Epsilon Archive for Student Projects

Abstract

Black soldier fly larvae (BSFL) bioconversion has emerged as a sustainable technology for addressing both food security and biowaste management challenges. A valuable by-product of this process, frass, has been increasingly recognized as a biofertilizer with plant growth-promoting and biostimulant properties. However, the microbial mechanisms underlying these effects remain poorly understood. This study aimed to investigate whether siderophore-producing Pseudomonas spp. contribute to the documented biostimulant properties of frass. It was hypothesized that plant growth promotion by frass is, in part, mediated by frass-associated microorganisms, and specifically that siderophore-producing Pseudomonas spp. in frass enhance plant iron nutrition. Iron nutrition is essential for various plant metabolic processes but can be limited by low bioavailability of iron in the soil. The study was conducted in two phases. In Phase 1, pig manure was bioconverted by BSFL, and the resulting frass was characterized and tested for phytotoxicity. Frass was also screened microbiologically for Pseudomonas spp. abundance, and isolates were tested for siderophore production. In Phase 2, a greenhouse experiment was conducted using four treatments: sterile soil (control, C), sterile soil with sterile frass (X), sterile soil with fresh frass (Y), and fresh soil with fresh frass (Z). After 38 days, plant growth parameters were assessed, and rhizosphere samples were analyzed for Pseudomonas spp. abundance and isolates were tested for siderophore production. The results showed that frass contained Pseudomonas spp. of which 59 % of screened isolates were positive for siderophore production. Soil amendment with BSFL frass promoted kale growth by up to 195 % increase in above-ground biomass compared to the negative control, although effects varied depending on whether frass and soil were sterile or fresh. Up to 93.3 % of the screened Pseudomonas spp. isolates from the rhizosphere soil were siderophore producers. These findings support the hypothesis that plant growth promotion by frass is, in part, mediated by frass-associated microorganisms. However, the specific role of siderophore-producing Pseudomonas spp. in enhancing plant iron nutrition remains unresolved based on the current data. To the authors' knowledge, this is the first study to examine the potential role of siderophore-producing Pseudomonas spp. from BSFL frass in plant growth promotion. If confirmed in future studies, this mechanism may be particularly relevant for the application of frass-based biofertilizers in iron-limited agricultural soils.