Modeling diffusion and reaction in soils: II. Atmospheric methane diffusion and consumption in a forest soil

A mixed hardwood forest with a maximum potential atmospheric methane consumption at 4 to 6 cm depth was investigated. Vertical variation of soil- water content, gas diffusivity and atmospheric methane uptake was measured with high spatial resolution in intact soil cores (2-5 cm depth intervals). Gas diffusivity increased rapidly with decreasing soil-water potential and a linear relationship between gas diffusivity, and the logarithm to the volumetric soil-water content was found (R² ≤ 0.98). Using this relationship in a simple, dynamic diffusion-reaction model, the vertical methane concentration profiles in intact soil cores were simulated. Only diffusion of methane in the soil air and variable methane consumption with depth was considered in the model. An excellent agreement between simulated and measured methane profiles indicated that a main control of methane consumption in non-waterlogged soils is methane diffusion in the soil air. Simulated methane uptake rates, calculated by summing up the methane oxidation at each 1-cm-depth interval, agreed well with measured methane fluxes into the soil cores. Model sensitivity analyses showed an accurate estimation of the effective gas diffusion coefficient at and above the zone of maximum methane consumption to be the most critical parameter for a realistic simulation of methane concentration profiles and total uptake rates.