Plants Drive Microbial Biomass and Composition but Not Diversity to Promote Ecosystem Multifunctionality in Karst Vegetation Restoration

Natural restoration has emerged as a prominent approach in recent decades for the rehabilitation of degraded ecosystems globally. However, the specific changes and underlying mechanisms by natural restoration that influence the multifunctionality of karst ecosystems remain poorly understood. In this study, soil, litter, and fine root samples were collected from four chronosequence stages of vegetation restoration—grassland (G), shrubland (SH), shrub-tree land (ST), and forest (F)—within a karst ecosystem in Southwestern China. The aim was to evaluate the impacts of vegetation restoration on ecosystem multifunctionality using an averaging approach. The results demonstrated that the indices of C-cycling functionality, N-cycling functionality, P-cycling functionality, and total ecosystem multifunctionality increased as vegetation restoration progressed, along with plant diversity. The structure of plant, bacterial, and fungal communities varied across different stages of vegetation restoration, exhibiting the highest microbial diversity indices in the SH stage. Additionally, the tightness and complexity of co-occurrence networks of bacteria and fungi increased with advancing vegetation restoration, and higher positive links were observed in fungi than bacteria. The four functional indices were significantly and positively correlated with increasing plant diversity, fine root and litter nutrient contents, fine root biomass, microbial biomass, fungal community, enzyme activities, and soil nutrient contents but not with bacterial and fungal diversities. Furthermore, Random Forest model results revealed that plants exerted a significantly greater influence on ecosystem multifunctionality compared to other factors. It is plausible that plants influence soil microbial biomass, fungal community and co-occurrence networks, enzyme activities, and nutrient levels through the input of root and litter nutrients rather than by altering microbial diversity to enhance karst ecosystem multifunctionality. Therefore, initiatives to increase plant diversity are beneficial for sustainable ecological restoration management in the karst regions of Southwestern China.