Fertilizer efficiency and induced chitinase activity of frass versus frass biochar amended to soil

Insect production of “black soldier fly” (BSF) larvae is an efficient and sustainable method to convert residual biomass into useful products. A byproduct from insect production, consisting of larval excrement, residual feed and larval exoskeletons, known as “frass,” contains essential macro- and micronutrients and can be advantageously used as fertilizer in agriculture. In addition to nutrients, frass may also contain plant biostimulants and beneficial microorganisms that may have pathogen suppressive effects. However, another potential value enhancement of frass is converting it into biochar via pyrolysis for carbon sequestration. In this study, we compare the effects of frass and the corresponding frass biochar on growth and nutrient uptake by wheat. In addition, we investigated the effect on chitinase activity as an indicator of the potential pathogen suppressive effects of frass compared to frass biochar. A pot experiment with wheat comparing the fertilizer efficiency of frass and frass biochar showed that frass was an efficient P fertilizer, resulting in comparable yields as to the NPK treatment given that N fertilizer was co-applied. In comparison, frass biochar also increased yields compared to the negative control, but not to the same extent as the raw frass. In an additional rhizobox setup, zymography was used to investigate the spatial distribution of chitinase activity in the rhizosphere of wheat. Chitinase activity was induced by frass application, but not by frass biochar, suggesting that the potential pathogenic suppressive effect of frass application is annihilated during pyrolysis. Frass could be an efficient biobased fertilizer, but further investigations into the effects on how frass affects the microbial processes in soil are needed. Frass biochar holds the potential for carbon sequestration and may function as a good soil conditioner. However, this might be at the cost of a more valuable product - the raw frass.