Enhancing cadmium resistance in Kandelia obovata seedlings via Sonneratia apetala: Rhizosphere microbial restructuring as a key mechanism

As critical blue carbon ecosystems, the mangrove forests in China's Pearl River Delta (PRD) are under severe pressure from cadmium (Cd) pollution and habitat degradation. The introduction of exotic mangrove species, such as Sonneratia apetala, has been adopted as a restoration strategy, yet it may compromise the ecological niche of native species. However, the physiological responses and rhizosphere microbial dynamics of native mangroves under these dual pressures, particularly during the critical seedling stage, are still largely unknown. In this study, pot experiments were conducted to evaluate how co-planting with S. apetala enhances Cd resistance in seedlings of the native species Kandelia obovata. Results showed that co-planting significantly improved the biomass of K. obovata seedlings (roots: +25.0 %; shoots: +23.0 %) and photosynthetic efficiency under high Cd stress (20 mg·kg⁻¹), while reducing Cd translocation to shoots (translocation factor: 0.293 vs. 0.359 in monocultures). Furthermore, co-planting and Cd pollution jointly reshaped the rhizosphere bacterial community, with microbial network analysis identifying three environmentally sensitive modules (Modules 1, 3, and 4) among five. Key bacterial families (Pseudomonadaceae, Comamonadaceae, Moraxellaceae, and Sphingomonadaceae) within these modules presented strong correlations with ammonium nitrogen, nitrate nitrogen, and Cd levels, simultaneously functioning as biomarkers in co-planting systems under high Cd pollution. Partial least squares path modeling demonstrated that the enhancement of Cd resistance in K. obovata seedlings was driven by both rhizosphere microbiome restructuring (total effect = 0.891) and synergistic improvements in photosynthetic performance and nutrient acquisition. These findings demonstrate that interspecific facilitation via S. apetala introduction alleviates Cd toxicity in native mangrove seedlings through multilevel mechanisms, suggesting a potential strategy for improving mangrove seedling establishment and restoration in Cd-polluted environments.