Page 133 - Soil and water contamination, 2nd edition

120 Soil and Water Contamination

a C:N ratio of greater than 20 brings about a net immobilisation. Immobilised nitrogen is
temporarily unavailable for plant uptake. Ammonification, nitrification , denitrification , and
assimilation of nitrate and ammonium are the major process of the nitrogen cycle. In general,
nitrogen fixation and ammonium volatilisation are relatively insignificant in comparison
with the other nitrogen fluxes. Figure 6.2 is a schematic diagram of the entire N-cycle
including N-fixation, ammonification , nitrification, and denitrification.
Human activities have considerably enhanced the natural global nitrogen cycle in a
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number of ways. First, the combustion of fossil fuels is currently adding 20-30 Tg yr of
reactive N species to the atmosphere. Second, legume crops are responsible for an additional
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40 Tg yr of N being fixed naturally from the atmosphere. Third, the invention of the
2
Haber-Bosch chemical process in 1913 has enabled the artificial (i.e. non-biological) fixation
+
of atmospheric N to ammonium (NH ) for the production of artificial fertilisers. As a
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result, the global artificial nitrogen fixation rate has increased from about 5 Tg yr just after
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the second World War to about 80 Tg yr at present (Galloway et al., 1995; Vitousek et al.,
1997a; Galloway, 1998). This means that nowadays more nitrogen is fixed artificially by
humans than by natural fixation processes in terrestrial ecosystems (Vitousek et al., 1997b;
Crutzen and Steffen, 2003).
6.2.3 External sources and sinks
Figure 6.2 demonstrates that volatilisation of NH , NO, and N O are sinks in the N-cycle of
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ecosystems. Other sinks include the harvest of N-containing biomass and leaching or runoff
of dissolved nitrogen to downstream areas. Leaching occurs mainly in the form of nitrate ,
because, as mentioned above, nitrate is very mobile. In addition to nitrogen fixation by soil
bacteria, other important external sources of nitrogen sources are fertiliser inputs, inflow
of contaminated water from upstream areas, and atmospheric deposition . Fertiliser inputs
depend on initial soil fertility, crop type and intended crop yield, and other possible sinks
in the N-cycle. In intensive cropping systems, the N input by fertilisers may range from 50
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to more than 250 kg N ha y . Inputs by atmospheric deposition are derived from nitrogen
emissions by industry and traffic (mainly nitrogen oxides (NO )) and agriculture (mainly
x
ammonia (NH )). Current rates of atmospheric N deposition in Europe and North America
3
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have increased by a factor of 5–20 compared to pre-industrial times (25–100 kg N ha y
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compared to 5–10 kg N ha y ) (Hatch et al., 2002). In areas with intensive agriculture,
the dominant form of atmospheric N deposition is the deposition of ammonia-N. In 2001,
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the average total N deposition in the Netherlands was 32 kg N ha y of which about 70
percent was ammonia-N (RIVM, 2002). Locally, deposition rates may depart considerably
from these average values.
By far the greatest emission of ammonia (83 percent) occurs during the decomposition
of manure . The volatilisation of ammonia from manure is positively related to the pH ,
temperature , and moisture content of the manure. Once in the atmosphere, ammonia is
one of the most important gases that neutralise atmospheric nitric acid and sulphuric acid
originating respectively from the oxidation of nitrogen oxides (NO ) and sulphur dioxide
x
(SO ). This process causes the formation of ammonium aerosols ((NH ) SO and NH NO ).
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Under normal atmospheric conditions in Europe, sulphuric acid occurs in hydrated form,
which accelerates the neutralisation reaction considerably. The formation of the (NH ) SO
4 2 4
aerosol is irreversible. The formation of the NH NO is reversible and slows down with
4 3
increasing temperature. Both ammonia and ammonium aerosols may be dissolved in the
droplets of moisture in clouds.
Ammonia and its derivatives reach the Earth’s surface via different mechanisms:
• Dry deposition: removal of gases and aerosols from the atmosphere without the
interference of precipitation (rain, snow, hail).

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