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.

Phosphorus (P) limitation is prevalent in terrestrial ecosystems. Plants can improve soil P availability through the exudation of organic acids and symbiotic interactions with microorganisms. However, associations between different plant functional groups and phosphorus cycling in P limited karst ecosystems remain poorly understood. To investigate this, the exudation rates of oxalic, citric and acetic acids from fine roots, the contents of carbon, nitrogen, and P in leaves and fine roots, and the contents of oxalic, citric and acetic acids, total P, available P (AP), and microbial biomass P in rhizosphere soils were measured across different plant functional groups in a karst ecosystem in southwestern China. Additionally, the activities of acid and alkaline phosphatases were also analyzed, as well as the relative abundance, community structure, diversity, and co-occurrence network patterns of arbuscular mycorrhizal fungi (AMF) and alkaline phosphatase-encoding (phoD) gene-harboring bacteria. The results showed that both the exudation rates and the contents of organic acids and AP were highest in the tree group, followed by the shrub and grass groups. The AP content of the legume group was significantly higher than that of the non-legume group. The exudation rates of oxalic acid were significantly greater than those of citric and acetic acids. AMF diversities were highest in the shrub and legume groups. The diversities of phoD-harboring bacteria decreased from the tree group to the shrub group and then to the grass group, yet there were no significant differences between the legume and non-legume groups. The communities of both AMF and phoD-harboring bacteria exhibited significant differences among these plant functional groups. The prevalent genera of phoD-harboring bacteria across all groups were Pseudomonas and Halomonas, with Halomonas being particularly prevalent in the legume group. The AMF community was dominated by Glomus, which attained its highest relative abundance in the tree and legume groups. Furthermore, the increased exudation rate and content of oxalic acid were associated with higher relative abundances of Glomus in AMF and Pseudomonas and Bacillus among phoD-harboring bacteria. Structural Equation Model (SEM) analysis demonstrated that plant-exuded organic acids, especially oxalic acid, were positively associated with P availability indirectly through their linkages with the diversity and abundance of AMF and phoD-harboring bacteria. The crucial role of oxalic acid was particularly prominent in the tree and legume groups. Our findings suggest that screening AMF and phoD-harboring bacteria with highly efficient P transformation activity and inoculating them into the rhizosphere of plants with high oxalic acid exudation could help improve plant resilience to P limitation and support sustainable restoration in karst ecosystems.