MOISTURE CONTROL OVER ATMOSPHERIC CH4 CONSUMPTION AND CO2 PRODUCTION IN DIVERSE ALASKAN SOILS

Moisture is an important control on atmospheric CH4 consumption and CO 2 production in soil. Wet conditions limit these microbial activities by restricting CH4 and O-2 diffusion and dry conditions limit microbia l activity due to physiological water stress. We examined the relation ship 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 po tential and percent of water-holding capacity (%WHC), were compared fo r their abilities to predict microbial activity in the different soils . We also examined the physiological responses of CH4 oxidizers and th e general microbial community to changes in water potential. The quant itative relationship between absolute water content and microbial acti vity varied widely among soils with different textures. The relationsh ip 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 sim ilar among the five soils. CO2 production also related to %WHC similar ly across soils. Maximum atmospheric CH4 consumption occurred between 20-40% WHC in all soils with a mean optimum of 34% WHC, whereas CO2 pr oduction was maximal above 50% WHC. For CH4 oxidation, optimum water p otential 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 expres sion for quantitatively relating microbial activity responses to moist ure across physically diverse soils and may be useful for modeling the response of biogeochemical processes, especially atmospheric CH4 cons umption, to climate change. Our data also suggest that CH4 oxidizers i n upland soils are adapted to growth on atmospheric CH4 and that CH4 c onsumption in upland taiga soils may be decreased by altered soil mois ture, regardless of whether conditions become wetter or drier. (C) 199 8 Elsevier Science Ltd. All rights reserved.