Control of nitrous oxide and methane emissions during nitrate reduction in sediments: Microbial mechanisms and...

Although nitrate dosing has been widely applied to remediate black-odorous sediments, its net effect on greenhouse gas emissions and the underlying microbial mechanisms remain unclear. Laboratory incubation experiments were conducted to elucidate methane (CH₄) and nitrous oxide (N₂O) emission responses during nitrate reduction in sediments and to evaluate the N₂O mitigation effect of batchwise nitrate dosing designed to promote preferential sulfide oxidation. Nitrate addition significantly suppressed CH₄ emissions, with cumulative CH₄ emissions in the treatment groups accounting for only 0.7-0.8% of those in the control by day 28. However, nitrate addition simultaneously promoted N₂O accumulation, thereby increasing greenhouse gas emission equivalents. Functional gene analysis suggested that nitrate reduction inhibited methanogenesis and altered the abundances of genes associated with methylotrophic C1 metabolism and denitrification. Further investigation showed that preferential sulfide oxidation markedly reduced N₂O accumulation under the tested laboratory conditions. This effect was associated with lower nitrite accumulation and stronger nitrogen-sulfur metabolic coupling. Microbial community analyses indicated that Rhodocyclaceae and Gallionellaceae were associated with the low-N₂O system, whereas Thermomonas was associated with N₂O accumulation. Overall, nitrate-based sediment remediation suppressed CH₄ but promoted N₂O accumulation, whereas optimizing the dosing strategy substantially improved its greenhouse gas mitigation potential. These findings provide a conceptual basis for low-carbon in situ sediment remediation.