Geoscience Reference
In-Depth Information
which is the final product of the nitrification process. Conversion of ammonia to ni-
trate is an important part of the Nitrogen Cycle in nature and
Figure 46
depicts a sim-
plified version. Nitrogen, the main gaseous component of the earth's atmosphere, can
be combined or fixed by various microorganisms, initially forming ammonia which
may then be incorporated into organic compounds in other organisms. The role of
Rhizobium
bacteria in root nodules was mentioned in
chapter 3
, but another important
group are the blue-green Cyanobacteria. These may be present in rice paddy fields in
vast numbers and may fix all the nitrogen required by the crop. This is an assimilation
process. Complex organic nitrogen substances, present in plants, animals or other or-
ganisms, in turn can be broken down or mineralized by microorganisms into simpler
substances including ammonia; the latter is then available to be converted into nitrate
by nitrifying bacteria. Nitrates are taken up by plants and assimilated into complex ni-
trogen compounds again. Many microorganisms can also reduce nitrate to nitrite and
then to ammonia; this is the reverse of the nitrification process. There are yet other
microorganisms which decompose nitrate to form gaseous nitrogen, a process termed
denitrification, to be described later, and so completing the cycle of nitrogen trans-
formations.
Nitrifying bacteria are strictly aerobic organisms and require free gaseous oxy-
gen for their respiration. They also need carbon dioxide to supply carbon and smaller
amounts of a few simple salts to provide phosphorus, iron, sulphur and trace elements,
e.g. perhaps copper. Nitrification occurs in nearly all soils. It proceeds best at a neutral
reaction, although nitrifying bacteria grow quite slowly and reproduce only once or
twice a day even under the optimum nutrient and temperature conditions possible in
a laboratory. In soil, where conditions are far from optimal, growth is much slower.
Ammonia that is released slowly
in situ
by the gradual breakdown of organic matter,
is converted into nitrate, which is taken up by plants. The size of populations of ni-
trifying bacteria is largely determined by the quantity of available ammonia and the
presence of oxygen and carbon dioxide. Consequently, nitrifying bacteria are gener-
ally to be found in the top two or three inches of soil. The application of ammonium
sulphate to soil as a fertilizer may cause some acidification of the soil due to liberation
of sulphuric acid and formation of nitric acid, but the extent of this depends on the
buffering or ion exchange capacity of the soil as well as the presence of lime. Nitri-
fication also takes place in the sea and other bodies of water, and especially in sewage
and ammonia-containing effluents.
Nitrates, which are readily soluble in water, are not adsorbed on the soil clay
minerals and so are easily washed out of soil by rain. Inorganic cations, including the
ammonium cation (the form adopted by ammonia when dissolved in water) are ad-
sorbed on clay minerals, and thus are not subject to leaching. To prevent nitrogen loss,
