Plant diversity increases microbial resistance to drought and soil carbon accumulation

Abstract Plant diversity is an important driver of many ecosystem functions, including decomposition and soil carbon (C) cycling. While it is well known that climate extremes can negatively impact ecosystem functions, it remains unclear how plant diversity affects soil microbial tolerance to drought and the consequences this has for soil C cycling. We tested how plant diversity influences soil microbial biomass, respiration and growth under both moist conditions and during a standardised experimental drought. This was done using soils from the Jena Experiment, a long‐term grassland biodiversity experiment, at two different depths (0–10 cm and 10–30 cm). We found that under moist conditions higher plant diversity increased respiration rates, while microbial growth rates were stable. This may be explained by more high‐quality below‐ground inputs, as we found evidence for microbial use of more recently plant‐derived C with increasing plant diversity. The use of organic matter containing high‐quality C but low nitrogen content might have exacerbated microbial nitrogen limitation, constraining microbial growth rates. As less microbially derived organic matter was used as a source of C, this resulted in a build‐up of microbial biomass that could contribute to greater soil C accumulation of microbial origin. We also found a positive effect of plant diversity on microbial growth resistance to drought with increasing plant diversity. This suggests that plant diversity may have promoted drought resistance via higher availability of high‐quality C that could support stress tolerance strategies. Synthesis . Our results highlight that higher plant diversity can enhance microbial growth resistance to drought and C accumulation of both microbial and plant origin, which can strengthen soil C sequestration in grasslands.