Plant and soil biodiversity sustain root mycorrhizal fungal richness under drought stress.

Mycorrhizal phenotypes arise from interactions among plants, soil biota, and environmental factors, but disentangling these drivers remains a key challenge in ecology. Understanding how these interactions shape mycorrhizal community assembly and stability is essential for predicting and managing these relationships in both natural and agricultural ecosystems. Here, we designed a fully factorial experiment examining how plant and soil biodiversity impact arbuscular mycorrhizal fungal communities under drought conditions. We further examined the role of plant ecological strategies in shaping these communities by including 16 herbaceous plant species along a gradient of plant-mycorrhizal reliance. Specifically, we investigated how plant traits and functional groups affected root-associated arbuscular mycorrhizal fungal richness and composition. Although drought decreased arbuscular mycorrhizal fungal phylogenetic species richness in roots, this effect was mitigated by higher soil and plant biodiversity. Plants with traits indicating high mycorrhizal reliance, such as legumes, displayed lower arbuscular mycorrhizal fungal richness but maintained higher constancy over time and across treatments. Overall, our findings indicate that ecosystems with limited plant and soil biodiversity partially lose their ability to support diverse arbuscular mycorrhizal root colonization under drought conditions. If repeated, such a loss could have severe implications for both immediate plant functioning and long-term soil health. The varied responses of arbuscular mycorrhizal fungal communities to drought in plants with differing ecological strategies suggest diverse fitness outcomes for plants and their symbionts, underscoring the need to integrate plant-symbiont dynamics into ecosystem management approaches to address global change.