Mycorrhizal fungi stabilize bacterial growth and diversity in water-limited soils

Drought disrupts soil microbial activity and many biogeochemical processes. Although mycorrhizal fungi are known to impact plant functions and nutrient cycling during drought, their effects on drought-exposed soil microbial communities are not well resolved. We used H218O quantitative stable isotope probing (qSIP) to investigate microbial response to water limitation in the hyphospheres of two distinct mycorrhizal lineages ( Rhizophagus irregularis and Serendipita bescii ) grown with the bioenergy model grass Panicum hallii . In the absence of mycorrhizal inocula, water limitation resulted in significantly lower bacterial growth rates and lower diversity in the actively growing bacterial community. Water limitation also decoupled growth from CO2 efflux, resulting in a threefold lower growth efficiency. In contrast, both mycorrhizal lineages appeared to protect bacterial communities exposed to water limitation: bacterial growth rates, growth efficiency, and the diversity of the active bacterial community were similar in water-replete and water-limited soils inoculated with either fungus. Several of the bacterial taxa that responded positively to mycorrhizal inocula in water-limited soil belong to lineages that are considered drought-susceptible. Together, these results indicate that mycorrhizal-bacterial interactions can stabilize bacterial communities in water-limited environments. As drought frequency and severity increase, these synergistic relationships may support ecosystem resilience to moisture stress. ### Competing Interest Statement The authors have declared no competing interest.