Cow dung biochar amendment affects the bioconversion of spent mushroom substrate and fruit waste by black soldier fly (Diptera: Stratiomyidae) larvae.

Spent mushroom substrate (SMS) tends to emerge as an extensive problem in mushroom-consuming areas, which needs an immediate solution under a bioresource generation circular economy model. Meanwhile, biochar addition has shown significant improvements in waste bioconversion rates, which is less explored with the utility of black soldier fly (Hermetia illucens) larvae (BSFL) in such waste (SMS) valorization. Thus, in our study, bioconversion of SMS and fruit waste (FW) mixtures (30, 70, and 100%), spiked with cow dung biochar (BC) with different doses (2 and 5%), through BSFL was investigated. Changes in waste physicochemical properties and BSFL growth parameters were recorded until the appearance of flies in the setups. BSFL triggered a decrease in total organic carbon (4.22-51.7%), and C/N ratio (13.7-61.8%), while an increase in total nitrogen (2.8-25.6%), total phosphorus (13.7-62%) and total potassium contents (34.7-358%) in the waste mixtures. The GI value and NH₄⁺/NO₃^- ratio (0.24-0.62 g kg^-1) of residual waste (frass) suggested the requirement for further maturity of frass through aerobic composting. Best results for larval growth (133.96%), feed conversion ratio (FCR) (2.35), waste reduction index (WRI) (0.70), and relative growth rate (RGR) (0.06 g d^-1) were obtained in 100% FW with 2% BC while waste reduction (WR) (56.31%) was found in 100 FW (5%BC). BSFL were also analysed for pro-N and lipid contents, and respective values were found in the ranges of 4.72-6.13% and 6.76-19.27%, suggesting the feasibility of larval biomass as animal feed. In summary, BSFL could be a valuable tool to utilize organic wastes for generating a circular bioeconomy. The effect of BC addition on functional microbial communities during bioconversion may be taken as a problem for future research.