Black soldier fly larvae frass increases the soil's residual nutrient content and enzymatic activity - a lettuce production trial

The aim of this study was to evaluate the potential agronomic value of black soldier fly larvae frass (BSFF) as an organic fertiliser in short-cycle crops, using lettuce as the test plant. Treatments consisted in applying different fertilisers (BSFF and mineral) at different doses and combinations. The experiment was carried out for 42 days and plants were analysed in terms of biomass production, while the soil was chemically characterised before and after fertilisation, in order to assess the residual nutrient concentrations. In addition, soil microbial activity was assessed through the activity of the enzymes dehydrogenase and beta-glucosidase. The highest yields were obtained with an exclusive mineral fertilisation (162.5 +/- 61.8 g fresh weight) and with a mixture of organic/mineral fertilisation (144.5 +/- 16.8 g) in comparison to exclusive fertilisation with BSFF, probably due to the immobilisation and slow mineralisation rate of the N provided by frass, along with the choice of the short-cycle plant, which requires readily available nutrients. Nevertheless, the BSFF increased the soil's organic matter and residual nutrient content after 42 days of experiment, as well as the enzymatic activity of dehydrogenase and beta-glucosidase, by at least 121 and 24% in the soils fertilised with BSFF, respectively. Thus, despite not being effective as an exclusive fertiliser for a short cycle culture, the BSFF included in a mixture with mineral fertilisation, may compete with exclusive mineral fertilisation with the benefit of improving the sustainability of soil fertility and crop production.