We hypothesized that denitrifying bacteria vary grossly regarding their capability to achieve a balanced transition from oxic to anoxic metabolism, and to reduce NO3—N all the way to N2 (rather than emitting large amounts of NO and N2O). Our data seem to confirm this notion with some reservation.
For a while, we were convinced that no congruence would be found between phylogeny and phenotypic response patterns of denitrifying prokaryotes. That hypothesis seems to be refuted by our data; which suggest that efficient regulation adds significantly to the fitness of some phylotypes. This appears to be confined to their regulation of NO2- and NO concentrations, however; not the N2O emission.
For some time, we have suspected that denitrifying communities in soils are different regarding their ability to achieve a balanced transition from oxic to anoxic conditions. This view has been corroborated by new empirical data collected over several years. For some time we had the notion that fingerprinting of a community’s functional genes can be used to predict its propensity to emit N2O, hence for parameterization of an biogeochemical N2O emission model. We still believe that it is feasible in theory, but are convinced that much work remains to be done before such “wet dreams” of molecular ecologists can come true.
Our investigations of long term liming experiments strongly suggest that simple agronomic practices such as liming can significantly reduce N2O emission, both by a direct pH effect on the metabolism and by altering the denitrification community composition of the soil. Proof of concepts needs to be found at all scales, from the direct pH effect at the cellular level, at community level and in intact soils. Field fluxes in combination with modelling would represent ultimate proofs.
We also believe that much can be done to improve the performance of mineral fertilizers regarding their immediate effects on N2O emissions. Soluble salts perturbate the environment adjacent to the fertilizer particles, and may enhance transient accumulation of NO2- and hence emissions of both NO and N2O.
Several observations of communities and pure cultures have convinced us that post- translatoric regulation of electron flow to either O2 or NOx is a clue to understand N2O emissions from soils. Transcription of the denitrification genes appears to be universally repressed by oxygen, but aerobic denitrification is a significant phenomenon, provided that the denitrification proteome has been expressed during foregoing anoxia.
Thauera Aerobic denitrification in Thauera aminoaromatica (Mao et al ms in prep)
Illustration
The "post anoxic trauma" is a term used for a community exposed to oxygen after an anoxic spell. The figure above shows that Thauera aminoaromatica is unable to stop denitifying if given a sudden dose of oxygen. The same phenomenon was deomstrated for a denitrifying community by Morley et al 2008 (see publication list). In both cases, nitrous oxide reductase activity was inhibited by oxygen.
