Effects of Coupled Earthworm and Biochar Composting on Cu Forms and Cu Resistance Genes in Cattle Manure Mixtures

The change patterns of copper (Cu) forms and Cu resistance genes during vermicomposting are unclear. In this study, a coupled system of earthworms and biochar was adopted to treat a mixture of cow manure and corn stalks. The European Community Bureau of Reference (BCR) sequential extraction method combined with Cu gene chip detection technology was employed to investigate the dynamics of Cu fractions and Cu resistance genes throughout the composting process. The results revealed that the changes in earthworm biomass could be described well by a logistic model and that biochar addition significantly increased earthworm biomass. The combined earthworm–biochar treatment promoted more complete mixture decomposition. In the earthworm and biochar treatments, the proportions of acid-extractable and reducible Cu decreased, while those of oxidizable and residual Cu increased. A synergistic effect was observed, which significantly reduced Cu bioavailability. Earthworms and biochar individually reduced the abundance of Cu resistance genes at the end of composting, with the coupled treatment yielding the greatest reduction (except the cusA gene). Furthermore, cusR and pcoD jointly regulated acid-extractable Cu contents; tcrY influenced mainly reducible and residual Cu contents; tcrY and cueR jointly affected oxidizable Cu contents; and tcrY , cusR , and pcoD served as total Cu content indicators. These genes drove Cu form transformation through a dual direct–indirect action pathway. • Adding biochar to vermicompost significantly increased the earthworm biomass. • Cu in cattle manure mixtures was transformed from active to inert forms by coupled earthworm and biochar composting. • The implementation of coupled earthworm and biochar composting reduced the abundance of Cu resistance genes in cattle manure mixtures. • Cu resistance genes could directly or indirectly affect Cu forms through gene interactions.