Drought exacerbates biodiversity-loss-induced soil organic matter decline by altering soil fungal assemblages

Soil organic matter is essential for maintaining soil fertility, supporting nutrient cycling, and mitigating climate change. However, how soil organic matter responds to the interplay between plant diversity loss and environmental stress such as drought remains poorly understood. To address this knowledge gap, we conducted a three-year experiment manipulating plant species richness (1, 2, 4, and 8 species) and soil moisture conditions (ambient vs. drought) in grassland ecosystems. Our findings reveal that plant species richness had negligible effects on soil organic matter under ambient conditions. In contrast, under drought, reduced plant diversity led to a significant decline in soil organic matter content. Further analyses identified soil fungal diversity and the relative abundance of Glomeromycota-a group of arbuscular mycorrhizal fungi classified as K-strategists, characterized by slow growth and efficient resource use traits-as key contributors to soil organic matter accumulation under drought stress. Taken together, our study demonstrates that drought exacerbates the negative effects of plant diversity loss on soil organic matter and highlights the critical role of soil fungal assemblages in mediating these effects. These insights offer valuable implications for the sustainable management of grasslands in the face of global climate change and biodiversity loss.