Ferrihydrite-biochar augments ecological restoration in reclaimed coral islands via dual pathways: Antioxidant system stimulation and microbial metabolic mediated C/N cycling.

Reclaimed coral islands are facing serious challenges of vegetation restoration and ecological function reconstruction due to the salinity stress and nutrient scarcity. Ferrihydrite-, goethite-, magnetite-, and hematite-loaded biochars (FBC, GBC, MBC, HBC) were synthesized to enhance ecological restoration. Pot experiments with Ipomoea pes-caprae showed FBC outperformed controls and raw biochar, boosting plant fresh weight (63.9 %), height (18.1 %), and chlorophyll (111.3 %) while elevating SOD (407 %) and POD (143 %) activities, reducing MDA (41.2 %). FBC increased soil organic matter, enriched Micropruina and Pseudomonadota microbes, and enhanced microbial community stability. Functional genomics revealed upregulated glycolysis, TCA cycle, and nitrate to ammonium genes under FBC. MBC activated denitrification, removing nitrate effectively but causing nitrogen loss that limited chlorophyll synthesis. Iron oxide types critically influenced soil-plant-microbe interactions, with FBC balancing nutrient regulation, stress tolerance, and microbial-driven carbon/nitrogen cycling. This study revealed the effects of iron oxide types on soil-plant-microorganisms, and provided an integrated improvement strategy for coral island ecological restoration that balanced nutrient regulation, stress tolerance enhancement, and microbial function strengthening.