Drivers of Plot-Scale Variability of CH4 Consumption in a Well-Aerated Pine Forest Soil

While differences in greenhouse gas (GHG) fluxes between ecosystems can be explained to a certain degree, variability of the same at the plot scale is still challenging. We investigated the spatial variability in soil-atmosphere fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) to find out what drives spatial variability on the plot scale. Measurements were carried out in a Scots pine (Pinus sylvestris L.) forest in a former floodplain on a 250 m(2) plot, divided in homogenous strata of vegetation and soil texture. Soil gas fluxes were measured consecutively at 60 points along transects to cover the spatial variability. One permanent chamber was measured repeatedly to monitor temporal changes to soil gas fluxes. The observed patterns at this control chamber were used to standardize the gas fluxes to disentangle temporal variability from the spatial variability of measured GHG fluxes. Concurrent measurements of soil gas diffusivity allowed deriving in situ methanotrophic activity from the CH4 flux measurements. The soil emitted CO2 and consumed CH4 and N2O. Significantly different fluxes of CH4 and CO2 were found for the different soil-vegetation strata, but not for N2O. Soil CH4 consumption increased with soil gas diffusivity within similar strata supporting the hypothesis that CH4 consumption by soils is limited by the supply with atmospheric CH4. Methane consumption in the vegetation strata with dominant silty texture was higher at a given soil gas diffusivity than in the strata with sandy texture. The same pattern was observed for methanotrophic activity, indicating better habitats for methantrophs in silt. Methane consumption increased with soil respiration in all strata. Similarly, methanotrophic activity increased with soil respiration when the individual measurement locations were categorized into silt and sand based on the dominant soil texture, irrespective of the vegetation stratum. Thus, we suggest the rhizosphere and decomposing organic litter might represent or facilitate a preferred habitat for methanotrophic microbes, since rhizosphere and decomposing organic are the source of most of the soil respiration.