Moisture control over atmospheric CH4 consumption and CO2 production in diverse Alaskan soils

Moisture is an important control on atmospheric CH4 consumption and CO2 production in soil. Wet conditions limit these microbial activities by restricting CH4 and O2 diffusion and dry conditions limit microbial activity due to physiological water stress. We examined the relationship between soil moisture and these biogeochemical activities in five Alaskan soils with varying physical properties. Three expressions of soil moisture, absolute water content (g H2O g-1 dry soil), water potential and per-cent of water-holding capacity (%WHC), were compared for their abilities to predict microbial activity in the different soils. We also examined the physiological responses of CH4 oxidizers and the general microbial community to changes in water potential. The quantitative relationship between absolute water content and microbial activity varied widely among soils with different textures. The relationship between microbial activity and water potential was asymmetrical and differed between upland and wetland soils. In contrast, the parabolic relationship between %WHC and CH4 consumption was symmetrical and similar among the five soils. CO2 production also related to %WHC similarly across soils. Maximum atmospheric CH4 consumption occurred between 20-40% WHC in all soils with a mean optimum of 34% WHC, whereas CO2 production was maximal above 50% WHC. For CH4 oxidation, optimum water potential was -0.3 to -0.2 MPa in upland soils, and about -0.02 MPa in a wetland soil. Our results demonstrate that %WHC is a powerful expression for quantitatively relating microbial activity responses to moisture across physically diverse soils and may be useful for modeling the response of biogeochemical processes, especially atmospheric CH4 consumption, to climate change. Our data also suggest that CH4 oxidizers in upland soils are adapted to growth on atmospheric CH4 and that CH4 consumption in upland taiga soils may be decreased by altered soil moisture, regardless of whether conditions become wetter or drier.