Black soldier fly frass from seed waste of nitrogen-rich legumes - How long-term maturation affects the fertilizer properties?

Expanded insect production represents a source of post-breeding residues (frass) that can potentially be used as a soil additive. These types of biofertilizers are carriers of recirculated nutrients, as well as organic matter. In the present study, we investigated whether the bean waste (BW) and pea waste (PW) in the form of crushed seeds and post-production leftovers, naturally rich in proteins, were suitable as a substrate for rearing black soldier fly (Hermetia illucens) larvae. The main objective of this study was to characterize fertilizing properties of the resulting frasses (after 1 month of larvae breeding), as well as to investigate how aerobic maturation (lasted 10 months) would affect their properties. The experiment demonstrated that larval bioconversion increased the concentrations of ammonium and nitrate ions, as well as concentrations of macro- and micronutrients in the frass compared to the substrates. Raw PW frass can already be used as a fertilizer, as indicated by the appropriate ranges of N_tot, EC and C/N values, while raw BW frass had low C/N and high EC, which could contribute to phytotoxic effects. Maturation of frass improved the properties of BW frass, but worsened some characteristics of PW frass, such as an excessive increase in the C/N ratio, which could result in nitrogen immobilization. At the same time, maturation caused further increase in micro- and macronutrients concentration, which meant that both variants of the mature frass were rich in elements necessary for plant development.