Coral reef islands face escalating degradation due to extreme substrate infertility (coral sand) and waste management challenges in isolated marine environments. High-oil food waste composting offers a circular economy solution, yet uncontrolled application risks marine pollution through salt leaching into adjacent waters. This study establishes an evidence-based ecological safety threshold for mature high-oil food waste compost amendment in South China Sea coral sand. The compost achieved full maturity (germination index 157.5%, C/N 11.37, oil degradation rate 92.7%, residual oil 1.84%, NH4+/NO3- 0.21, humification index 7.70). Through 45-day controlled pot experiments (0%, 5%, 10%, 15%, and 20% amendment), segmented regression identified 10% as the critical nonlinear threshold. This dosage optimized substrate moisture (16.5-fold increase), alleviated alkaline stress (pH 8.65), and maintained marine-safe salinity (EC 0.53 mS cm-1, below the 1.0 mS cm-1 vegetation stress limit) while maximizing plant biomass and soil enzymatic activity. Mechanistically, 10% amendment uniquely enriched functional bacteria (Solirubrobacter) and saprophytic fungi (Aspergillus and Melanocarpus), constructing balanced microbial co-occurrence networks. Integrated PICRUSt2-enzyme activity correlation validated the coupling of predicted carbon/nitrogen metabolic potential with measured soil enzyme activities (Spearman ρ = 0.74-0.84, FDR q < 0.001). Dissolved organic matter analysis confirmed optimal humification (82.19% humic acid) with stable aromatic core structures, and life cycle assessment verified negligible marine eutrophication burdens (EP: 0.0275 kg PO43- eq t-1). These findings provide the first evidence-based ecological limit for organic amendments in tropical reef island restoration and propose microbial network stability as a correlative biological indicator of coastal ecosystem functional status.
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