Page 131 - Soil and water contamination, 2nd edition

118 Soil and Water Contamination

groundwater, or surface water until it is taken up by biota or transformed through redox
reactions (see next section).
Although nitrogen is a key nutrient, inorganic aqueous nitrogen, like any substance,
is toxic to plants, animals, and humans at high concentrations. Elevated levels of nitrate
or nitrite in drinking water have been known to cause a potentially fatal blood disorder in
infants below six months of age, called methemoglobinemia or ‘blue-baby syndrome ’, in
which the oxygen-carrying capacity of blood is reduced. High concentrations of ammonium
in the soil solution are toxic to plants. Furthermore, free ammonia formed from ammonium
under basic conditions in surface water is highly toxic to fish. The role of nitrogen as a key
factor in the process of eutrophication and the development of algal blooms was discussed
above.

6.2.2 Nitrogen cycle
Primary producers and some bacteria absorb nitrogen mainly in the form of aqueous nitrate
or ammonium . Because ammonium is essentially toxic inside cells, it is assimilated into
organic nitrogen very quickly. Some plants (papilionaceous legumes, such as lupine, lucerne,
and clover, and some other plants) host Rhizobium bacteria or cyanobacteria , which are able
to fix nitrogen (N ) from the atmosphere. This process is known as nitrogen fixation . For
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this reason, legumes are commonly included in agricultural crop rotations on poorly fertile
soils to increase the amount of soil nitrogen and to reduce the need for fertiliser applications.
Organic nitrogen is made available by the decomposition and subsequent mineralisation
of organic matter by heterotrophic bacteria (i.e. bacteria that derive their cell carbon from
organic carbon ) and fungi. Organic nitrogen is first oxidised to amino acids and finally to
ammonium (ammonification ).
Under oxic conditions, autotrophic bacteria oxidise ammonium (3- oxidation state ) to
nitrate (5+ oxidation state ) in a multiple-step nitrification process (see Figure 6.2). The
overall nitrification process can be summarised as a two-step process:

2 NH + + 3 O 2 NO + 4 H + + 2 H O (6.2a)
4 2 2 2
(6.2b)

Nitrosomonas bacteria carry out the first step, the oxidation of ammonium to nitrite, and
Nitrobacter bacteria carry out the second step, the final oxidation to nitrate . Nitrite is
very reactive, and the second reaction step (Equation 6.2b) proceeds very fast. Therefore,
nitrite generally only occurs in small amounts in soil and the primary nitrogen species in
aqueous solutions are nitrate and ammonium. The nitrification reaction requires oxygen ,
so the reaction only takes place in well-aerated soils or surface waters. Furthermore, because
the nitrification reaction is carried out by live bacteria, the reaction rate depends greatly
on environmental factors such as temperature and pH . At temperatures below 10 °C
the reaction is inhibited. Between 10 °C and about 32 °C the reaction rate increases with
temperature. The optimal pH for nitrification is between 6.6 and 8. The reaction is slowed at
pH less than 6 and comes to a standstill at pH less than 4.5.
Under slightly reducing conditions, for example due to the biologically mediated
decomposition of organic matter (see Section 4.3.4) or pyrite oxidation (see Section 5.11),
the process of denitrification breaks down nitrate (5+ oxidation state ) to N (0 oxidation
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state ) through intermediates including nitrite, nitric oxide , and nitrous oxide :

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