Microbial mechanisms enhancing humification and nitrogen retention during co-composting of black soldier fly frass and pig manure

The bioconversion of food waste by black soldier fly larvae (BSFL) has garnered increased attention in recent years. However, nutrient-rich BSFL frass produced after larval harvesting does not meet the standard for organic fertilizer and requires secondary composting. This study investigated the co-composting of BSFL frass with different types of pig manure, including fresh (FR) and stored for 10 days (FM), to explore the microbial mechanisms enhancing humification and nitrogen retention using rice bran as a bulking agent. Results indicated FM achieved the longest thermophilic phase, exceeding 50 °C for 15 days and 55 °C for 8 days, along with the highest germination index (64.41 ± 2.13 %) and nitrogen content (4.09 ± 0.21 %). Ultraviolet–visible (UV–Vis) spectroscopy results demonstrated that FM facilitated the most effective humification process, with three-dimensional fluorescence spectrometer (3D-EEM) analyses confirming a greater humic acid formation. Microbial profiling revealed a predominant bacterial succession, with key populations including Bacillus spp., Micrococcales, Saccharomonospora, and uncultured Sphingobacteriaceae. Functional predictions based on PICRUSt2 analysis indicated nitrogen assimilation and fixation were enhanced, while denitrification was suppressed, thereby minimizing nitrogen loss. This study provides a viable strategy for integrating insect-based bioconversion with livestock manure composting to achieve high-value organic waste recycling.