Composition overweighs abundance: bacterial communities mediates plant performance under drought and post recovery
Within the biodiversity-ecosystem functioning framework, greater soil biodiversity is generally linked to enhanced multifunctionality and stability under environmental stresses in natural ecosystems. However, in intensively managed cropping systems, which are characterized by distinct intrinsic properties than natural ecosystems, empirical tests of this assumption remain limited, especially under drought stress and post-drought recovery. This research empirically tested whether soil biodiversity influences crop performance at peak drought stress and after soil rewetting. Soil biodiversity gradients were created with dilution-to-extinction approach and barley plants were grown in these soils, subjected to 21 days drought period followed by a 15 days recovery period after soil rewetting. At peak drought, decreasing biodiversity with increasing soil dilution led to a reduction in bacterial ASV richness, Shannon diversity, and Pielou evenness. Drought further negatively affected these bacterial alpha-diversity indices, except for Pielou evenness which was not affected by drought. Such soil dilution and drought mediated effects on bacterial alpha-diversity indices remained evident even after the recovery phase. Interestingly, shoot and root biomass increased with decreasing soil diversity (increasing soil dilutions) at peak drought, showing a negative relationship between soil diversity and plant productivity. Moreover, shoot biomass remained unaffected by drought, whereas root biomass declined by 24 percent. After the recovery period, soil diversity continued to negatively influence shoot biomass, whereas no detectable effects of either drought or soil diversity were observed on root biomass. We also observed that growing barley in diluted soils led to enrichment of various beneficial bacterial taxa (such as Arthrobacter, Burkholderia s.l., Massilia, Mucilaginibacter) through community successions that may have positively influenced plant performance. Moreover, bacterial genome copy number (GCN) and the decomposition rate of a standard litter increased as soil diversity decreased. These findings suggest better nutrient cycling and quick colonization of empty soil niches by fast-growing, r-strategist bacteria which may have reduced plant-microbial competition for resources, thereby contributing positively to plant performance. Altogether, these results suggest a decoupling of soil diversity from crop productivity and stability, where plant biomass increased with decreasing soil diversity, and no evidence that higher soil diversity buffered drought effects on plant performance. These results hints toward context-dependency of biodiversity-ecosystem functioning framework within intensively managed cropping systems, and challenge the common assumption that higher soil diversity is directly associated with increased crop productivity and drought mitigation.