References

160 L. Philippot and J.C. Germon
2
Nitrogen Transformations in the Soil
With 79% of gaseous N2 and a total mass of 3.8 × 10 21 g of nitrogen atoms,
the atmosphere is the largest reservoir of nitrogen. However, this N2 is
metabolically unavailable to the higher plants which do not have the ability
to break the triple covalent bond of N2. Nitrogen from the air enters the
nitrogen cycle through the action of several unique types of microorganisms
that can convert N2 gas to inorganic forms available to plants. This
conversion of molecular nitrogen into the available form is known as nitrogen
fixation. Nitrogen is the nutrient element most frequently found
to be limited to plants because of its continual loss by denitrification, soil
erosion, leaching, chemical volatilization, etc. Therefore, biological nitrogen
fixation (BNF) is important in agriculture because it provides a source
of fixed nitrogen for the growth of plants that does not require fossil fuels
for production. Biological nitrogen fixation is mainly performed by
microorganisms called diazotrophs and can be represented by the following
equation, in which 2 moles of ammonia are produced from 1 mole of
nitrogen gas:
N2 +8H + +8e − +16ATP → 2NH3 + H2 +16ADP +16Pi .
The product of nitrogen fixation is generally used directly by free nitrogenfixing
bacteria or exported to the plants by symbiotic nitrogen-fixing bacteria.
Exported nitrogen is used by plants to synthesize organic nitrogen
compounds. These organic nitrogen compounds are used by animals as
a nitrogen source. After the death of an organism (plant, animal, fungi,
bacteria etc.) organic nitrogen, i.e., proteins, amino acids and nucleic acids,
is degraded by microorganisms into ammonia. This decomposition of dead
organic matter by saprophytic microorganisms is termed ammonification
or mineralization. Urea and uric acid excreted by living animals are also
mineralized by microorganisms in the soil. Once ammonia is produced, it
can be (1) fixed by clays or by soil organic matter, (2) volatilized as NH3,
(3) assimilated by plants and microorganisms or finally (4) converted to
nitrate by highly specialized bacteria during a two-step process: nitrification.
In the first step of nitrification, ammonia is oxidized into nitrite via
NH2OH:
NH3 + O2 +2H + +2e − → NH2OH + H2O → NO − 2 +5H+ +4e − .
Inthesecondstep,nitriteisoxidizedintonitratewithoutdetectableintermediates:
NO − 2 + H2O → NO − 3 +2H+ +2e − .