Soil microbial responses to soil drying and wetting are primary controllers of CO2 pulses in arid and semi-arid soils

Background/Question/Methods .  The rapid increase in microbial activity that occurs when a dry soil is rewetted mediates production of large pulses of atmospherically reactive trace gases; changing patterns of precipitation may substantially alter these gas-flux dynamics.  We followed the gross population dynamics of bacteria and fungi after soil wet up of a seasonally dried California annual grassland soil by stable isotope probing with H₂¹⁸O coupled with quantitative PCR to characterize new growth, survival, and mortality.  To understand the impacts of changing precipitation patterns on the microbial communities, we imposed dry-down periods of different lengths and we followed community response to the first rainfall using DNA and RNA extracted from soil to characterize the physiological status of the soil communities.

Results/Conclusions .  Microbial activity, as determined by CO₂ production, increased significantly within 3 hours of wet-up, yet new growth was not detected until after 3 hours, suggesting a pulse of non-growth activity immediately following wet-up likely due to osmo-regulation and resuscitation from dormancy in response to the rapid change in water potential.  We identified three metabolic response strategies in response to wet up events based on when taxa had the highest relative ribosomal quantity: rapid-responders (within 1 hour of wet-up), intermediate-responders (between 3 and 24 hours following wet-up), and delayed-responders (24 to 72 hours post wet-up).  Throughout the 7-day post-wet up period there was substantial turnover of both bacterial and fungal populations with bacteria and fungi differing in mortality and survival characteristics during the post wet-up period.   Cell death, occurring either during the extreme dry down period (preceding 5 months) or during the rapid change in water-potential due to wet-up, generates a significant pool of available C that likely contributes to the large pulse in CO₂ associated with wet-up.  Longer periods of dry-down before rainfall significantly increased the soil CO₂ efflux.   A long preceding dry period results in destruction of more bacterial and fungal cells, which may be fueling the larger pulses of CO₂.    A dynamic assemblage of organisms with different response strategies, some growing and some dying, controlled trace gas pulses.  Interestingly, the balance between death and growth resulted in relatively stable total population abundances even after this profound and sudden change in environment.