Methane emission mitigation in hypoxic freshwater triggered by oxygen-carrying dual-modified sediment-based biochar

Abstract

Anthropogenic deoxygenation of freshwater systems has attracted widespread global attention due to its potential to threaten ecosystem biodiversity, increase internal nutrient loading, and enhance greenhouse gas emissions. However, the exploration of effective, economical, and green solutions available to combat hypoxia and its impacts on freshwater remains a longstanding challenge. Herein, oxygen-carrying dual-modified sediment-based biochar (O-KN-SBC) was developed as a new approach to sustainably restore hypoxic freshwater and synchronously mitigate methane (CH4) emission. The O-KN-SBC was observed in column incubation experiments to elevate dissolved oxygen (DO) concentration by 6 times and reduce CH4 emission by 18 times compared to the control. Electrochemical analysis confirmed the ability of modified biochar to promote electron transfer and its role as an electron shuttle to mediate biological processes. Microbial analysis suggested that the restored oxygen-enriched ambiance could greatly promote the expression of pmoA gene with an 8-fold increase and the shift of methanotrophs from Type I dominance to Type I and II co-dominance. Last but not least, partial least squares path modeling revealed that in the presence of O-KN-SBC, CH4 fluxes had strong associations with functional genes namely pmoA and mcrA, which mainly depended on the ambient water quality (especially DO, nitrate, and organic carbon). Our study not only develops a promising approach to sustainably address hypoxia and warming related issues in freshwater systems, but also demonstrates the roles of water quality improvement and microbial community reconstruction in the greenhouse gas emission reduction process.