Mg-modified biochar mitigates carbon and nitrogen losses and enhances humification by reshaping core microbial community function in distilled grain waste composting

Although biochar has been widely used to regulate carbon-nitrogen transformations and promote compost humification, its performance is highly variable across feedstocks and pyrolysis conditions, and current studies largely emphasize emission mitigation or nitrogen conversion alone. This study introduces Mg-modified biochar (MMB) into distilled grain waste (DGW) composting to quantify carbon-nitrogen synergistic retention and humification. Three treatments were established (CK, pristine biochar (T1), and MMB (T2)) to systematically evaluate their impacts on compost maturity, coupled C/N transformations and losses, and humification dynamics. Results indicate that T2 significantly accelerated the composting process, reducing maturation time by 7 days compared to T1. Self-Organizing Map (SOM) analysis visually confirmed its faster succession trajectory. Compared with the CK treatment, T1 reduced nitrogen and carbon losses by 26.42% and 13.24%, respectively. Notably, T2 exhibited superior performance in carbon-nitrogen synergistic retention, achieving reductions of 30.15% and 19.61% in nitrogen and carbon losses, while increasing humic acid carbon (HA-C) content by 97.72%. Furthermore, T2 significantly reshaped the topological structure of microbial communities through increased network complexity and modularity, while directionally enriching lignocellulose-degrading and nitrogen-retaining functional communities represented by Thermobifida and Luteimonas . The XGBoost-SHAP machine learning model further elucidated that MMB drives ecological functions by optimizing physicochemical conditions (pH, EC) and differentially regulating key species. This primarily involves inhibiting Alcaligenaceae to control nitrogen loss and promoting Saccharomonospora proliferation to accelerate humification. This study provides clear microbiological mechanisms and practical evidence for developing efficient, low-emission technologies for DGW resource utilization. • Effects of Mg-modified (T2) vs pristine (T1) biochar on composting were studied • Self-organizing map confirmed T2 reached maturity 7 days earlier than T1 • Nitrogen and carbon losses in T2 decreased by 30.15% and 19.61% versus CK • HA-C content in T2 increased by 97.72%, the highest among treatments • T2 reshaped the microbial topology and enriched functional communities