Global changes in precipitation and soil water availability affect microbial, plant and ecosystem functions, and create uncertainty in terrestrial carbon balance predictions. Beyond precipitation level, rainfall timing affects biological activity, because thriving through variable conditions requires different physiological traits and carbon allocation patterns than living in stable conditions. Building upon a >15-year field experiment that altered the timing, not the amount, of precipitation to >2X the size of, and duration between, rainfall events, we asked whether precipitation timing history affected soil microbial responses to drying/rewetting. Based on the hypothesis that a “droughty” history selects for drying tolerant microbial populations, we predicted that in soils with Altered rainfall history, microbial survival through dry-rewet events would be higher while respiration would be lower, and that soil microbial structure and function would show lower sensitivity and faster recovery from an extreme field-scale drought. To evaluate these predictions, we ran a laboratory experiment to measure total bacterial abundance, 16S rRNA gene and rRNA composition, and CO2 release preceding and following repeated drying-rewetting; and we measured soil microbial respiration, biomass carbon, and community turnover before and after a rainfall event that followed 2 years of <30% average precipitation at the field experiment.
Results/Conclusions
In the laboratory experiment, we used differential abundance of 16S rRNA and rRNA genes to define populations that exhibited mortality, survival and “ribosome maintenance” through drying-rewetting events. Survival was higher in altered precipitation history soils, and mortality was 10-100x higher in ambient soils. This provides direct evidence of a legacy of selection for drought-tolerant microbial populations due to drying-rewetting mortality, despite the same total historical precipitation amount. Also, in ambient-history soils, more CO2 was mineralized (despite the same bacterial abundance), and more populations showed increased ribosome levels during drying. This could suggest that lower-maintenance, constitutively expressed, drying tolerance mechanisms are prevalent in altered-history populations. In the field experiment, through the largest rainfall event in two years, the microbial community was stable in altered-history soils but microbial diversity clearly decreased, concomitant with community turnover, in the ambient treatment. Yet, precipitation history did not affect field soil microbial respiration, and microbial biomass carbon increased more in the ambient field treatment. Altogether, event-based selection for microbial drying-rewetting survival did reduce microbial community sensitivity to extreme field-scale drought, but, microbial activity seemed linked to ecosystem-scale changes in carbon availability. Future research must link plant and soil microbial drought responses to accurately improve carbon forecasts.