Background/Question/Methods

We have invstigated CH₄ oxidation by soil methanotrophic bacteria and N₂O emissions from soils in multi-year experiments in two pasture ecosystems that were managed at different intensity. Our objective was to study the patch-level dynamics of soil trace gas fluxes and the underlying microbial processes, both under current climatic conditions and under simulated summer drought, at scales ranging from whole ecosystem flux measurements, over potential microbial activities in laboratory assays, to the genetic structure of the underlying soil microbial community. Experimental treatments applied consisted of an N treatment (cattle urine, cattle feces, ammonium nitrate, no fertilizer at all), that was factorially combined with a rain exclusion treatment inducing severe drought.

Over the two-year treatment, we measured soil-atmosphere fluxes of methane and nitrous oxide, the concentration of these gases in the soil atmosphere, soil microbial biomass, soil enzyme activities, soil mineral N concentrations, and the community structure of the underlying microbial communities. We also mapped the micro-scale distribution of soil methanotrophs with a novel imaging technique yielding a resolution better than 100 micrometers.

Results/Conclusions

Simulated drought significantly reduced N₂O emissions, whereas mineral N and urine application stimulated N2O fluxes; dung had less of an effect. Potential denitrification significantly dropped under mineral N fertilization, an effect that progressively developed and which was reflected in a reduction in abundance of bacterial genes encoding for nitrite reducing enzymes (nirS and nirK).

Methane oxidation significantly increased under drought, mainly due to lower diffusive limitation of methane diffusion. The micro-scale distribution of soil methanotrophic bacteria was correlated with soil aggregate structure and was analysed in dependence of treatments.

More generally, our results demonstrate that treatment effects occurred at different hierarchical levels, some of which did not propagate to changes at the whole-ecosystem level; understanding the hierarchy of controlling mechanisms is therefore crucial to predict changes in trace gas exchange rates under altered environmental conditions. This hierarchy, however, depends on the ecosystem and time scales investigated.