N2O consumption: ecosystem function and proposed mechanisms

Pascal Boeckx1, Dries Roobroeck2, Joachim Desloover1, Korneel Rabaey1, Nico Boon1
1 Ghent University

Invited Talk 3 in Fluxes between the atmosphere and ecosystems

17.07.2014, 10:00-10:30, H17

Recently the median value of 4 mg m-2 h-1 for nitrous oxide (N2O) uptake in soils of natural ecosystems has been put forward (Schlesinger et al. 2013). The highest values are nearly all associated with soils of wetland and peatland ecosystems. Global N2O consumption in soils is estimated to be below 0.3 Tg N yr-1, which is less than 2% of the current estimated N2O sources. Nevertheless large knowledge gaps exist on the magnitude and biogeochemistry of N2O consumption. Here we report on 1) in situ N2O uptake rates from pristine fens in the Biebrza national park in Poland, 2) N2O consumption pathways as identified in Pseudomonas stutzeri, and 3) an hypothesis for an electron shuttling mechanisms that facilitates the transfer of electrons to soil denitrifying microorganisms promoting the reduction of N2O to di-nitrogen (N2) (Cayuela et al. 2013).

Fen ecosystems in Biebrza exhibited moderate N2O removal under oxic conditions (-0.1 to -3.5 mg N m-2 h-1), whereas very strong N2O removal under anoxic conditions (-41 to -67 mg N m-2 h-1) occurred. A multi-response permutation procedure of canonical analysis of principal coordinates put forth that N2O removal in undisturbed fen ecosystems were not strongly linked to the abundance of nirK and nosZ denitrifiers, but rather the activity of NirK and NosZ triggered by physical parameters such as O2 availability (controlled via WFPS) and pH.

Microbiologically the consumption of N2O results from its reduction to N2 as part of the denitrification process. However, there is on-going debate regarding an alternative pathway, namely reduction of N2O to NH4+, or assimilatory N2O consumption. To date, this pathway is poorly investigated and lacks unambiguous evidence. Enrichment of denitrifying bacteria using a mineral nitrogen-free medium rendered a mixed culture capable of anoxic and oxic N2O consumption. Dilution plating, isolation and DNA fingerprinting identified a collection of Pseudomonas stutzeri strains as dominant N2O consumers in both anaerobic and aerobic enrichments. An isotope tracing experiment with one specific P. stutzeri isolate showed that consumption of N2O via assimilatory reduction to NH4+ was absent. Conversely, respiratory N2O reduction was directly coupled to N2 fixation.

In Nelissen et al. (2014) we showed that biochar application to soils decreased cumulative N2O emission by 52 to 84% upon nitrate addition. N2O emissions were more decreased at high compared to low temperature pyrolysis biochars. Several hypotheses for our observations exist. However, electron shuttling mechanisms that facilitate the transfer of electrons to soil denitrifying microorganisms could promote the reduction of N2O to N2.

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last modified 2014-06-19