Microbes, moisture, and metabolic activity: Is there a soil moisture threshold for microbial activity?

Growing evidence suggests that climate-driven changes in precipitation regulate microbially mediated ecosystem processes, such as CO₂ flux from soils to the atmosphere. Changes in precipitation can impact the activity of soil microbial communities by altering water availability and diffusion. Low water conditions pose a physiological stress which microorganisms may overcome by transitioning into a dormant state, permitting survival and protection during desiccation while requiring low maintenance energy. But at what water potentials do microbes transition to this state? In order to better understand the relationship between soil water potential and microbial activity, we deployed rainfall shelters in replicated successional grassland plots. We measured soil water potential and the rate of soil CO₂ production (a proxy for microbial activity) on soils collected from reduced-rainfall and ambient-rainfall treatments over a two-month period. The rainfall shelters successfully reduced incoming precipitation by 75% over the course of the experiment. This reduction in rainfall led to a significant decrease in water potential as well as a reduction in soil respiration. Our preliminary results suggest that these moisture-mediated changes in CO₂ production were accompanied by shifts in the composition of bacteria detected by RNA, which may be correlated to changes in activity. In addition, the active microbial community (detected by RNA) appears to reflect the current soil water potential while the present microbial community (detected by DNA) maintains a signature of past conditions. Thus, we may be able to use shifts in relative ribosomal quantity (RNA) to identify when taxa transition out of the active pool. Ultimately, if we can understand and identify soil moisture thresholds, we should be able to predict corresponding declines in microbial activity in different ecosystems with varying plant communities and soil types.