Biochars modify the degradation pathways of dewatered sludge by regulating active microorganisms during gut digestion of earthworms

Biochar can accelerate the degradation and mineralization of organic matter during vermicomposting of sludge and the resulted vermicompost is termed as vermi-char containing active enzymes and microorganisms. However, the mechanisms by which biochars affect vermicomposting of the dewatered sludge during gut digestion of earthworms remain unclear. This study aimed to investigate the effects of biochar on the degradation pathways of organic matter and the involved active microorganisms in dewatered sludge during gut digestion of earthworms. The earthworms Eisenia fetida were fed on three sludge substrates; 1) sludge mixed with 5% corncob biochar, 2) sludge mixed 5% rice husk biochar, and 3) sludge without biochar. The results showed that dissolved organic carbon significantly decreased by 5.65%–21.81% after the 5-day gut digestion of earthworms (P < 0.05) and that biochar addition could accelerate the decomposition of aromatic protein-like substances. Contrarily, the nitrate in earthworms casting with biochars significantly increased by 47.32%–122.64% (P < 0.05) compared with the control. The numbers of active bacteria and eukaryotes in earthworm castings with biochars significantly enhanced by 1.34–1.45 times and 0.45–5.91 times, respectively, than the control (P < 0.05). Active Actinobacteria and Bacteroidetes dominated the castings with biochars significantly enriched by 76.18%–88.83% and 4.02%–18.59% (P < 0.05), respectively, compared to control. As for eukaryotes, the biochars amendment increased Cercozoa abundance by 114.23%–136.31% but decreased Annelida by 55.61%–75.88% in the castings. The partial least squares path model revealed that the biochars could change the content and structure of organic matter in earthworm castings during vermicomposting of sludge by affecting environmental factors, microbial abundance, and microbial community composition.