Enhanced nitrogen retention with a two-stage composting strategy: Synergistic effects of exogenous additives and eco-friendly insect on microbial regulation.

The increasing mushroom residues present a significant challenge to environmental sustainability. Aerobic composting is an effective approach for utilizing mushroom residue, but traditional methods suffer from issues such as severe nitrogen loss. Unlike previous studies that have primarily focused on single additives, this study introduced a modified two-stage composting strategy combining microbial agent-biochar (RB) and Protaetia brevitarsis larvae (PBL) to enhance nitrogen transformation and retention. Four treatments were tested over 40 days: CK (no additives), RB (0.01% microbial agent and 12% biochar), PBL (CK + 1200 PBL), and RBPBL (RB + 1200 PBL). The results showed that the combined addition of RB and PBL exhibited the best performance, with the most significant nitrogen retention efficiency. Compared to CK, RBPBL significantly increased total Kjeldahl nitrogen (TKN) and nitrate nitrogen (NO 3 - -N) contents by 50.52% and 32.31%, respectively, and facilitated the conversion of ammonium nitrogen (NH 4 + -N) into stable organic nitrogen. Meanwhile, N 2 O and NH 3 cumulative emissions were reduced by 20.58% and 23.48%. Additionally, metagenomic analysis revealed that RBPBL enriched functional microbes (e.g., Luteimonas , Xylanimonas ) and strengthened positive microbial interactions, shifting the community toward a more cooperative relationship. RBPBL also upregulated the abundance of genes associated with nitrification ( amoA , amoB, amoC, hao ) and ammonia assimilation ( gltB , gltD ), while inhibiting the contribution of denitrifying bacteria to nitrogen loss, thereby improving nitrogen transformation pathways and retention. This research elucidates the microbial mechanisms of nitrogen cycling and improves nitrogen retention in insect-assisted mushroom residue composting, thereby contributing to sustainable waste valorization and circular bioeconomy. • Two-stage RBPBL composting increased TKN by 33.51% and reduced N 2 O emissions. • RBPBL enhanced NH 4 + -N conversion into stable organic nitrogen, reducing N loss. • RBPBL enriched functional microbes ( Luteimonas , Xylanimonas ) and strengthened positive interactions. • RBPBL upregulated nitrification and ammonia assimilation genes. • This study reveals microbial N-retention mechanisms for sustainable mushroom residue valorization.