Microbial resistance and resilience to drought across a European climate gradient

Abstract

Drought and rainfall events are becoming more frequent and intense. At the same time, soil moisture is one of the main factors controlling soil microbial processes. Microorganisms release carbon into the atmosphere via respiration, but also accumulate carbon in the soil via microbial growth. When challenged with drought, microbial communities are thought to have two different responses: (1) they can maintain growth rates during drought (i.e., resistance) and (2) growth rates can recover faster when the drought ends (i.e., resilience). Microbial communities are also shaped by many other aspects of the soil environment, however how those interact with drought remains unclear. Here we investigated how differences in climate and soil properties determine microbial drought resistance and resilience. To do so, we used a climate gradient of 40 soils across Europe ranging from the Arctic to Southern Mediterranean. Sites were also selected to represent a wide range of soil properties that were likely drivers of microbial responses, includingsoil organic matter, pH, and soil texture. Microbial community composition was estimated at the start of the experiment. Microbial growth was measured during dry-down to determine resistance and up to 72h after rewetting to determine drought resilience. We found that bacterial alpha diversity was the strongest driver of both bacterial resistance and resilience. This was most probably due to changes in pH, where higher pH coincided with higher alpha diversity, which increased bacterial resistance and resilience. This suggests that high diversity helps maintain bacterial functions during drought. Climate was the second most important driver, where bacterial communities from arid climates showed higher resistance and resilience than those from humid climates. Moreover, the bacterial community composition was linked to resilience and could be associated with shifts in the relative abundance of specific taxa. We also found that fungal communities were both more resistant and resilient compared to bacteria, but fungal resistance and resilience were unaffected by the climate and measured soil properties. These findings show that bacteria are more sensitive to drought and rainfall events than fungi. In addition, we found no evidence of a trade-off between resistance and resilience for bacteria or fungi, meaning that a community with high resistance does not have low resilience or vice versa. Taken together, higher diversity and drier climates favoured both more resistant and resilient bacterial communities which could be linked to differences in community composition. Our study provides a space-for-time substitution that can be used to predict microbial responses to a changing climate. Our results imply, that if soils become drier or are managed to promote bacterial diversity, that increased bacterial growth during drought perturbations will promote soil carbon storage.