Soil microbes increase switchgrass germination but not seedling growth under drought

Abstract Purpose Soil microbial communities can mitigate the negative effects of drought on early-stage plant growth. However, the magnitude of this benefit may depend on both the microbial community’s previous host associations and the plant’s developmental stage.Methods We conducted a greenhouse experiment to investigate how microbial presence (autoclaved bulk vs. live bulk soils) and the microbial community’s association history (bulk soil vs. rhizosphere soil) affect germination and seedling growth during drought, as well as how drought and life-stage alter the assembly of the inoculated communities. Our focal plant was switchgrass (Panicum virgatum), a target bioenergy crop frequently used in native prairie restorations.Results We found that drought reduced growth by 59% and germination by 41% compared to ambient conditions, and that microbial presence altered drought responses. Seeds with microbes (live bulk soil) had 83% greater germination and 72% higher survival than seeds in autoclaved soils, and these effects were similar under both precipitation regimes. In contrast to microbial presence, the inoculated communities’ association history did not affect plant responses. We did find that plant growth-stage altered bacterial community assembly; bulk and rhizosphere bacterial communities were initially similar, and responded similarly to drought, but they diverged at the end of the experiment only with a germinating seed.Conclusion We show that soil microbes can increase germination and mitigate early-stage drought stress but that microbial association history may not strongly affect plant drought responses. Furthermore, interactions between soil community history and germination may be a critical, yet understudied, driver of microbiome assembly.