions from reduced to oxidized forms and back. An excess of nitrates in food is a dangerous consequence of excessive nitrogen fertilization of soils. During assimilation, nitrates are reduced, passing through the stage of toxic nitrites. Denitrification is a multi-step process that proceeds through nitrite and nitrous oxide (N2O) to molecular nitrogen. The bacterium Pseudomonas obtains the oxygen it needs through this process if there is no oxygen in the soil! Denitrification can also occur without the involvement of living organisms. Nitrogen fixers include free-living bacteria Azotobacter (aerobe) and Clostridium (anaerobe), legume symbionts bacteria Rhizobium, symbionts of many plant groups (e.g., alder) actinomycetes, and cyanobacteria Anabaena and Nostoc. The enzyme that fixes nitrogen is nitrogenase. Its work requires high energy expenditure: about 10 g of glucose per 1 g of fixed nitrogen. Nitrogen can also be fixed abiotically (outside of connection with organisms). For example, during a lightning strike, the temperature in its channel reaches such a level that atmospheric nitrogen is oxidized by oxygen in the air through which the electric discharge has passed. Since agricultural plants experience nitrogen starvation in many soils, humans intensively produce nitrogen fertilizers, carrying out industrial fixation of atmospheric nitrogen. Industrial nitrogen fixation is approximately equal to natural abiotic fixation. Ukrainian / Russian
2.06. Biogeochemical Cycle of Carbon
D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 2. Biospherology
2.08. Biogeochemical Cycle of Sulfur