Impact of water table depth on forest soil methane turnover in laboratory soil cores deduced from natural abundance and tracer C-13 stable isotope experiments

We investigated turnover of methane (CH4) in soils from a poorly drained UK forest. In situ, this forest exhibited a negligible soil-atmosphere CH4 flux, whereas adjacent grassland plots were sources of CH4. We hypothesised that the forest plots exhibited reduced anaerobic CH4 production through water-table draw down. Consequently, we exposed soil cores from under oak to high and low water-table conditions in the laboratory. Methane fluxes increased significantly in the high water-table (1925±1702 μg CH4 m-2 h-1) compared to the low one (-3.5±6.8 μg CH4 m-2 h-1). Natural abundance δ13C values of CH4 showed a strong depletion in high water-table cores (-56.7±2.9 ‰) compared to methane in ambient air (-46.0 ‰) indicative of methanogenic processes. The δ13C values of CH4 from low water-table cores (δ13C-46.8±0.2 ‰) was similar to ambient air and suggested little alteration of headspace CH4 by the soil microbial community. In order to assess the CH4 oxidizing activity of the two treatments conclusively, a 13CH4 spike was added to the cores and 13CO2 production was measured as the by-product of CH4 oxidation. 13CH4 oxidation rates were 57.5 (±12.7) and 0.5 (±0.1) μg CH4 m-2 h-1 for high and low water-tables, respectively. These data show that the lower water-table hydrology treatment impacted methanogenic processes without stimulating methanotrophy.