Submitted by, SREEJITH P S4 EEE ROLL NO:- 54 NITROGEN CYCLE Submitted by, SREEJITH P S4 EEE ROLL NO:- 54

CONTENTS Introduction Processes of the nitrogen cycle Human influences on the nitrogen cycle References

INTRODUCTION Earth's atmosphere is about 78% nitrogen. Nitrogen is essential for many biological processes and is crucial for any life on Earth. It 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.

Nitrogen cycle The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a gaseous cycle.

Processes of the nitrogen cycle Conversion of N2 Assimilation Ammonification Nitrification Denitrification Anaerobic ammonium oxidation

CONVERSION OF N2 The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals and humans is an important step in the nitrogen cycle, that determines the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms: 1.Biological fixation 2.Industrial N-fixation 3.Combustion of fossil fuels  4.Other processes

1. Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen and assimilate it as organic nitrogen. An example of nitrogen fixing bacteria is the Rhizobium bacteria, which live in legume root nodules. An example of the free-living bacteria is Azotobacter. 2.Industrial N-fixation : in the Haber-Bosch process, N2 is converted together with hydrogen gas (H2) into ammonia (NH3) which is used to make fertilizer and explosives.

3.Combustion of fossil fuels : automobile engines and thermal power plants, which release various nitrogen oxides (NOx). 4.Other processes : Additionally, the formation of NO from N2 and O2 due to photons and especially lightning, are important for atmospheric chemistry, but not for terrestrial or aquatic nitrogen turnover.

Assimilation Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants which have a mutualistic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. Animals, fungi, and other heterotrophic organisms absorb nitrogen as amino acids, nucleotides and other small organic molecules.

Ammonification When a plant or animal dies, or an animal excretes, the initial form of nitrogen is organic. Bacteria, or in some cases, fungi, converts the organic nitrogen within the remains back into ammonia, a process called ammonification or mineralization.

Nitrification The conversion of ammonia to nitrates is performed primarily by soil-living bacteria and other nitrifying bacteria. The primary stage of nitrification, the oxidation of ammonia (NH3) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-).

Denitrification Denitrification is the reduction of nitrites back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.

Anaerobic ammonium oxidation In this biological process, nitrite and ammonium are converted directly into dinitrogen gas. This process makes up a major proportion of dinitrogen conversion in the oceans.

SCHEMATIC REPRESENTATION OF THE FLOW OF NITROGEN THROUGH THE ENVIRONMENT.

Human influences 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. In addition, humans have significantly contributed to the transfer of nitrogen trace gases from Earth to the atmosphere, and from the land to aquatic systems. N2O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources.

N2O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. Ammonia (NH3) in the atmosphere has tripled as the result of human activities. It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging on to water droplets, eventually resulting in acid rain. Fossil fuel combustion has contributed to a 6 or 7 fold increase in NOx flux to the atmosphere.

NOx actively alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems. Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants. Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species diverse heathlands.

Reference http://en.wikipedia.org/wiki/nitrogen cycle http://www.ucsusa.org/clean_energy/nitrogenbasics/ http://www.aquariumdomain.com/guideTheNitrogenCycle.asp

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