Sodicity Thresholds Alter Biodiversity–Multifunctionality Relationships Through Fungal Dominance and Microbial Trait‐Based Strategies

Increasing soil sodicity represents a critical threat to global agroecosystem health, but how exchangeable sodium percentage (ESP) modulates relationships between biodiversity and ecosystem multifunctionality (BEF) is unresolved. We surveyed 378 soil samples from 189 paired saline-sodic lands and adjacent farmlands across four major saline-sodic regions of China spanning ~2000 km. Random forest models demonstrated that ESP emerged as the primary abiotic predictor of soil multifunctionality, defining sharp thresholds ~13% for cropped systems and ~44% for natural saline-sodic habitats beyond which BEF relationships undergo fundamental reorganization. These breaks coincide with significant shifts toward fungal dominance within microbial communities. Notably, under hyper-sodic conditions, fungal diversity emerges as essential for sustaining ecosystem functions. Metagenomic and trait-based analyses further characterized three functional dimensions of microbial trait-based strategies-environmental responsiveness, metabolic capacity, and nutrient recycling. We then mechanistically linked microbial life-history strategies to soil multifunctionality. Our results showed that in farmland soils, nutrient recycling was positively associated with multifunctionality, whereas metabolic capacity was negatively correlated with multifunctionality, and in saline-sodic soils metabolic capacity exhibited a positive association with multifunctionality. Collectively, this study establishes ESP as a key regulator of BEF relationships and microbial eco-evolutionary adaptations, providing mechanistic insights for managing saline-sodic soils under escalating climate change.