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172 L. Philippot and J.C. Germon
to estimate the respective contributions of nitrification and denitrification
to these emissions, indicating that both mechanisms can be alternatively the
dominant processes depending on the environmental conditions (Stevens
et al. 1997). The denitrification part in N2O emissions was determined in
several studies: in their review Pratt et al. (1997) consider that 5–10% of
denitrified nitrogen is on average emitted as N2O while Aulack et al. (1992)
mention that the proportion of N2O in gaseous denitrification products
can vary from 0 to 100%. However, the specific contribution for a given soil
can be less variable (Germon and Jacques 1990). Hénault et al. (2001) determined
this contribution by comparing denitrification and N2O emission
kinetics in different situations. They showed that the transient accumulation
of N2O during denitrification in laboratory conditions appeared to be
a relevant indicator of soil in situ N2O emissions and suggested an empirical
index. Recent studies underlined that the quality of the denitrifying
community (size, composition, enzyme induction) must be taken into account
when trying to understand and to model N2Ofieldfluxesfromsoils
(Holtan-Hartwig et al. 2000; Cavigelli and Robertson 2001).
In conclusion, soil microorganisms carry out processes that are important
to ecosystem function, such as maintaining soil fertility by cycling the
nitrogen. With the development of molecular genetic tools in microbial
ecology, rapid progress has been made in the field especially for bacteria
contributing to the N cycle (Bothe et al. 2000; Kowalchuk and Stephen 2001;
Zehr et al. 2003). However, formulating meaningful conclusions about the
importance of diversity within these functional groups is still difficult and
the relative contribution of the different bacterial populations to the N
transformation processes remains unclear. This brings a new challenge for
soil microbiologists, i.e., relating microbial diversity to ecosystem functioning.
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