Model analysis of the influence of gas diffusivity in soil on CO and H-2 uptake

CO and H-2 uptake by soil was studied as a diffusion process. A diffusion m odel was used to determine how the surface fluxes (net deposition velocitie s) were controlled by in-situ microbial uptake rates and soil gas diffusivi ty calculated from the 3-phase system (solid, liquid, gas) in the soil. Ana lytical solutions of the diffusion model assuming vertical uniformity of so il properties showed that physical properties such as air-filled porosity a nd soil gas diffusivity were more important in the uptake process than in t he emission process. To incorporate the distribution of in-situ microbial u ptake, we used a 2-layer model incorporating "a microbiologically inactive layer and an active layer" as suggested from experimental results. By numer ical simulation using the 7-layer node, we estimated the effect of several factors on deposition velocities. The variations in soil gas diffusivity du e to physical properties, i.e., soil moisture and air-filled porosity, as w ell as to the depth of the inactive layer and in-situ microbial uptake, wer e found to be important in controlling deposition velocities. Tills result shows that the diffusion process in soil is critically important for CO and H-2 uptake by soil, at least in soils with higher in-situ uptake rates and /or with large variation in soil moisture. Similar uptake rates and the dif ference in deposition velocity between CO and H-2 may be attributable to di fferences in CO and H-2 molecular diffusivity. The inactive layer is resist ant to diffusion and creates uptake limits in CO and H-2, by soil. The coup ling of high temperature and a thick inactive layer, common in arid soils, markedly lowers net CO deposition velocity. The temperature for maximum upt ake of CO changes with depth of the inactive layer.