Bacterial and fungal diversity and species interactions inversely affect ecosystem functions under drought in a semi-arid grassland.

Extreme climatic events, such as drought, can significantly alter belowground microbial diversity and species interactions, leading to unknown consequences for ecosystem functioning. Here, we simulated a drought gradient by removing 30 %, 50 %, and 70 % of precipitation in a semi-arid grassland over five years. We assessed the effects of drought on bacterial and fungal diversity, as well as on their species interactions. We also evaluated the impact of drought on ecosystem individual functions (e.g., plant biomass and microbial activity), and on multifunctionality (EMF). Finally, we linked the drought-induced changes in microbial communities with the variations in EMF. Drought significantly increased fungal diversity and intensified species interactions, but it decreased bacterial diversity and species interactions. Both plant and microbial biomass significantly decreased with increasing drought severity, while microbial activity showed the opposite trend. Only the -50 % rainfall treatment notably reduced EMF. Bacterial diversity and species interactions positively correlated with most ecosystem functions. However, fungal parameters were negatively associated with these functions. Structural equation modeling indicated that bacterial diversity had a strong direct positive effect on EMF (standardized path coefficient: 0.52), and that bacterial diversity was indirectly suppressed by drought through decreasing soil water content and bacterial phospholipid fatty acids (PLFAs). In contrast, fungal species interactions had a significant direct negative effect on EMF with the highest standardized path coefficient (-0.6) and were directly enhanced by fungal diversity. Drought had indirect positive effects on fungal diversity by decreasing soil water content and stimulating fungal PLFAs. Our results highlight the importance of considering soil microbial species interactions when evaluating the ecological impacts of drought. Furthermore, the divergent regulatory pathways of bacterial and fungal communities to EMF suggest that improving ecosystem functionality may be achieved by enhancing bacterial diversity while mitigating fungal species interactions through reducing fungal diversity.