From feed to field: substrate-specific black soldier fly (Hermetia illucens) frass profiles modulate soil microbiota and tomato performance

Abstract

Background and aims Insect frass, a by-product of insect farming, consists of larval faeces, insect remains, and residual feed. It contains agronomically important nutrients and diverse microbiota, making it a promising organic fertiliser. However, its utilisation as fertiliser has only been tested for few frass types from different feed materials, with little attention to their influence on soil microbiota or application mode (solid or liquid), which we address in this study. Methods We investigated frass from black soldier fly larvae (BSFL, Hermetia illucens ) reared on rapeseed cake (RC), potato peelings (PE) or chicken feed (CF) as fertiliser for tomato cultivation and their impact on soil microbiota. Elemental analysis of pure frass and microbial community profiling of the frass/soil blends were performed to explore potential mechanistic explanations for observed differences. Results The frass type exhibited distinct nutrient profiles and effects on total above-ground dry green biomass and total tomato fruit weight. RC frass had the highest NPK levels and lowest C/N ratio, whereas CF resulted in the highest tomato fruit weight, followed by RC, while PE performed poorly. Liquid forms produced higher total above-ground dry green biomass, especially with RC frass. Frass type also significantly impacted both bacterial and fungal community variation. Conclusion Our study provides insights into the effectiveness of different types and forms of frass fertiliser in tomato pot experiments and highlights its economic potential and influence on soil microbiota. The versatility of frass underscores the importance of tailoring frass for specific applications based on its nutrient profile and microbial community. Graphical abstract Frass derived from black soldier fly larvae reared on chicken feed, potato peelings, and rapeseed cake was collected (A) and subjected to elemental analysis (B). The impact of frass as a fertiliser for tomato cultivation was investigated in both solid and liquid forms (C). The influence of frass on bacterial and fungal soil microbiota at different sampling depths was assessed via amplicon sequencing (D).

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