References

170 L. Philippot and J.C. Germon
In order to adjust nitrate availability to plant requirements and to reduce
nitrogenlossesbyleachingorbydenitrification,manyauthorshavetried
to manage the nitrification rate with the help of nitrification inhibitors:
the most frequently used products are nitrapyrine (2-chloro-6-(trichloromethyl)pyridine)
and dicyandiamide (DCD). However, numerous studies
have shown that nitrification inhibitor efficiency largely varies and it is
difficult to draw a conclusion on a beneficial and reliable effect on nitrogen
balance and fertilizer efficiency (Keeney 1986).
Nitrification is also a source of the gaseous nitrogen oxides N2O and
NO directly or indirectly involved in the greenhouse effect; the 100-year
global warming potential of N2O is about 300 times as strong as that of
carbon dioxide with a lifetime of approximately 120 years (IPCC 2001) and
N2O emissions are taken into account in the global balance of greenhouse
gas. N2O is produced by nitrifying bacteria during ammonium oxidation
into nitrite, on the one hand, and by nitrite reduction into nitrogen gas in
oxygen limiting conditions, i.e., nitrifier denitrification, on the other. In the
absence of denitrification in different soils, Garrido et al. (2002) assert that
N2O emissions are in direct relation to the amounts of nitrified nitrogen
with a variable proportion between 0.03 and 1% depending on the soils.
Under similar conditions, NO production varied from 0 to 2.5% of nitrified
nitrogen depending on the soils and the water potential. The significance of
N2O loss via nitrifier denitrification in soils is still a matter of speculation
and varies between insignificant amounts (Robertson and Tiedje 1987) to
30% of the total N2O production (Webster and Hopkins 1996).
4.4
Dissimilatory Nitrate Reduction to Ammonium
Dissimilatory nitrate reduction to ammonium is developed under anaerobic
conditions and can be considered as an alternative mechanism to
denitrification,allowingnitrogentobemaintainedinsoilasanavailable
form for the vegetation. Several studies have tried to evaluate the respective
contribution of denitrification and DNRA during nitrate reduction in
soils in relation to different conditions, particularly oxygen and carbon
availability. They demonstrated that the indigenous labile soil carbon is the
key factor influencing the partitioning of nitrate reduction. In cultivated
and normally aerated soils kept under anaerobic conditions, DNRA corresponds
to only a few percent of denitrified nitrogen, while it can reach
15% of reduced nitrate in soils naturally rich in biodegradable organic C
(Yin et al. 2002). DNRA can be increased strongly in soil receiving fermentable
C source with a DNRA/denitrification ratio increasing with the
biodegradable-C/N-NO− 3 ratio (Fazzolari et al. 1998). Moreover, DNRA can