Plant and bacterial diversity co-regulate ecosystem multifunctionality in subtropical grasslands of southwestern China.

Grasslands are critical for ecosystem multifunctionality but face severe degradation from anthropogenic pressures. However, previous studies have typically examined plant or microbial diversity in isolation, leaving a critical gap in understanding how their combined effects regulate ecosystem multifunctionality (EMF). In addition, most existing research has been conducted over short timeframes, failing to capture how diversity-EMF relationships may shift across years. To address these gaps, we established a gradient of plant diversity using eight plant species categorized into three functional types (grasses, legumes, and herbs) across four levels of species richness (1, 3, 5, and 7 species) in a field experiment. Results revealed that plant diversity significantly influenced individual ecosystem function especially productivity and support functions across experimental years. The relationship between plant diversity and EMF shifted from negative in 2023 to positive in 2024, with the shift being statistically significant (P < 0.05). By 2024, the 7 S treatment increased EMF by 72.6% compared to monocultures. Plant diversity was strongly and positively correlated with EMF (P < 0.001). Similarly, soil bacterial diversity and richness were also significantly positively correlated with EMF (P < 0.001), whereas soil fungal diversity and richness exhibited no significant relationship with EMF (P > 0.05). Piecewise structural equation modeling (SEM) revealed that plant diversity had a significant positive direct effect on EMF. It also exerted a significant positive indirect effect on EMF, mediated through changes in soil properties. Both plant properties (β = 0.43) and soil properties (β = 0.34) were identified as significant direct drivers of EMF. Collectively, these results indicate that EMF in cultivated grasslands is best predicted by plant and bacterial diversity, alongside key plant and soil properties, rather than by fungal diversity. Our findings demonstrate that scaling up diverse grassland systems offers a synergistic strategy to simultaneously support biodiversity conservation and enhance ecosystem multifunctionality.