soil - Lublin

ASSIMILATORY REDUCTION OF NITRATE
In nitrate assimilation, the first step is the reduction to nitrite, which is accomplished
by the enzyme nitrate reductase. Subsequently, the nitrite is reduced to hydroxylamine
by the enzyme nitrite reductase to finally be reduced to ammonia (Payne,
1973). The net reaction is shown in the following equation:
NO 3
-
NO 2
-
[H N O ]
2 2 2 [NH OH] 2 NH 3
NO 2
where N 2 O rather than N 2 may be produced as a by-product from the indicated intermediate
(hyponitrite). The reaction shown is essentially the same as that which occurs
during NO 3 - reduction to NH 4 + and involves the same precursor of N 2 O, again probably
hyponitrite (Freney et al., 1979; Mosier et al., 1983).
DISSIMILATORY REDUCTION OF NITRATE
NO(+II)
nitrate reductase nitrite reductase nitric oxide reductase nitrous oxide reductase
When the dissimilative reduction produces the gaseous dinitrogen or nitrous oxide
compounds, the process is termed denitrification.
Biological denitrification is the last step in the N-cycle, where N is returned to
the atmospheric pool of N 2 . It is an anaerobic process. Biological denitrification is a
respiratory process in which N-oxides (electron acceptors) are enzymatically reduced
under anaerobic conditions to nitrous oxide and dinitrogen for ATP production by
organisms that normally use O 2 for respiration. The process of denitrification (including
rhizobial denitrification) can be presented as follows: Conditions influencing ni-
NO
3(+V)
NO
2
(+III) [X] N2O(+I)
N(0)
2
trous oxide production in soils
OXYGEN The oxygen status in soil, which is inversely, proportional to the amount of
soil moisture, appears in many studies to be one of the key factors influencing nitrous
oxide production (McKenney et al., 2001), (Fig. 1. Włodarczyk, 2000).
ORGANIC MATTER AVAILABILITY Denitrification is a respiratory process, which
requires an easily oxidisable organic substrate. The presence of readily metabolized
organic matter and the availability of water soluble organic matter are closely correlated
with the rate of biological denitrification and hence with the potential production
of N2O from soil. There is observed very high correlation between N2O emission and
organic matter content (Fig. 2 Włodarczyk, 2000).
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