While most terrestrial primary productivity is returned to the atmosphere as carbon dioxide (CO2) via soil microbial respiration, some carbon (C) is stabilized in soil as microbial biomass. The amount of C that is stabilized depends on how effectively microbes convert C to biomass relative to how much C is lost as CO2 (carbon-use efficiency [CUE]). Biomass growth and respiration are sensitive to environmental conditions such as soil moisture. As soils dry, substrate supply becomes limited by slow diffusion along the tortuous paths of water films. As soils dry more, microbes physiologically adjust to desiccating conditions by increasing internal solute concentrations. Diffusion and desiccation may result in decreased growth and increased respiration, lowering CUE. However, the relative importance of both processes is unknown. We conducted a series of moist soil and soil slurry incubations using isotopically labeled C and nitrogen substrates to investigate the effects of soil moisture on microbial CUE in four semiarid soils of different textures. We hypothesized that decreased moisture will reduce CUE, but diffusion will be more important in high to mid-range water potentials whereas desiccation will be more important in low water potentials. Additionally, diffusional limitations will be more severe in coarse-textured than fine-textured soils.
Results/Conclusions
In general, soil microbial CUE decreased from high to low water potentials as respiration increased relative to biomass growth. The decline in CUE due to substrate diffusion was most important in soil with high water potentials whereas the decline in CUE due to desiccation was most important in soil with low water potentials. However, this relationship between the effects of diffusion and desiccation on CUE with water potential varied by soil texture. At a given water potential, CUE was lower in coarse-textured than fine-textured soils. More, the diffusional limitation to CUE was a relatively more important factor across the soil moisture gradient for coarse-textured than fine-textured soils. Overall, we were able to partition the effects of soil moisture on CUE into the effects of substrate diffusion limitation versus physiological desiccation stress, which was dependent on soil texture. We argue that quantitatively partitioning the effects of soil moisture on CUE will provide a flexible way of modelling processes in soils of different textures, and thus soils with different water content-water potential relationships. Our results will allow us to modify existing biogeochemical models so that we can better predict the biogeochemical consequences of a changing climate.