Oxidation of atmospheric methane in soil: Measurements in the field, in soil cores and in soil samples

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Abstract
Methane fluxes and vertical profiles of CH4 mixing ratios were measured in different German soils both in situ and in soil cores. Atmospheric CH4 was oxidized in the soil by microorganisms resulting in an average CH4 flux of −1.39±1.5 μmol‐CH4 m−2 h−1. Methane deposition showed only a weak positive correlation (r2 = 0.38) with soil temperature but a relatively strong negative correlation (r2 = 0.61) with soil moisture indicating limitation of the CH4 flux by gas transport. Diffusion experiments in soil cores showed that gas transport between atmosphere and soil was faster than microbial CH4 oxidation. However, the diffusion from the gas‐filled soil pores to the CH4 oxidizing microorganisms may have been limiting. The main CH4− oxidizing activity was located in a few centimeter thick subsurface soil layer at the top of the Ah horizon, whereas no activity was found in the overlying O horizons and in deep soil below about 20‐cm depth. In contrast, the highest CO2 production was found in the topmost O horizon. The effective diffusion coefficient of CH4 in soil was determined using a method based on relaxation experiments with argon. The diffusion coefficient was used to model the CH4 oxidation in soil cores from the vertical profiles of CH4 mixing ratios. The thus calculated CH4 oxidation rates and their localization in the soil profile compared fairly well with those determined directly from incubated soil samples. Fluxes were similar within a factor of 2–4 whether derived from the model, calculated from the measured CH4 oxidation rates of soil samples, or measured directly.

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