Effects of soil moisture content and temperature on methane uptake by grasslands on sandy soils

Aerobic grasslands may consume significant amounts of atmospheric methane ( CH4). We aimed (i) to assess the spatial and temporal variability of net CH 4 fluxes from grasslands on aerobic sandy soils, and (ii) to explain the va riability in net CH4 fluxes by differences in soil moisture content and tem perature. Net CH4 fluxes were measured with vented closed flux chambers at two sites with low N input on sandy soils in the Netherlands: (i) Wolfheze, a heather grassland, and (ii) Bovenbuurtse Weilanden, a grassland which is mown twice a year. Spatial variability of net CH4 fluxes was analysed usin g geostatistics, In incubation experiments, the effects of soil moisture co ntent and temperature on CH4 uptake capacity were assessed. Temporal variab ility of net CH4 fluxes at Wolfheze was related to differences in soil temp erature (r(2) of 0.57) and soil moisture content (r(2) of 0.73). Atmospheri c CH4 uptake was highest at high soil temperatures and intermediate soil mo isture contents. Spatial variability of net CH4 fluxes was high, both at Wo lfheze and at Bovenbuurtse Weilanden. Incubation experiments showed that, a t soil moisture contents lower than 5% (w/w), CH4 uptake was completely inh ibited, probably due to physiological water stress of methanotrophs. At soi l moisture contents higher than 50% (w/w), CH4 uptake was greatly reduced, probably due to the slow down of diffusive CH4 and O-2 transport in the soi l, which may have resulted in reduced CH4 oxidation and possibly some CH4 p roduction. Optimum soil moisture contents for CH4 uptake were in the range of 20 - 35% (w/w), as prevailing in the field. The sensitivity of CH4 uptak e to soil moisture content may result in short-term variability of net atmo spheric CH4 uptake in response to precipitation and evapotranspiration, as well as in long-term variability due to changing precipitation patterns as a result of climate change.