Drought impacts on microbial trait distribution and feedback to soil carbon cycling

1. Quantifying the impact of drought on microbial processes and its consequences for soil carbon cycling is hindered by the lack of underlying mechanistic understanding. Therefore, there is a need to scale up the physiological response to changing water status from individual soil microbes to collective communities across different ecosystems. 2. Here we propose the use of a framework that incorporates trait-based ecology to link drought-impacted microbial processes to rates of soil carbon decomposition and stabilisation. We briefly synthesise existing knowledge on the effects of drought on microbial physiology at the individual to community scale, before integrating this understanding within a framework incorporating life-history strategy, ecological strategy and biochemistry. 3. This framework highlights a dynamic allocation to high yield (Y), resource acquisition (A) and stress tolerance (S) pathways as environmental conditions change. Y-A-S strategies represent sets of traits that tend to correlate due to physiological or evolutionary trade-offs. This framework enables assessment of microbial processes along two key environmental gradients of water and resource availability, both of which are constrained by drought. 4. The variable chemistry of biomass and necromass produced under different physiological strategies in response to drying-rewetting impacts organic matter decomposition and stabilisation in soils, and should also be considered when quantifying soil carbon balance. We highlight that diversion of resources away from microbial growth can alter soil organic matter chemistry and its persistence depending on the kind of microbial compounds produced. 5. To advance such a framework, we highlight avenues of research that would enable the further identification and quantification of traits linked to Y-A-S strategies and the physiological outcomes at the community level under drought and rewetting, and conclude by hypothesising how ecosystem-level changes might feedback on to the soil carbon cycle. A scalable understanding of microbial drought-response mechanisms affecting soil carbon cycling will transform the way microbial physiology is represented in ecosystem studies.