Turning waste into resilience: the effect of black soldier fly larvae frass and wheat straw biochar on bell pepper growth, soil fertility and rhizosphere bacteria under water stress

Abstract Background and aims Drought significantly limits crop productivity, highlighting the need for sustainable soil management. Black soldier fly larvae frass and biochar (a nutrient- and carbon-rich amendment) can increase plant productivity, but their combined effects under drought stress are poorly understood. Methods The experiment was designed to evaluate the effects of three factors on bell pepper ( Capsicum annuum L.) growth, soil fertility, and rhizosphere bacteria: frass type (vegetable- and manure-derived), biochar presence (with or without), and water supply levels (well-watered at 70% and water-stressed at 35% field capacity). The experiment was conducted in a temperature-controlled glasshouse for 10 weeks. Results At harvest, shoot dry weight significantly increased by both frass types under both water supply conditions compared to the control. However, vegetable-derived frass resulted in greater shoot dry weight than manure-derived frass under well-watered conditions, whereas no significant difference was observed under water-stressed conditions. Biochar significantly increased shoot dry weight and shoot N content than without biochar. Frass increased shoot N content, soil NH 4 + -N, NO 3 − -N and dissolved organic carbon. Water stress decreased microbial biomass nitrogen, whereas both frass types increased the microbial biomass nitrogen and carbon. Co-applying biochar with manure-derived frass increased soil pH than frass alone. Under water-stressed conditions, combining frass with biochar enriched the relative abundance of bacterial genera, such as Porphyrobacter and Sphingomonas , which are associated with drought stress resilience. Conclusion This study highlights the potential of integrating frass and biochar as sustainable soil management strategies to enhance crop production under varying water conditions. Graphical Abstract