Historical Drought Affects Microbial Population Dynamics and Activity During Soil Drying and Re-Wet.

A history of drought exposure promoted by variable precipitation regimes can select for drought-tolerant soil microbial taxa, but the mechanisms of survival and death of microbial populations through the selective stresses of soil drying and re-wet are not well understood. We subjected soils collected from a 15-year field drought experiment ("Altered" precipitation history with extended dry periods, versus the "Ambient" field control) to a laboratory drying/re-wetting experiment, to learn whether selective population survival, death, or maintenance of protein synthesis potential and microbial respiration through variable soil water conditions was affected by field drought legacy. Microbial community composition, as measured by Illumina MiSeq sequencing of the 16S rRNA and 16S rRNA gene, shifted with laboratory drying/re-wet and field drought treatments. In Ambient soils, there was a higher proportion of reduced OTU abundance (indicative of mortality) during re-wet, whereas Altered soils had a greater proportion of stable OTU populations that did not change in abundance (indicative of survival) through drying/re-wet. Altered soils also had a lower proportion of rRNA:rRNA genes (lower protein synthesis potential) during dry-down, a greater weighted mean rRNA operon number (potential growth rate and r-selection) which was associated with higher abundance of Firmicutes (order Bacillales), and lower average microbial respiration rates. These data demonstrate that soils with a weaker historical drought legacy exhibit a higher prevalence of microbial water-stress mortality and differential survival and death at OTU levels following short-term dryingand re-wetting, concurrent with higher carbon loss potential. This work provides novel insight into the mechanisms and consequences of soil microbial changes resulting from extended drought conditions.