Soil amendment with insect frass and exuviae affects rhizosphere bacterial community, shoot growth and carbon/nitrogen ratio of a brassicaceous plant

Aims: In terrestrial ecosystems, deposition of insect frass and cadavers in the soil influences soil characteristics, including microbial community composition, with consequences for plant growth and development. Insect frass and exuviae are also a major residual stream from insect production for food and feed, that may be used as soil amendment. However, only few studies have thoroughly examined the effect of soil amendment with insect frass and exuviae on rhizosphere microbial communities and plant growth. Methods: We studied the effects of soil amendment with frass and/or exuviae originating from three insect species, Tenebrio molitor, Acheta domesticus, and Hermetia illucens, at three different concentrations, compared to synthetic fertiliser. At several time points we analysed the rhizosphere bacterial community and assessed multiple plant-growth parameters of a brassicaceous plant. Results: Soil amendment with frass and/or exuviae improved plant growth at least as well as synthetic fertiliser, A. domesticus exuviae having the strongest impact. The origin (insect species), type (frass or exuviae) and concentration of soil amendment influenced the effects on plant traits. The rhizosphere bacterial community differed between amended and unamended soil. Bacterial genera that contain plant growth-promoting species were more abundant in the rhizosphere of plants grown in amended soil. Conclusions: Addition of insect frass and/or exuviae to the soil differentially affects the bacterial rhizosphere community and promotes plant growth in these soils, underlining their unique roles in the aboveground-belowground feedback loop, and their potential use as soil amendment in circular agriculture.