Water quality improvement and consequent N2O emission reduction in hypoxic freshwater utilizing green oxygen-carrying biochar

Declines in dissolved oxygen (DO) levels in aquatic systems worldwide negatively influence biodiversity, nutrient biogeochemistry, drinking water quality, and greenhouse gas emission. As a response, oxygen-carrying dual-modified sediment-based biochar (O-DM-SBC) as a green and sustainable emerging material was utilized for simultaneous hypoxia restoration, water quality improvement, and greenhouse gas reduction. Column incubation experiments were carried out using the water and sediment samples from a tributary of the Yangtze River. The application of O-DM-SBC effectively increased the DO concentration from ~1.99 mg/L to ~6.44 mg/L and decreased the concentrations of TN and NH4+-N by 61.1 % and 78.3 %, respectively, during the 30-day incubation period. Moreover, the N2O emission was apparently inhibited by O-DM-SBC with a 50.2 % decrease in daily flux under the functional coupling of biochar (SBC) and oxygen nanobubbles (ONBs). Path analysis supported that the treatments (SBC, modification, and ONBs) had joint effects on N2O emission by changing the concentration and composition of dissolved inorganic nitrogen (e.g., NH4+-N, NO2--N and NO3--N). The nitrogen-transforming bacteria were found to be significantly promoted by O-DM-SBC at the end of the incubation, while the archaeal community seemed to be more active in the SBC groups without ONB, confirming their different mechanisms. The PICRUSt2 prediction results revealed that most nitrogen metabolism genes including nitrification (i.e., amoABC), denitrification (i.e., nirK and nosZ), and assimilatory nitrate reduction (i.e., nirB and gdhA) were largely enriched in O-DM-SBC, indicating the active nitrogen-cycling network was established, thus achieving simultaneous nitrogen pollution control and N2O emission reduction. Our findings not only confirm the beneficial effect of O-DM-SBC amendment on nitrogen pollution control and N2O emission mitigation in hypoxic freshwater, but also contribute to a more comprehensive understanding of the effect of oxygen-carrying biochar on nitrogen cycling microbial communities.