Nitrogen in the environment

  • Nitrogen is an essential component of many organic molecules such as DNA, RNA and proteins, the building blocks of life.  Air is the major reservoir of nitrogen that constitutes 79% of nitrogen gas (N2). Although the majority of the air we breathe is N2, most of this is unavailable for use. This is because of the strong triple bond between the N atoms in the N2 molecules that make it relatively inert. Therefore, in order for the plants and animals to use nitrogen, N2 gas must be converted to either ammonium (NH4+) or nitrate (NO3-) or organic nitrogen such as urea - (NH3)2CO.
  • Nitrogen (N), a macronutrient, is most frequently found limiting for plant growth. This is due to the continual loss of nitrogen from the reserve of combined or fixed nitrogen, which is present in soil and available for use by plants. It is continually depleted by such processes as microbial denitrification, soil erosion, leaching, chemical volatilization, and most important, removal of nitrogen-containing crop residues from the land. The nitrogen reserve in agricultural soils must therefore be replenished periodically in order to maintain an adequate (non-growth limiting) level for crop production. This replacement of soil nitrogen is generally accomplished by the addition of chemically fixed nitrogen in the form of commercial inorganic fertilizers or by the activity of biological nitrogen fixation (BNF) systems.
  • Nitrogen is a versatile element that exists in both organic and inorganic forms as well as in many different oxidation states. The movement of nitrogen between the atmosphere, biosphere, and geosphere is described in the nitrogen cycle (Figure 1), one of the major biogeochemical cycles.

Nitrogen cycle

  • Five main processes cycle nitrogen through the biosphere, atmosphere and geosphere: nitrogen fixation (N2 -> NH4+), nitrogen uptake (NH4+ -> Organic N), nitrogen mineralization (Organic N -> NH4+), nitrification (NH4+ -> NO3-)  and denitrification (NO3 -> NO2 -> NO -> N2O -> N2).
  • Nitrogen fixation: Three processes are responsible for most of the nitrogen fixation in the biosphere:

Atmospheric Fixation
The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
Industrial Fixation (Haber-Bosch process)
Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of it is further processed to urea and ammonium nitrate (NH4NO3).
Biological Nitrogen Fixation (BNF) and its importance
Nitrogen fixing bacteria such as Rhizobium form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g. beans, peas, and clover). In this relationship, nitrogen fixing bacteria inhabit legume root nodules  and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix.

  • The most important contribution to BNF comes from the symbiotic association of certain micro-organisms with the roots of higher plants. A classic example is that of the bacteria (Rhizobium) which characteristically infect the roots of leguminous plants (e.g., bean, soybean, clover, and peanut) with a high degree of host specificity. Small nodules are formed on the roots and these become filled with an altered form of the bacteria (bacteroides) which fix appreciable amounts of nitrogen. This symbiosis alone accounts for 20% of global biological nitrogen fixed annually. The legumes represent a major direct source of food for man and forage for livestock and therefore represent a critical contribution to world food production.

Human impact on the N cycle and its effect on the environment

  • Humans are currently fixing as much N2 as the biosphere through the Haber-Bosch process for fertilizer production. Because of this, the nitrogen emitted extensively by industrial companies has increased the nitrate and nitrite supplies in soil and water as a consequence of reactions that take place in the nitrogen cycle.
  • Sewage dumping and agricultural practices that cause soil erosion and fertilizer runoff have increased the N input to the oceans. Anthropogenic N flux can lead to eutrophication which in turn promotes algal blooms (some of these species are toxic); bacterial decomposition of the decaying biomass results in depleted O2; and both these effects, have tremendous impact on fish and invertebrates in the system.
  • Biomass and fossil fuel burning produce nitrogen oxides that take part in ozone chemistry.

Effects of nitrogen on human health

    • Reactive nitrogen (like NO3- and NH4+) present in surface waters and soils can also enter the atmosphere as the smog-component nitric oxide (NO). NO is also a major factor in the formation of smog, which is known to cause respiratory illnesses like asthma in both children and adults.
    • Excess nitrate in the soil can leach into the groundwater supplies and contaminate wells. This nitrate is therefore a potential human health threat especially to infants, causing the condition known as methemoglobinemia, also called "blue baby syndrome". Nitrate is converted in the gut to nitrite, which then combines with hemoglobin to form methemoglobin, thus decreasing the ability of the blood to carry oxygen. Infants are more susceptible to nitrate toxicity than older children or adults.


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