Food waste-derived fertilizers as sustainable bio-based nutrient sources: Enhancing pak choi growth, nutrient concentrations, and physiological function

• Frass boosts digestate by enhancing nutrient synergy and availability. • Digestate improved pak choi photosynthesis; frass alone reduced growth. • High Na, K, and NH₄⁺ in frass negatively impacted pak choi physiology. • Digestate increased fertilizer efficacy with higher microbial abundance. • Frass-digestate mix with minerals ensures effective, sustainable fertilization. Managing post-consumer food waste requires strategies that enhance circularity within the food system. Anaerobic digestion of food waste produces digestate, a nutrient-rich byproduct, while black soldier fly (BSF) bioconversion generates frass, nutrient-rich larval excreta with potential agricultural applications. This study evaluated food-waste-derived solid digestate and frass as sustainable, bio-based nutrient sources for pak choi in a greenhouse pot trial using a soil–sand mixture. Results demonstrated that sole application of solid digestate improved plant growth and physiological performance, whereas frass alone reduced growth performance, indicating potential phytotoxicity. Specifically, the foliar CO 2 assimilation rate in frass fertilized plants was 13 µmol CO 2 m –2 s –1 whereas plants receiving digestate or mineral fertilizer exhibited higher rates, ranging from 18 to 20 µmol CO 2 m –2 s –1 . Germination bioassays showed greater seedling vigour with digestate than with frass, suggesting differences in nutrient availability and toxicity. The poor growth associated with frass application was attributed to high Na, K, and NH₄⁺ levels compared to digestate, causing possible ionic stress and nutrient imbalances. Illumina metabarcoding (16S/ITS) further revealed that digestate harboured richer bacterial and fungal diversity across phylum, family, and genus levels, likely supporting improved plant performance via bio-stimulatory activity. Importantly, blending 25 % digestate and frass (N) with 75 % mineral N fertilizer maximized biomass yield, demonstrating that up to 25 % of mineral N inputs can be substituted by these bio-based resources. These findings underscore that selectively integrating organic amendments with mineral fertilizers can recycle food-waste streams into effective nutrient sources while maintaining productivity, advancing sustainability, and circular bioeconomy.