Composition outweighs abundance: Soil bacterial communities mediate plant performance under drought and post-recovery

Within the biodiversity-ecosystem functioning framework, soil biodiversity is linked to stability under environmental stress. However, in intensively managed cropping systems, empirical tests of this assumption remain limited, especially under drought and post-drought recovery. For this, we used dilution-to-extinction (DTE) approach to create gradients that simultaneously reduced microbial diversity and abundance, mimicking community depletion in soil. Barley plants were grown in these soils, exposed to a 21-day drought, followed by a 15-day recovery. The DTE successfully reduced bacterial ASV richness, Shannon diversity, and Pielou's evenness, and drought further reduced these indices. These effects persisted after recovery. Shoot and root biomass increased with community depletion at peak drought. Shoot biomass was unaffected by drought, whereas root biomass declined by 24%. After recovery, microbial communities continued to influence shoot biomass, whereas no effects of either drought or community depletion were observed on root biomass. We further observed that depleted soils were enriched with beneficial bacterial taxa, including Arthrobacter , Burkholderia s.l., Massilia , and Mucilaginibacter . Bacterial genome copy number and litter decomposition rates increased, indicating enhanced nutrient cycling and colonization by fast-growing, r-strategist bacteria. These shifts may have reduced plant-microbial competition and improved plant growth. Overall, our results suggest a decoupling of soil biodiversity from crop productivity and drought mitigation. Importantly, our results should not be interpreted as evidence that microbial diversity is not important for agroecosystem functioning nor that decreasing microbial diversity would benefit plant productivity. Instead, these findings indicate that community composition may have significantly mediated plant productivity in our experimental context.