Plant diversity and root depth modulate microbial resistance and resilience to drought

Soil microorganisms play a central role in terrestrial carbon (C) cycling, yet our mechanistic understanding of how environmental change alters their functioning remains limited. In a world increasingly affected by climate variability, drought and subsequent rewetting events, and the resistance and resilience of microbial functions to them, are particularly important. Rewetting of dry soils triggers a strong biogeochemical response, characterized by a large pulse of microbial CO2 emissions (the Birch effect) and a progressive recovery of microbial growth. The magnitude and temporal dynamics of these responses provide valuable information on microbial community functioning and have important implications for soil C dynamics. Recent studies indicate that climatic history shapes microbial resistance and resilience through ecological memory: communities frequently exposed to drying-rewetting cycles tend to recover growth more rapidly and exhibit sharper respiration peaks, whereas less adapted communities show delayed growth recovery and prolonged and more complex respiration responses. However, how plants modulate microbial perception of drought-rewetting events and provide the resources that enable microbial adaptation and response remains poorly understood.

We analysed how plant diversity and root length influence microbial growth, respiration, and carbon-use efficiency during drought-rewetting events across the soil vertical profile (at different depths). We used complementary experimental settings including gradients of plant species richness (1-60 species and different plant functional groups) and a comparison between wheat and kernza; a conventional annual crop versus a deep-rooted perennial capable of reaching depths of up to 2.5 m. Our results highlight the importance of plants in modulating microbial responses to drought, with the potential to enhance microbial performance and to strengthen soil C sequestration. Higher plant diversity positively affected microbial resistance and resilience, likely by increasing the availability of high-quality C that supports microbial stress tolerance strategies. Longer root systems promoted greater microbial biomass and C cycling at depth, with a tendency towards increased resistance and resilience. The effects on long-term soil C storage remained uncertain as enhanced microbial activity at deep layers may increase the accumulation of persistent organic matter through microbial necromass formation, but also stimulate soil organic matter decomposition via priming.