Molecular and microbial insights towards understanding the effects of hydrochar on methane emission from paddy soil

Directly returning rice straw to the paddy soil would significantly stimulate methane emission, and hydrochar has potential to be used as soil conditioner. However, the effects of hydrochar on the methane emission from paddy soil and the related mechanisms are still unclear. In the present study, straw-based hydrochar obtained at 200 °C (HC200), 250 °C (HC250) and 300 °C (HC300) and hydrochar after removal of bio-oil at these temperatures (CHC200, CHC250, and CHC300) were prepared and added to the paddy soil. The application of HC200, HC250 and HC300 resulted in the enhanced methane production compared to the control, showing 4.3, 1.6 and 1.5-fold higher methane production, respectively. It was related to the large amount of dissolved organic matter (DOM) released from hydrochar. Excitation-emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC) showed that the hydrochar-derived DOM mainly included humic-like, phenolic and less aromatic structures, and with the increase of hydrothermal temperature, the content of humic-like substances and phenols increased, while biodegradable organics decreased. This was consistent with the maximum methane production by HC200. After incubation, there was no low-aromatic structures observed in the soil leachate, and the residual organics were mainly humus. The EEM-PARAFAC results were supported by compositional characterization of soil leachate by high-resolution mass spectrometry, and the refractory organics released from hydrochar was mainly lignins or (CRAM)-like structures in the range of H/C = 0.8–1.6 and O/C = 0.1–0.5. The organics dissolved from the washed hydrochar was significantly reduced, and some washed hydrochar (CHC250 and CHC300) even inhibited methane emission possibly due to their ability to adsorb organics. Microbial analysis further showed that the increased methane production resulted from hydrochar was associated with the enrichment of Janibacter, Anaeromyxobacter, Anaerolinea and Sporacetigenium. This present study provided a better understanding to the effect of hydrochar on methanogenesis in paddy soil.