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The Nitrogen Cycle

The nitrogen cycle represents one of the most important nutrient cyclesThe cycling of a single element by various abiotic and biotic processes through the various stores found in the biosphere, lithosphere, hydrosphere, and atmosphere. found in terrestrial ecosystemsa community of organisms, including humans, interacting with one another and the environment in which they live.. Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acidsOrganic nitrogen containing acids which are used to construct proteins., proteinsOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds., and nucleic acidsIs an organic compound composed primarily of different combinations of carbon, hydrogen, nitrogen, oxygen, and phosphorus. They are very complex compounds being created by the atomic linking of thousands of individual atoms. DNA or deoxyribonucleic acid, the genetic blueprint of life, is an example of a nucleic acid.. The store of nitrogen found in the atmosphere, where it exists as a gas (mainly N2), plays an important role for life. This store is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include organic matterCompounds that contain carbon and hydrogen covalently bonded together in molecules; molecules from living matter. Organic wastes in sewage and runoff from lawns and farms in fresh waters can cause oxygen-depletion and degradation of water quality. in soil and the oceans. Nitrogen is the limiting nutrient for most plant growth. Most plants can only take up nitrogen in two solid forms: ammoniumChemical compound composed of nitrogen and hydrogen (NH4). Component of the nitrogen cycle. Product of organic matter decomposition. Can be fixed to clay minerals and later exchanged. ionAn atom, molecule or compound that carries either a positive (cation) or negative (anion) electrical charge. (NH4+) and the ion nitrateForm of nitrogen commonly found in the soil and used by plants for building amino acids, DNA and proteins. It is commonly produced by the chemical modification of nitrite by specialized bacteria. Chemical formula for nitrate is NO3-. (NO3 -). Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution. Ammonium is used less by plants for uptake because in large concentrationsThe amount of a component in a given area or volume. it is extremely toxicPoisonous, a substance that reacts with specific cellular components to kill cells.. Some algae, for example Lyngbya sp., can fixThe process by which atmospheric nitrogen is converted for use by an organism nitrogen themselves. Animals receive the required nitrogen they need for metabolism, growth, and reproduction by the consumption of living or dead organic matterCompounds that contain carbon and hydrogen covalently bonded together in molecules; molecules from living matter. Organic wastes in sewage and runoff from lawns and farms in fresh waters can cause oxygen-depletion and degradation of water quality. containing molecules composed partially of nitrogen. It is an important constituent of farm and domestic fertilizers.

A diagram illustrating the nitrogen cycle is shown at PhysicalGeography.net (The Nitrogen Cycle) also, nitrogen isotopes

Useful Links:
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Nitrogen Geochemistry

Mostly gas, Nitrogen (released by plants) constitutes more than 78 percent of the atmosphere. It is a vital element in many classes of compounds essential to living systems. Nitrogen is a primary nutrient for all green plants, but it must be modified before it can be readily utilized by most living systems.

Nitrogen fixation is one process by which molecular nitrogen is reduced to form ammonium. This complex process is carried out by nitrogen-fixing bacteria present in the soil. Some algae fix nitrogen themselves. The ammonium ion (the conjugate acid of ammonia, NH3) that is produced by this fixation is the form of nitrogen that is used by living systems in the synthesis of many bio-organic compounds.

Another way by which ammonium may be formed is by the process called nitrification. In this process compounds called nitrates and nitrites, released by decaying organic matter are converted to ammonium ions by nitrifying bacteria present in the soil. The process carried out by these bacteria is also a complex series of oxidation-reduction reactions.

Another way in which molecular nitrogen is modified is via the discharge of lightning. The energy released by the electrical discharges in our atmosphere breaks the rather strong bonds between nitrogen atoms, causing them to react with oxygen. Note in this process, nitrogen is oxidized and oxygen is reduced. Acids produced in the lightning reaction release hydrogen to form nitrate and nitrite ions which can readily be used by plants and micro organisms.

Denitrifying bacteria, act on ammonium as well as nitrates produced by death and decay, recycling these compounds as free nitrogen (N2). The nitrogen that is fixed by the processes described above is eventually returned to the atmosphere by this denitrification process, to complete what is commonly referred to as the "nitrogen cycle".

Man and Nitrogen

Graham Hancock writing in his 2002 book, "Underworld" on pages 175ff draws attention to Mergarh in Pakistan on the alluvial Kachi plains beside the Bolan river. On page 177, he writes about the first people, circa 9000 years ago, and describes them as being accomplished farmers and quotes the archeologist Gregory Possehl, that their use of pulses, annual legumes cultivated for their seed, are an especially interesting group of plants because they are able to fix atmospheric nitrogen in symbiosis with the bacterium Rhizobium found on their roots. They add nitrogen to the soil, rather than consume it, and if these plants are rotated and mixed with the food grains, higher yields are achieved through increased soil fertility.

Nitrogen also vents into the atmosphere from the burning of fossil fuels for power generation, for example.

Rainfall transports nitrogen from the atmosphere to the ground surface where most is taken up by growing plant cover.

Of course, when urban developments replace green areas with concrete and buildings the plant's take up of nitrogen is interrupted. However, stormwater run off conveys the nitrogen together with other contaminants collected from land surfaces to eventually deliver it into the groundwater.

Several other pathways allowing nitrogen to enter groundwater in addition to the rainfall are: decaying organic and vegetative deposits in lake systems and rivers, natural organic decay, domestic turf fertilization, golf courses, effluent and sludge disposal from sewage treatment facilities, septage spreading, drain fields of septic tanks, agricultural row crops, citrus, and animal husbandry (dairy, beef, pork, poultry).

It is important to note the travel time taken by depositions to the land surface and direct percolation into groundwater to reach surface water bodies via spring discharges.

Other factors to note in regard to nitrogen in groundwater spring discharges are:

  • Point source discharges from industrial or waste water treatment operations which present a steady stream of contaminants into water systems.
  • Unlike point sources non-point sources driven by rainfall are more diffuse and cannot be centrally collected or readily controlled.
  • Eutriphication is the acceleration of the natural aging process which degrades water quality due to increasing nutrient, and sedimentation loads (mostly due to nitrogen or phosphorus loading).

Water Quality Degradation

The source of the degradation is in plain sight and all around us. Kings Bay springs have had their water flow reduced by one third over the past few decades, from the 630 million gallons a day (mgd) , to the 410 mgd measured by a scientific study published in 2010. The result has been progressively to reduce flow rates across the water body and extend residence times for pollutants to magnify their impact upon biota of all kinds growing in the waters. This conclusion is averred to in a recent Florida Geographical Survey report, as due to drought and over pumping from the aquifer.

Unfortunately, as water flow rates have receded the chief polluting culprit - Nitrogen - has not. A few facts from scientific studies may help to understand the phenomenon.

The major source of Nitrogen is in the air we breathe which falls to ground in rainfall over Citrus County to the tune of some 1,150 tons a year on average. Traditionally most is absorbed by soils to be taken up by vegetation. However, replacing soils and vegetation with concrete and other impermeable surfaces interrupts the natural functions and waters run off to convey Nitrogen and other pollutants swept along with the flow into the groundwater source of spring outflows. Nevertheless, all but 80 or so tons of atmospheric Nitrogen are returned to the atmosphere as described above.

The next most prevalent source of Nitrogen in spring discharges is in turf fertilizers applied to domestic areas and golf courses in Citrus County which account for more than 300 tons per year of Nitrogen. This results from over application of fertilizers at levels greater than can be absorbed by the turf, allowing surpplus to percolate into ground waters in areas having poorly or un-confined aquifer systems.

The next most prevalent source of nitrogen delivered deiectly underground is from on site wastewater treatment systems (OWTS or septic systems). The amount reaching groundwaters from the totality of all the thousands of OWTS throughout Citrus County exceeds 212 tons of Nitrates a year. Whilst it is important to realize that correctly designed and installed OWTS are inherently efficient in preventing coliform and other potential dangers to human health from entering groundwater supplies. Residual nitrate depositions are finite and it is this aspect of an OWTS operation that contributes excess nitrogen as the cause of surface water degradation discussed above. The use of sewer utility systems are not without risk as transporting pipe work is subjected to serious stress especially in karstic areas having sharply varying near surface rock variations and variable groundwater levelsand tidal influences. This in addition the internal stresses created the passage of liquid plugs bounded by air. The same disastrous impact upon the protected waters from conveyance failures as those currently being reported in the Press regularly elsewhere in Florida. (Especially as the Crystal River sewage system was first installed in 1926).

Other sources of Nitrogen applied to the land surface from agriculture together with the application of sewage effluent to the land exceed 320 tons per year.

See also the following which focuses upon Sewer Systems.

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