Protein- or fat-rich substrates have divergent effects on waste reduction, biomass yield, and gas emissions in black soldier fly bioconversion

The black soldier fly larvae’s (BSFL) yield, nutrient quality, and gas emissions are often difficult to align due to the complicated and variable chemical constituents in substrates. Especially, the synergistic or trade-off mechanism triggered by fiber, fat, and protein components for system optimization is still unclear. Thus, this study evaluated the production efficiency and gas emissions in a BSF conversion system fed with various types of substrates. The treatment included chicken manure-straw substrate (fiber-rich, CMS, reference diet), substitution of 33.3% CMS with tofu dregs (protein-rich, CMST), discarded fish waste (protein-rich, CMSO), slaughtered chicken waste (fat-rich, CMSS), and former food products (fat-rich, CMSF), respectively. Larvae in protein-rich group (CMST and CMSO) maintained higher activity than those in CMS. This greatly increased gross production (+85.1% and +92.2%), bioconversion rate (BCR; +39.9% and +93.6%), and substrate reduction rate (+34.3% and +42.5%), respectively. This enhanced activity also accelerated protein mineralization, leading to elevated NH 4 + accumulation and subsequent NH 3 volatilization, while simultaneously stimulating nitrification-denitrification processes that increased non-CO 2 greenhouse gas (GHG) emissions. Besides the benefits in biomass yield and growth rate, fat-rich group showed greater improvement in insect quality (CMSS) but reduced BCR in CMSF. Additionally, CMSS increased N 2 O and NH 3 but decreased CH 4 , resulting in enhanced non-CO 2 GHG emission. In contrast, CMSF increased N 2 O and CH 4 but decreased NH 3 , and did not affect non-CO 2 GHG emission. The increment of GHG emissions per unit system output in CMSO was lower than other treatments and exhibited a higher N retention rate and maturity of frass. Overall, substrate nutrition components differentially drive BSFL conversion. CMSO (protein-rich animal waste) optimally balanced biomass production, GHG reduction, and waste minimization. From a GHG mitigation perspective, organic waste composition should combine readily utilizable animal-derived protein with structuring agents (e.g., straw) that ensure favorable aeration, while avoiding excessively fat-rich and poorly aerated mixtures that increase GHG emissions per unit product. In the future, the recombination of substrate for BSFL production should target the market requirements (e.g., waste reduction, yield maximization, high-quality insect) to achieve the largest circular economy benefits.