References

Contribution of Bacteria to Initial Input and Cycling of Nitrogen in Soils 161
The conversion by nitrification of the relatively immobile nitrogen form
NH + 4 tothehighlymobileformNO−3 provides opportunities for nitrogen
losses from soil. Thus, nitrification in soil can lead to losses of nitrogen
byrunoffandleachingwhichcanresultinanaccumulationofnitratein
lakes, groundwater and rivers. Alternatively, nitrification is of significance
in aerobic sewage treatment in which ammonium from ammonification is
converted to nitrate. The nitrate formed by nitrification can be assimilated
by plant roots and bacteria or used as a terminal electron acceptor by
microorganisms when oxygen is limited. Therefore, nitrification can be
considered to be central for the flow, transfer or loss of nitrogen in soil.
Due to the fact that the end product of nitrification is NO− 3 –theprecursor
forpathwaysofmajorlossesofnitrogen–ithasbeensuggestedthatthe
only realistic approach for controlling the nitrogen cycle in an agricultural
ecosystem is to inhibit this process (Hauck 1983).
The reduction of nitrate into gaseous nitrogen in an anaerobic environment
is performed by a four-step reaction respiratory process: denitrification.
Denitrification is a microbial respiratory process in which soluble nitrogen
forms, nitrate and nitrite, are reduced into gas, nitric oxide, nitrous
oxide and dinitrogen when oxygen is limited:
NO − 3 → NO− 2 → NO → N2O → N2 .
The importance of denitrification in the nitrogen cycle is depicted in Figure
1: it is the main biological process responsible for the return of fixed
nitrogen to the atmosphere, thus completing the N cycle. Denitrification
is also of interest for several reasons. First, reduction of nitrate to gaseous
nitrogendepletesthesoilofnitrateandisthusresponsibleforthelossof
an essential plant nutrient. It is also used to remove nitrate from water,
accumulated mainly as a result of agricultural nitrogen-fertilizer. Third,
denitrification contributes to the modification of the global atmospheric
chemistry, essentially through the greenhouse effect (Lashof and Ahuja
1990) and destruction of the Earth’s ozone layer (Waibel et al. 1999) by
emitting N2O.
Alternatively, nitrate produced by nitrification can be also reduced into
ammonium by either respiratory or dissimilatory processes:
NO − 3 → NO− 2 → NH+ 4 .
In respiratory nitrate reduction to ammonium, reduction of nitrite is coupled
to the generation of an electrochemical proton gradient across the
membrane. This reduction of nitrite to ammonia is performed without
the release of intermediate products (Simon 2002). Similarly to denitrification,
this respiratory nitrate ammonification is carried out under anaerobic
conditions.