Towards circularity for agro-waste: Minimal soil hazards of olive pomace bioconverted frass by insect larvae as an organic fertilizer.

As global populations escalate and the demand for food and feed intensifies, the generation of agri-food waste is becoming an increasingly critical issue. Addressing this challenge is crucial for optimizing food production and advancing sustainable waste management practices. In this context, insects, including the Black Soldier Fly (BSF, Hermetia illucens), present opportunities for circularity through the bioconversion of organic waste. Olive pomace (OP), a by-product of the olive oil industry, is known for its phytotoxic properties due to its high phenolic content and acidic pH. Using BSF for OP bioconversion could mitigate the environmental disposal of this by-product while producing valuable resources such as protein, fats, and insect frass. Insect frass is the excrement of insects that can be used as an entomofertilizer. Building from BSF feeding on OP, this study aimed to evaluate the safety of applying the resultant frass in soil amendment applications for the first time. Here are explored the effects of olive pomace-derived frass (OP-BSF) on soil health and plant growth by evaluating the soil model invertebrate Enchytraeus crypticus and phytotoxicity bioassays using the forage crop ryegrass (Lolium perenne) and the agricultural species broccoli (Brassica oleracea). Our methodologies included direct soil applications and aqueous extract tests, with a range of OP-BSF concentrations (from 0 to 9.8% w/w) and observation periods (2 and 32d). Despite initial concerns over the phytotoxic nature of OP, our findings revealed that OP-BSF did not adversely affect the survival of E. crypticus and even enhanced its reproductive success. Furthermore, while higher frass concentrations elicited some adverse effects on plant germination and growth, these were limited to levels unlikely to be used in practical applications. The outcomes of this study suggest that OP-BSF could be safely integrated into the soil as a fertilizer, promoting a circular bio-economy by converting waste into economically and environmentally friendly products. This study underscores the potential of insects in transforming waste management paradigms and enhancing food security, particularly in regions like the Mediterranean, thus contributing to a more sustainable and resilient agricultural sector.