Nitrogen Cycle Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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The nitrogen ( N ) cycle describes the transformation of nitrogen into its various organic and inorganic forms and its movement between different deposits in the atmosphere, soils, vegetation, and living organisms of the micro and macro fauna. The nitrogen cycle is one of the most important nutrient cycles within the world’s ecosystems, since nitrogen is indispensable for the constitution of living organisms, which synthesize proteins, peptides, amino, and nucleic acids from nitrogen. However, the nitrogen cycle has to be managed carefully, since abundance of nitrogenous forms like nitrates in water or food can cause adverse effects on human health. Reactive nitrogenous gases in the atmosphere can contribute to climate warming and the destruction of the ozone layer. The terrestrial nitrogen balance of the nitrogen cycle with its major components can be described by the following equation:

dN/dt = N(R) + N(F) + N(bio) + N(H) + N(M)

N(P) N(L) N(D) ± N(Er) N(fix).

t = time

N(R) = nitrogen content in precipitation

N(C) = nitrogen input through combustion

N(bio) = nitrogen fixed by the nodules of leguminous plants and autotrophic bacteria

N(F) = nitrogen in organic and inorganic fertilizers

N(M) = mineralized nitrogen from soils

N(P) = nitrogen uptake by plants

N(Er) = nitrogen inand out-flow by erosion

N(L) = leached nitrogen

N(D) = gaseous losses of nitrogen through denitrification

N(fix) = nitrogen fixed in clay minerals

Nitrogen (N) constitutes 78 percent of the atmosphere, and the whole atmospheric store is about a million times larger than all other N stocks. Atmospheric nitrogen inputs into the nitrogen cycle enter the soil-plant-water system through electrical processes, fires, combustion, and precipitation, processes by which molecular nitrogen, N2 , is combined with H2 or O2 . The main natural nitrogen input from atmosphere to soil is induced by biological nitrogen fixation, which refers to the fixation and incorporation of atmospheric N2 by the nodules of leguminous plants through a symbiosis with bacteria of the species rhizobium, as well as by nonsymbiotic fixation by autotrophic microorganisms as blue-green algae.

Nitrogen mineralization is the transformation of organic nitrogen mainly located in the topsoil into inorganic forms as ammonium NH4+ through the process of ammonification, frequently followed by the subsequent process of nitrification, which means the oxidation of the ammonium ions into nitrate, NO3-. The processes are mainly performed by species of Nitrosomonas and Nitrobacter and its kinetics are mainly dependent on the soil temperature.

The reverse process is immobilization, which refers to the transformation and subsequent incorporation of inorganic nitrogen forms into proteins, peptides, and amino acids of microor macroorganisms in the soil fauna. Mineralization is also frequently considered as the resulting net rate of all these processes. The highest mineralization rates in the history of soil ecosystems have been created by large-scale transformations of virgin land into arable land due to population pressure. In general this releases about 50 percent of the nitrogen that has been organically bound in soils.

Transport of nitrogen-containing soil matter through erosion can add to or reduce the amount of nitrogen cycling in a system. Further reduction of nitrogen in soils occurs through the fixation of ammonium in a biologically unavailable form in certain expanding clay minerals and, in higher quantities, through nitrogen uptake by plants. In natural ecosystems, the nitrogen uptake by plants is adjusted to the mineralization rate, which is the same if fertilizers are added to soils in combination with agricultural crops. If nitrogen in the soil solution is below the demands of the plant cover, the plants will be undernourished; if it exceeds the demands of the plants, it will either be leached into the groundwater or be denitrified. It can also be enriched in the plant solution.

All nitrogen that is not absorbed by sinks as vegetation, soils, and microand macro-faunae is available for leaching into groundwaters. From there, dependent on physical conditions, it flows into adjacent aquifers that determine the general water flow in soils. Denitrification involves the metabolicreduction of nitrate (NO3-) into nitrogen (N2) or nitrous oxide (N2O) gas.Both of these gases then diffuse into the atmosphere. Soluble carbon is used as an energy source; therefore the denitrification rate is limited by its amount and favored by anaerobic conditions. Denitrification can take place in every part of the soil profile where these conditions prevail and also in groundwater. Similar processes occur in marine ecosystems; however, there are still uncertainties about them, and estimates of N fixation by organisms vary, ranging from less than 30 to more than 300 teragrams (million tons) per year.

Human activities have interfered with the natural nitrogen cycle, which was in a dynamic equilibrium in preindustrial times. Then, annual fluxes of nitrogen from the atmosphere to the land and aquatic ecosystems were 90-130 teragrams per year. This was balanced by reverse denitrification, since the C and N ratio necessary for denitrification was also in equilibrium because almost all inputs were from organic origins.

Industrial combustion increases the emission of reactive N gases (NOx) to the atmosphere, where they contribute to the production of tropospheric ozone before depositing either as a gas in the form of nitrate or ammonia ions, a nitric acid dissolved in precipitation, or as dry aerosols on land or sea.

Production and use of synthetic nitrogen fertilizer, produced by the Haber-Bosch process, together with expanded planting of nitrogen fixing crops and the deposition of nitrogen-containing air pollutants, have created an additional flux of about 200 teragrams a year, only part of which is denitrified. The addition of a major new flux from atmosphere to land, by way of industrial and crop nitrogen fixation, has created an imbalance leading to increased flows to the ocean. In the process this contributes to eutrophication of rivers and lakes. Some of the nitrogen oversupply leads to increased emissions of N2O and NOx , which are increasing in the atmosphere and contributing to global warming, tropospheric pollution, and stratospheric ozone depletion.

Bibliography:

N. Galloway, “The Global Nitrogen Cycle: Changes and Consequences,” Environmental Pollution (v.102/S1, 1998);
Hans Jenny, The Soil Resource: Origin and Behaviour (Springer, 1980);
W.V. Reid et , Millennium Ecosystem Assessment: Ecosystems and Human Well-Being (World Resources Institute, 2005);
L. Tisdayle et al., Soil Fertility and Fertilizers (Macmillan, 1997).