Dual Engines of Adsorption and Biodegradation: Ammonium Nitrogen Removal and Mechanism Analysis by EM-Modified Corn Straw Biochar in Aqueous Solution

Agricultural ammonium pollution from farmland drainage and low-value crop straw utilization are two critical rural environmental problems that cannot be solved by single remediation approaches. Herein, a novel composite was prepared by immobilizing effective microorganisms (EM) on corn straw biochar to construct a synergistic adsorption–biodegradation system, and its nitrogen removal mechanism was systematically investigated at structural and molecular levels. Metagenomic analysis detected a complete set of heterotrophic nitrification–aerobic denitrification (HN-AD) functional genes (amoA, hao, napA, nirK, norB, nosZ) in the isolated strain Bacillus thuringiensis A1, revealing the genetic potential of this strain for ammonium biodegradation. EM modification optimized biochar pore structure and increased the equilibrium adsorption capacity to 1.215 mg/g, which was 66.4% higher than that of pristine biochar (0.73 mg/g). Sterilization control tests indicated that physicochemical adsorption occupied the dominant position in ammonium removal, while microbial biodegradation acted as an auxiliary removal pathway. Importantly, the synergistic relationship between the two pathways should be interpreted cautiously, since autoclaving may subtly alter biochar physicochemical properties, and direct paired characterization of viable composites before and after sterilization is technically unavailable. Kinetic and thermodynamic results further validated the improved adsorption performance after modification. Overall, EM immobilization promoted ammonium adsorption via pore optimization, while pore-confined microbes achieved sustainable HN-AD biotransformation, jointly realizing synergistic nitrogen removal. This study provides a mechanistic reference for the optimized design and application of biochar–microbe composites in agricultural nitrogen pollution control.