Growth and death of bacteria and fungi underlie rainfall-induced carbon dioxide pulses from seasonally dried soil

The rapid increase in microbial activity that occurs when a dry soil is rewetted has been well documented and is of great interest due to implications of changing precipitation patterns on soil C dynamics. Several studies have shown minor net changes in microbial population diversity or abundance following wet-up, but the gross population dynamics of bacteria and fungi resulting from soil wet-up are virtually unknown. Here we applied DNA stable isotope probing with H₂¹⁸O coupled with quantitative PCR to characterize new growth, survival, and mortality of bacteria and fungi following the rewetting of a seasonally dried California annual grassland soil. Microbial activity, as determined by CO₂ production, increased significantly within three hours of wet-up, yet new growth was not detected until after three hours, suggesting a pulse of nongrowth activity immediately following wet-up, likely due to osmo-regulation and resuscitation from dormancy in response to the rapid change in water potential. Total microbial abundance revealed little change throughout the seven-day post-wet incubation, but there was substantial turnover of both bacterial and fungal populations (49% and 52%, respectively). New growth was linear between 24 and 168 hours for both bacteria and fungi, with average growth rates of 2.3 × 10⁸ bacterial 16S rRNA gene copies·[g dry mass]⁻¹·h⁻¹ and 4.3 × 10⁷ fungal ITS copies·[g dry mass]⁻¹·h⁻¹. While bacteria and fungi differed in their mortality and survival characteristics during the seven-day incubation, mortality that occurred within the first three hours was similar, with 25% and 27% of bacterial and fungal gene copies disappearing from the pre-wet community, respectively. The rapid disappearance of gene copies indicates that cell death, occurring either during the extreme dry down period (preceding five 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. A dynamic assemblage of growing and dying organisms controlled the CO₂ pulse, but the balance between death and growth resulted in relatively stable total population abundances, even after a profound and sudden change in environment.