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| Titel |
Multiple approaches to understand regulation of N2O from denitrification: the case of pH |
| VerfasserIn |
Peter Dörsch, Umb Nitrogen group |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052775
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| Zusammenfassung |
| Despite many decades of field research, no clear strategy has emerged for reducing soil N2O
emissions associated with food production. To the contrary, atmospheric N2O continues to
rise (0.3% yr-1) as anthropogenic N fixation increases to meet human population and
prosperity growth. Any reactive nitrogen added to the biosphere will eventually return to the
atmosphere via microbial N transformations that produce N2O in variable proportions. We
believe that it is the stoichiometry of these processes (nitrification and denitrification) that
ultimately determines atmospheric N2O. Recent evidence suggests that denitrifying
prokaryotes differ widely in their ability to perform complete denitrification in response to
anoxia and hence display specific ratios in their denitrification products. The UMB nitrogen
group uses high-resolution gas kinetics (O2, NO, N2O and N2) with batch incubations to
explore such product ratios in search for distinct ‘denitrifier regulatory phenotypes’ (DRP).
To understand which factors are critical for denitrification regulatory phenotypes,
gene transcription analyses (nirS, nirK, nosZ) are run in controlled experiments
together with the gas kinetics. Pure culture experiments with denitrifying model
organism are used to generate hypotheses regarding the underlying mechanisms of
regulation, while extracted complex bacterial communities from soils are used to test the
hypotheses. In the present contribution, we report combined functional/molecular
approaches seeking to unravel the effect of pH on denitrifier performance, functional
taxonomic composition and relative N2O production. Acidic soils are wide spread
and acidification may exacerbate with further intensification of agriculture and
increased use of chemical fertilizers. There is abundant circumstantial evidence
that acidity increases the denitrification product ratio (N2O/N2), but nothing is
known about the mechanism of pH control. Moreover, since pH affects virtually all
biochemical processes in soil, confounding of direct and indirect control by pH is
formidable. To explore direct effects of pH, we designed experiments with the denitrifier
model organism Paracoccus denitrificans and found high N2O accumulation at
suboptimal pH. Expression studies suggested that pH interferes with the assembly of
the N2O reductase enzyme at the post-transcriptional level. This mechanism was
corroborated indirectly in a study with intact soil from a long-term liming experiment and
with bacteria extracted from these soils, suggesting pervasive pH control on N2OR
functioning on a cellular level, i.e. a DRP common to all bacterial denitrifiers. In contrast,
when running experiments with soil slurries and extracted bacterial consortia from
geographically remote organic soils with contrasting pedogenic pH, congruent
differences in denitrifier taxonomic composition/abundance and function were found,
suggesting that pH selects for specific DRPs on a community level with clear-cut
consequences for the propensities to emit N2O. As proof of concept, we present first
results from a field study recently initiated in a long term liming experiment in
Norway where we found dramatic differences in fertiliser induced N2O emissions
depending on liming history. Based on the mechanistic understanding of direct and
indirect pH control as well as the empirical evidence from field experiments, we
hypothesized that liming acid agricultural soils may be an efficient way to reduce N2O
emissions. Moreover, our data call for a refinement of biogeochemical models, many of
which do not account for pH control and denitrification regulatory phenotypes. |
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