Interactive effects of warming and drought on seasonal dynamics of soil microbial communities and functions

Soil microbial communities are known to drive key processes such as carbon and nutrient cycling. These microbes have developed physiological and metabolic adaptations to cope with the constraining conditions found in soils. However, their responses to climate change, such as increased temperatures and drought, remain uncertain.  In addition, the physiological adaptations, interactions, and feedback mechanisms within microbial communities during such perturbations, as well as the mechanisms driving the temporal dynamics of microbial responses and recovery remain understudied.

To address these gaps, we assessed soil microbial responses in an outdoor mesocosm experiment, where warming and drought are simultaneously manipulated. We characterized microbial community composition, and we quantified extracellular enzyme activity and microbial biomass, at four time points over the course of a year: before drought (early-growing season), immediately after drought (peak of the growing season), one month after drought (peak of the growing season), and three months after drought (end of the growing season), on 4 different temperature regimes. We further associate both soil biotic (e.g., microbial diversity and composition) and abiotic variables (e.g., organic matter quality) to better understand enzymatic shifts due to warming and drought.

Our results reveal distinct post-drought recovery patterns in fungal and bacterial diversity under various warming scenarios. Fungal diversity seems more resistant to drought and warming than bacterial diversity. The activity of soil microorganisms declined immediately following drought, with recovery varying based on the type of enzymatic substrate. Oxidative enzymes were highly sensitive to the combined effects of warming and drought, and drought hindered their activity in soils exposed to periodic heatwaves 3 months after drought. On the other hand, constant warming enhanced the recovery of hydrolytic enzymes 3 months after drought, but this recovery was obstructed by periodic heatwaves. These findings suggest that hydrolytic enzymes seem to recover after drought likely due to fungal and bacterial diversity recovery.

These results suggest that soil microbial activity may recover after drought in the short term under warming, but repeated periodic heat waves could disrupt this recovery, by changing microbial community composition and potentially leading to shifts in functional capabilities, having detrimental impacts on carbon and nutrient cycling. By examining drivers such as soil organic matter quality, moisture, and nutrient availability, we aim to obtain critical insights into the stability of soil microbial activity under the combined effects of warming and drought, with implications for predicting and mitigating ecosystem changes in a warming world.