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

Abstract

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