Nitrogen cycle. The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. Lecture Nitrogen Cycle. Sources: Atlas and Bartha, Chapter Brock Biology of Microorganisms, general reference. Fenchel et al. Bacterial. The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple Print/export. Create a book · Download as PDF · Printable version.
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Since the mids, humans have been exerting an ever-increasing impact on the global nitrogen cycle. Human activities, such as. Nitrogen Cycle. Sources. Lightning; Inorganic fertilizers; Nitrogen Fixation; Animal Residues; Crop residues; Organic fertilizers. Forms of Nitrogen. PDF | The nitrogen cycle is the shift between different chemical forms of nitrogen through biologic, physical, and geologic processes on Earth.
For example, Azospirillum species have been shown to fix nitrogen when growing in the root zone rhizosphere or tropical grasses, and even of maize plants in field conditions.
Similarly, Azotobacter species can fix nitrogen in the rhizosphere of several plants. In both cases the bacteria grow at the expense of sugars and other nutrients that leak from the roots.
However, these bacteria can make only a small contribution to the nitrogen nutrition of the plant, because nitrogen-fixation is an energy-expensive process, and large amounts of organic nutrients are not continuously available to microbes in the rhizosphere.
This limitation may not apply to the bacteria that live in root nodules or other intimate symbiotic associations with plants. The nitrogen cycle The diagram below shows an overview of the nitrogen cycle in soil or aquatic environments.
17.2B: Nitrogen Cycle
At any one time a large proportion of the total fixed nitrogen will be locked up in the biomass or in the dead remains of organisms shown collectively as "organic matter". So, the only nitrogen available to support new growth will be that which is supplied by nitrogen fixation from the atmosphere pathway 6 in the diagram or by the release of ammonium or simple organic nitrogen compounds through the decomposition of organic matter pathway 2.
Some of other stages in this cycle are mediated by specialised groups of microorganisms and are explained below. Nitrification The term nitrification refers to the conversion of ammonium to nitrate pathway This is brought about by the nitrifying bacteria, which are specialised to gain their energy by oxidising ammonium, while using CO2 as their source of carbon to synthesise organic compounds.
Organisms of this sort are termed chemoautotrophs - they gain their energy by chemical oxidations chemo- and they are autotrophs self-feeders because they do not depend on pre-formed organic matter. In principle the oxidation of ammonium by these bacteria is no different from the way in which humans gain energy by oxidising sugars. Their use of CO2 to produce organic matter is no different in principle from the behaviour of plants.
The nitrifying bacteria are found in most soils and waters of moderate pH, but are not active in highly acidic soils.
They almost always are found as mixed-species communities termed consortia because some of them - e. Nitrosomonas species - are specialised to convert ammonium to nitrite NO2- while others - e. Nitrobacter species - convert nitrite to nitrate NO In fact, the accumulation of nitrite inhibits Nitrosomonas, so it depends on Nitrobacter to convert this to nitrate, whereas Nitrobacter depends on Nitrosomonas to generate nitrite.
The nitrifying bacteria have some important environmental consequences, because they are so common that most of the ammonium in oxygenated soil or natural waters is readily converted to nitrate.
Most plants and microorganisms can take up either nitrate or ammonium arrows marked "1" in the diagram. However, process of nitrification has some undesirable consequences. In contrast, the negatively charged nitrate ion is not held on soil particles and so can be washed down the soil profile - the process termed leaching arrow marked 7 in the diagram.
In this way, valuable nitrogen can be lost from the soil, reducing the soil fertility.
The nitrates can then accumulate in groundwater, and ultimately in drinking water. There are strict regulations governing the amount of nitrate that can be present in drinking water, because nitrates can be reduced to highly reactive nitrites by microorganisms in the anaerobic conditions of the gut. In addition, both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota , that convert ammonia to nitrites.
They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle. Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification - converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.
Denitrification The three processes above remove nitrogen from the atmosphere and pass it through ecosystems. Denitrification reduces nitrates and nitrites to nitrogen gas, thus replenishing the atmosphere.
They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
All of these processes participate in closing the nitrogen cycle. Are the denitrifiers keeping up? Agriculture may now be responsible for one-half of the nitrogen fixation on earth through the use of fertilizers produced by industrial fixation and the the growing of legumes like soybeans and alfalfa.
This is a remarkable influence on a natural cycle.
Are the denitrifiers keeping up the nitrogen cycle in balance? Probably not. Certainly, there are examples of nitrogen enrichment in ecosystems. One troubling example: the "blooms" of algae in lakes and rivers as nitrogen fertilizers leach from the soil of adjacent farms and lawns.
The sinking results in ammonia being introduced at lower depths below the euphotic zone. Bacteria are able to convert ammonia to nitrite and nitrate but they are inhibited by light so this must occur below the euphotic zone. Nitrification can then occur to convert the ammonium to nitrite and nitrate.
N2 can be returned to the atmosphere through denitrification. Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve a redox reaction and therefore requires little energy. Nitrate requires a redox reaction for assimilation but is more abundant so most phytoplankton have adapted to have the enzymes necessary to undertake this reduction nitrate reductase.
There are a few notable and well-known exceptions that include Prochlorococcus and some Synechococcus. The nutrients in the ocean are not uniformly distributed. Areas of upwelling provide supplies of nitrogen from below the euphotic zone. Coastal zones provide nitrogen from runoff and upwelling occurs readily along the coast.
However, the rate at which nitrogen can be taken up by phytoplankton is decreased in oligotrophic waters year-round and temperate water in the summer resulting in lower primary production. Nitrate is depleted in near-surface water except in upwelling regions. Coastal upwelling regions usually have high nitrate and chlorophyll levels as a result of the increased production.
However, there are regions of high surface nitrate but low chlorophyll that are referred to as HNLC high nitrogen, low chlorophyll regions. The best explanation for HNLC regions relates to iron scarcity in the ocean, which may play an important part in ocean dynamics and nutrient cycles. The input of iron varies by region and is delivered to the ocean by dust from dust storms and leached out of rocks.
Iron is under consideration as the true limiting element to ecosystem productivity in the ocean. Ammonium and nitrite show a maximum concentration at 50—80 m lower end of the euphotic zone with decreasing concentration below that depth. This distribution can be accounted for by the fact that nitrite and ammonium are intermediate species.
They are both rapidly produced and consumed through the water column. It can be produced during nitrate assimilation, nitrification, and denitrification; however, it is immediately consumed again. New vs. Outside sources are upwelling from deep water and nitrogen fixation. New production is an important component of the marine environment. One reason is that only continual input of new nitrogen can determine the total capacity of the ocean to produce a sustainable fish harvest.
This will have a negative effect on the system. However, if fish are harvested from areas of new nitrogen the nitrogen will be replenished. Human influences on the nitrogen cycle Main article: Human impact on the nitrogen cycle As a result of extensive cultivation of legumes particularly soy , alfalfa , and clover , growing use of the Haber—Bosch process in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into biologically available forms.
Human alterations to the global nitrogen cycle are most intense in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest. Nitrous oxide is also a greenhouse gas and is currently the third largest contributor to global warming , after carbon dioxide and methane.
While not as abundant in the atmosphere as carbon dioxide, it is, for an equivalent mass, nearly times more potent in its ability to warm the planet. It is a reactant in the atmosphere, where it acts as an aerosol , decreasing air quality and clinging to water droplets, eventually resulting in nitric acid H NO3 that produces acid rain.
Atmospheric ammonia and nitric acid also damage respiratory systems.The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas, which is inert and unavailable to plants. It can be produced during nitrate assimilation, nitrification, and denitrification; however, it is immediately consumed again.
Sources, Recent Trends, and Damage Costs". Since groundwater often serves as the primary domestic water supply, nitrate pollution can be extended from groundwater to surface and drinking water in the process of potable water production, especially for small community water supplies, where poorly regulated and unsanitary waters are used.
Fossil fuel combustion is a primary contributor, but so are biofuels and even the burning of hydrogen. Human alterations to the global nitrogen cycle are most intense in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest.
They also form symbiotic associations with other organisms such as the water fern Azolla, and cycads.
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