N2O production by denitrification in an urban river: evidence from isotopes, functional genes, and dissolved organic matter

Rivers are important sources of N 2 O emissions into the atmosphere. Nevertheless, N 2 O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N 2 O, NH 4 + , NO 2 − , and NO 3 − in surface water to identify the microbial processes of N 2 O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers ( amoA -bacteria, nirK , nirS , nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N 2 O production processes. The observed nitrogen (δ 15 N) and oxygen (δ 18 O) of N 2 O were within the expected isotopic range of N 2 O produced by nitrate reduction, indicating that N 2 O was dominantly produced by denitrification. The positive significant correlation between N 2 O Net concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N 2 O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N 2 O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N 2 O. Measurement of isotope ratios of N 2 O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N 2 O production along the Tama River.