Synergistic Remediation of Coastal Wetlands: Identifying Optimal Substrate Amendment and Incorporation Ratio for Enhanced Kandelia obovata Growth and Nutrient Management

Substrate amendment is a promising strategy to enhance phytoremediation in degraded coastal wetlands, yet the selection of optimal materials and their incorporation ratios remains challenging. This study systematically investigated the effects of five amendments, viz., manganese sand, maifan stone, bentonite, iron–carbon (Fe-C), and vermiculite, across an incorporation ratio gradient (5–40%) on the growth of the mangrove, Kandelia obovata, and the physicochemical properties of coastal wetland substrate. Results demonstrated material-specific and dose-dependent responses. Four amendments (vermiculite, Fe-C, manganese sand, and maifan stone) promoted Kandelia obovata growth to varying degrees, while bentonite exhibited significant inhibition. All amendments ensured the physical stability of the substrate. Nutrient removal efficiency followed the order: Fe-C > vermiculite > maifan stone > manganese sand, with 10% Fe-C showing the highest comprehensive nutrient removal. Conversely, bentonite functioned as a nutrient enrichment agent. The amendments differentially influenced redox potential, CO2 emissions, and electrical conductivity, yet all maintained a stable substrate pH. A comprehensive evaluation considering plant growth, nutrient removal, and CO2 sequestration identified maifan stone as the optimal amendment, with the 40% incorporation ratio delivering the most favorable integrated performance. This study provides critical, ratio-specific guidance for selecting and applying substrate amendments in coastal wetland restoration. This study provides critical, ratio-specific guidance for selecting and applying environmentally sustainable amendments, supporting the development of nature-based solutions for long-term coastal wetland restoration.