Elucidating the role of earthworms in N2O emission and production pathway during vermicomposting of sewage sludge and rice straw.

Vermicomposting is a sustainable option for the recycling of biodegradable organic waste. However, it also produces nitrous oxide (N2O), which is a highly potent greenhouse gas. In this study, the N2O stable isotope and functional genes for nitrogen cycling were determined to investigate the sources of N2O during vermicomposting. The results showed that vermicomposting promoted the organic degradation and nitrogen nitrification, and the presence of earthworms increased the emission of N2O during vermicomposting compared to that during the control treatment with no earthworms. The site preference analysis of N2O stable isotope showed that both nitrification and denitrification were present during the early stages of vermicomposting, while nitrification was the dominant contributor to N2O production in the later stages. Moreover, earthworms increased the gene copies of amoA, and stimulated the nitrifying bacteria, and hence, increased the N2O emission via nitrification. In addition, the activity of earthworms reduced the gene number of nosZ during vermicomposting, while the denitrification was the main source of N2O in the earthworm gut, as the conditions inside the gut inhibited nosZ. Overall, nitrification was the major pathway (55.8-88.7 %) for N2O production, which was promoted by the introduction of earthworms through nitrification.