Abstract: To address the limitation of insufficient carbon sources in conventional ecological gravel beds treating effluent from wastewater treatment plants, which restricts their denitrification efficiency, alkali-modified corn cob (CC-AL) was prepared as enhanced denitrification media. This material was combined with steel slag to construct a novel enhanced ecological gravel bed. The study focused on investigating the effects of media type and hydraulic retention time (HRT) on nitrogen and phosphorus removal efficiency in actual effluent from wastewater treatment plants. Microbiome characteristics were analyzed using metagenomic methods. Results indicated that CC-AL demonstrated superior carbon release capacity compared to unmodified corn cob (CC-UN). The CC-AL ecological gravel bed outperformed the CC-UN bed in denitrification and phosphorus removal, achieving removal rates of 64.72±4.20% for TN, 85.93±2.18% for NO₃⁻-N, and 31.32±1.25% for TP (at HRT 6h). At HRTs of 6h, 4h, and 2h, TN and NO₃⁻-N removal rates gradually decreased, but TP removal peaked at 40.08±2.30% when HRT was 4h. CC-AL and CC-UN enhanced denitrification efficiency in the ecological gravel bed by intensifying the denitrification pathway, increasing the relative abundance of denitrifying bacteria (Bacteroidetes, Thermodesulfobacteriota, etc.) and the concentration of various denitrifying reductases. EC.1.7.99.- and EC.1.7.5.1 nitrate reductases in the CC-AL ecological gravel bed were 2.10-fold and 2.35-fold higher than in the control group. At a HRT of 6 hours, the CC-AL ecological gravel bed achieved load reduction rates of 5.12 g N/(m²·d) for TN and 0.46 g P/(m²·d) for TP, demonstrating high nitrogen and phosphorus removal efficiency and application potential. Considering both nitrogen and phosphorus removal efficiency, CC-AL is the preferred filler material for the ecological gravel bed, with an optimal HRT of 6 hours.