Manure-derived black soldier fly frass enhanced the growth of chilli plants (Capsicum annuum L.) and altered rhizosphere bacterial community

Sustainable manure management is crucial for minimising environmental impacts as the livestock industry expands to meet the increasing demand for protein. Black soldier fly ( Hermetia illucens L.) larvae (BSFL) farming is an emerging waste management method that efficiently processes large volumes of organic waste, including manure, to produce valuable protein, oil and chitin products. Frass, a nutrient-rich by-product of black soldier fly farming, has potential as an organic fertiliser. However, research has primarily focused on frass derived from food waste, with little exploration of manure-derived BSFL frass. This study aimed to determine whether frass derived from manure could enhance the growth of chilli ( Capsicum annuum L.) over 14 weeks under controlled glasshouse conditions. The impacts of manure-derived BSFL frass on soil properties and soil bacterial communities were characterised. The results indicated that a 0.6 % w /w application rate yielded the highest chilli plant biomass, with reduced growth observed at higher rates. The enhanced growth at optimal manure-derived BSFL frass application rates was due to increased nitrogen content, whereas reduced growth at higher rates was likely caused by phytotoxicity from not completely decomposed frass. Soil microbial biomass carbon and nitrogen also increased with manure-derived BSFL frass, implying microbial carbon and nitrogen immobilisation. Additionally, the changes in pH and nutrients due to manure-derived BSFL frass caused shifts in the bacterial community in the chilli plant rhizosphere, enriching the relative abundance of bacteria with potential growth-promoting properties. This study highlights the potential of integrating manure into black soldier fly waste management processes, demonstrating that manure-derived BSFL frass can be used as an organic fertiliser with circular economy benefits. • Manure-derived BSFL frass is a potential organic soil improver for crop production • Increased growth of chilli plants was primarily due to the provision of nitrogen from manure-derived BSFL frass • A positive link between chilli plant growth and plant growth-promoting rhizobacteria was observed • Decreased plant growth above 0.6 % w/w may be associated with non-mature manure-derived BSFL frass