LAUSR

Abstract Agricultural drainage ditches are subjected to high anthropogenic nitrogen input, leading to eutrophication and greenhouse gas emissions. Nitrate-dependent anaerobic methane oxidation (N-DAMO) could be a promising remediation strategy to remove methane (CH4) and nitrate (NO3−) simultaneously. Therefore, we aimed to evaluate the potential of N-DAMO to remove excess NO3− and decrease CH4 release from agricultural drainage ditches. Microcosm experiments were conducted using sediment and surface water collected from three different sites: a sandy-clay ditch (SCD), a freshwater-fed peatland ditch (FPD), and a brackish peatland ditch (BPD). The microcosms were inoculated with an N-DAMO enrichment culture dominated by Candidatus Methanoperedens and Candidatus Methylomirabilis and supplemented with 13CH4 and 15NO3−. A significant decrease in CH4 and NO3− concentration was only observed in the BPD sediment. In freshwater sediments (FPD and SCD), the effect of N-DAMO inoculation on CH4 and NO3− removal was negligible, likely because N-DAMO microorganisms were outcompeted by heterotrophic denitrifiers consuming NO3− much faster. Overall, our results suggest that bioaugmentation with N-DAMO might be a potential strategy for decreasing NO3− concentrations and CH4 emission in brackish ecosystems with increasing agricultural activities where the native microbial community is incapable of efficient denitrification.