Recirculation enhances the fertilizer efficiency of black soldier fly larvae frass

Frass produced from biowaste treatment with black soldier fly (BSF) larvae is a promising fertilizer, but its short bioconversion time can lead to phytotoxicity. A recently proposed method called ‘frass recirculation’ was found to improve frass quality by reintroducing fresh frass into subsequent rearing cycles. This study evaluated the effects of frass produced through different levels of recirculation on soil nitrogen (N) dynamics and plant performance. Five frass types were tested: fresh frass and frass recirculated once, twice, three, or four times. In the first experiment, the fertilizers were applied to the soil, and N mineralization was monitored over 84 days. The same treatments were then used to cultivated pak choi, where plant growth and photosynthetic activity were assessed. Frass recirculated four times had a higher intrinsic concentration of N-NH₄⁺ and produced significantly higher soil mineral N after 84 days compared to fresh frass. This increased N availability allowed for lower application rates while still providing sufficient nutrients for supporting plant growth. Pak choi fertilized with highly recirculated frass achieved biomass comparable to that of plants supplied mineral fertilizer. In addition, nutrient concentrations and accumulation in plant tissues were similar between the highly recirculated frass and mineral fertilizer treatments. Notably, frass-fertilized plants exhibited higher chlorophyll content and greater CO₂ assimilation rates, indicating an enhanced photosynthetic performance. Overall, frass recirculation enhanced N mineralization and improved the fertilization efficiency of BSF frass. These results demonstrate that recirculated frass have fertiliser potential comparable to that of mineral fertilizers, highlighting its potential as a sustainable organic alternative for greenhouse crop production.