Microbial resistance and resilience to short-term drought varies among grasslands spanning a broad climatic gradient

Drought is increasing in severity and duration across the United States with disproportionately drier conditions expected in arid and semi-arid regions. Aboveground productivity stands to be heavily impacted by intensified drought, and there is evidence that these effects will be greater in drier vs. wetter grasslands. In contrast, belowground responses, such as nitrogen availability and soil microbial activity and composition, tend to be much more variable in their response to drought in different environmental contexts. Our study sought to determine if belowground responses to drought were uniform or differed with respect to precipitation. To accomplish this goal, we collected intact soil mesocosms, each 9 cm diameter by 5 cm in depth, from four grassland sites spanning an ~500 mm precipitation gradient in the Central US. The soil mesocosms were brought back to the greenhouse and half were subjected to a 30-day drought while the other half experienced control (well-watered) conditions. We assessed drought resilience (i.e., recovery to control conditions after drought) at one and two weeks after the experimental drought ended. Resistance and resilience of several key belowground responses were measured, including inorganic nitrogen, potential extracellular enzyme activity, and microbial community composition via sequencing. We found that the wettest grassland site (tallgrass prairie) was the most sensitive to the short-term drought with decreases in alpha diversity for fungi, differences in beta diversity for fungi, and decreases in all soil enzymes measured. Post-drought there was evidence for increased enzyme activity for most enzymes at all sites, but these differences were quickly resolved after two weeks indicating high resilience to short-term drought. Overall, our study found that belowground responses to short-term drought were site-specific and variable, which could limit our ability to predict how belowground processes will be altered as drought intensifies in the future with changing climate.