Drought intensity shapes legacy effects on grassland plant and soil microbial communities and their response to a subsequent drought

Drought has long-lasting legacy effects on grassland ecosystem functioning, which can manifest as shifts in soil microbial community structure and function and plant productivity that persist long after a drought passes. Increasing drought intensity causes abrupt shifts in plant productivity, plant-soil carbon and nitrogen dynamics, and soil microbial communities. However, very little is known about the role that drought intensity plays in the formation of drought legacies, and in plant and microbial responses to a subsequent drought. In a two-year experiment, we studied soil legacies associated with drought intensity in two model grassland plant communities with contrasting resource acquisition strategies (i.e., a fast- and a slow-strategy community). In the first year of the experiment, communities experienced a gradient of increasing drought intensity from well-watered to severely drought-stressed. In the second year, we determined soil microbial community composition and function, and plant community above-ground biomass in response to a subsequent drought. We found that the drought in the first year affected soil prokaryote and fungal community composition, microbial network structure, and soil function in the following growing season, and these effects were dependent on the past drought’s intensity. Soil drought legacy effects significantly altered plant community resilience to the subsequent drought: increasing intensity of the initial drought reduced plant community productivity resistance in slow-strategy plant communities, and decreased productivity overshoot seven weeks after re-wetting in fast-strategy plant communities. Our study shows that drought intensity causes distinct legacies in soil microbial community composition and function and alters the resilience of plant productivity to subsequent drought.