Modified biochar mitigates nitrogen loss in distilled grain waste composting by modulating microbial community...

Pristine biochar (DB)-assisted composting can enhance product maturity and mitigate nitrogen loss, but its efficacy varies and is limited by feedstock variability and preparation conditions, highlighting the need for surface modifications to optimize performance. This study systematically investigated the effects of DB and KOH-modified biochar (DBK) on compost maturity, nitrogen loss, and the related microbial mechanisms during the composting of distilled grain waste, using a group without biochar addition (D) as the control. Results indicated that DBK exhibited a specific surface area of 644.33 m²/g and was rich in pore structures and functional groups. DBK significantly promoted compost maturity, with the seed germination index (GI) reaching 70 % by 23 d and biological nitrification occurring earlier (16 d). Additionally, compared to D and DB, DBK reduced nitrogen loss by 34.13 % and 10.47 %, respectively. DBK accelerates critical nitrogen transformation processes by increasing the abundance of nitrogen-fixing bacteria and associated functional genes. Neutral community modeling and symbiotic networks indicated higher microbial community complexity and stochasticity, thereby promoting functional redundancy and improving nitrogen retention. Furthermore, Actinomadura and Chryseolinea were identified as key microbial drivers of nitrogen transformation, with their nxrABC and hao genes playing crucial roles in establishing efficient 'microbe-gene' synergistic mechanisms. Finally, economic analysis indicated that DBK generated a net profit of up to 63.63 RMB/t. These findings provide a theoretical basis for using modified biochar to promote maturity and control nitrogen loss during composting.