Migration and Transformation of Cd in Pig Manure–Insect Frass (Hermetia illucens)–Soil–Maize System

Little is known about the fate of heavy metals in the recycling system of animal manure–black soldier fly larvae (BSFL) transformation-larval frass application. In this work, BSFL-transformed pig manure with different concentrations of exogenous cadmium (Cd) (0, 3, 15, 30 mg kg−1), and the obtained BSFL frass fertilizer were further used in pot experiments of maize planting to explore Cd migration during the whole recycling system. Results showed that Cd addition to pig manure had no significant effects on BSFL growth or BSFL transformation performance. The Cd concentrations in BSFL frass were 10.9–19.8% lower than those in pig manure, while those in BSFL bodies were 2.3–4.0-times those of pig manure. For maize planting, only 30 mg kg−1 of Cd treatment significantly inhibited maize growth. The BSFL frass application (under exogenous Cd treatment) enhanced Cd contents in the aboveground and underground parts of maize (3.3–57.6-times) and those in soil (0.5–1.7-times) compared with CK (no Cd addition). Additionally, 61.2–73.5% of pig manure-sourced Cd was transformed into BSFL frass and the rest entered BSFL bodies. Only a small part (0.31–1.34%) of manure-sourced Cd entered maize plants. BSFL transformation decreased the proportions of weak acid-dissolved Cd from 44.2–53.0% (manure) to 37.3–46.0% (frass). After frass application, the proportions of weak acid-dissolved Cd in soil were further decreased to 17.8–42.5%, while the residual fractions of Cd increased to 27.2–67.7%. The findings provided a theoretical basis for the rational application of BSFL frass fertilizers sourced from heavy-metal-contaminated manure.