<p>Methane production from anaerobic digestion of chicken manure requires addressing the challenges posed by high nitrogen-containing substrates. One potential solution is the use of modified biochar to adsorb inhibitors and enhance the anaerobic digestion process. This study focused on preparing magnesium-modified biochar (Mg-BC), iron-modified biochar (Fe-BC), and magnesium-iron modified biochar (Mg/Fe-BC) through specific methods, and comparing their performance in simulated solutions for ammonia nitrogen adsorption. The results indicated that Mg/Fe-BC exhibited superior specific surface area, crystallinity, and functional groups compared to Mg-BC and Fe-BC. Its maximum adsorption capacity (87.83&#xa0;mg/g) surpassed that of Mg-BC (26.77&#xa0;mg/g) and Fe-BC (14.57&#xa0;mg/g). Furthermore, the addition of 2%, 3%, and 5% Mg/Fe-BC to the biogas slurry resulted in NH₄⁺-N removal efficiency of 16.38%, 32.85%, and 38.57%, respectively. In the anaerobic digestion test using chicken manure, adding 2%, 3%, and 5% Mg/Fe-BC effectively mitigated ammonia inhibition, leading to a substantial increase in cumulative methane production by 116%, 196%, and 229%, respectively. The addition also decreased the oxidation reduction potential, promoted the decomposition of volatile fatty acids and COD, and kept ammonia nitrogen within a final concentration range of 2.67–3.63&#xa0;g/L, which helped maintain the stable operation of the system. Mg/Fe-BC facilitated the enrichment of acetogenic bacteria and enhanced the abundance of predominant archaea, including <i>Methanosarcina</i> and <i>Methanosaeta</i>. These findings provided valuable insights into the ammonia nitrogen adsorption characteristics of Mg/Fe-BC and its effectiveness in enhancing methane production from chicken manure anaerobic digestion.</p> Graphical Abstract <p></p>