Effect of gas bubbles on the diffusive flux of methane in anoxic paddy soil

Abstract

The emission of CH4 from paddy soil is driven by CH4 concentration gradients in the submerged soil. CH4 concentration gradients between anoxic methanogenic and oxic methanotrophic soil layers were measured in paddy soil microcosms by using gas diffusion probes with a spatial resolution of 1 mm. The CH4 emission rate was measured by placing a microcosm into a chamber containing an atmosphere of either synthetic air (80% N2, 20% 02) or N2. The CH4 flux was 1.6 ± 5.4 nmol cm−2 d−1 under synthetic air and 288 ± 10 nmol cm−2 d−1 under N2. The difference between the oxic and the anoxic CH4 fluxes was due to CH4 oxidation. The vertical CH4 concentration gradients indicated CH4 oxidation at 2–3 mm depth. Below this depth CH4 concentrations increased steadily to about 10 mm depth, below which accumulation of gas bubbles was observed. The diffusive flux calculated by Fick's first law from the linear part of the gradient was 166 ± 14 nmol cm−2 d−1. Obviously, the flux calculated from molecular diffusion was smaller than the flux that was actually measured under N2. An important condition for the use of Fick's law is that the slope of the gradient used for the calculation is taken in the direction where the slope is steepest. This direction is not necessarily identical with the vertical direction if CH4 concentrations also change in horizontal direction. Measurement of horizontal and vertical CH4 profiles demonstrated that the gradients had a three‐dimensional structure. The reason for this structure was that the isopleths of identical CH4 concentrations followed the uneven surface of the gas bubble layer as the main direct source for the CH4 diffusion gradients. We conclude that gas bubbles do not only directly cause a CH4 flux by ebullition but also indirectly affect the diffusional flux of CH4 in soil or sediment. When the diffusive CH4 flux was calculated from the concentration gradient at 6–8 mm depth, it was larger (224 ± 70 nmol cm−2 d−1) than from that at 3–5 mm depth (125 ± 86 nmol cm−2 d−1). Thus, a transport process in addition to molecular diffusion seemed to be active in the upper soil layers, possibly bioirrigation.