Continuous measurements of CO and H2 deposition velocities onto an andisol: uptake control by soil moisture

Carbon monoxide (CO) and hydrogen (H₂) net deposition velocities from the atmosphere onto soil and carbon dioxide (CO₂) effluxes to the atmosphere have been measured in an andisol field in Tsukuba, Japan by the open-flow chamber method. The deposition velocities of CO and H₂ were closely correlated (R = 0.87), with a ratio of 1.55, which was attributed to the difference in molecular diffusivities. However, the deposition velocities did not exhibit a direct relationship with the CO₂ efflux. Deposition velocities of CO and H₂ ranged from 0.00 to 0.06 cm s^-1 and from 0.00 to 0.10 cm s^-1, respectively, and were closely related to the level of the surface soil moisture (0–5 cm) and were higher in plowed plots than in compacted plots. CO deposition velocity was slightly lower in the daytime due to higher production rates affected by the soil temperature. These findings indicate that microbial CO and H₂ consumption was limited by transport resistance in the soil and that the in situ CO and H₂ uptake rates may be limited by a higher soil moisture level. CO and H₂ deposition was estimated to be restricted to the surface soil (possibly only the top 2–3 cm). CH₄ and CO₂ gas profiles were also related to the variation of the soil moisture level.