Drought is common and, with climate change, appears to be becoming more so. Earlier thinking had suggested that the main constraint on microbial processes in dry soils was the physiological stress, but increasingly it appears that as soils dry, physical and chemical drivers on diffusion and C-availability are more important. We have been exploring microbial and biogeochemical processes in dry soils and following rewetting by using a wide array of methods including lab experiments and field manipulations that have included both multi-year manipulations of plant inputs & dry season length, and hourly estimates of available nutrients following rewetting (by using microdialysis) and short term gas flux measurements. We couple these distinct measurement approaches and time scales by using fine-scale modeling.
Results/Conclusions
In California mollisols, as soil dries, bioavailable C pools increase while microbial biomass remains high and sometimes increases. These increases appear to be driven by release of organic matter from physically protected forms rather than from breakdown of freshly dead roots. Following rewetting, the increased mobility of N allows microbes to rapidly use nitrate during the first several hours to fuel the flush of respiration, while mineralization appears to take roughly a day before NH4+ becomes available in soil solution. Yet the time courses of response to rewetting vary between soils of different mineralogy. Understanding the dynamics and soils dry and rewet clearly requires conceptual and simulation models that integrate chemical interactions of substrates with soil minerals, diffusion, and finally microbial activity and its responses to drought and rewetting.