Mass Balance and Nitrogen Isotopic Determination of Sources for N2O in a Eutrophic River.
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Concentration of dissolved N<SUB>2</SUB>0 in the Minami-Asakawa River, Tokyo, varied from 15.5 ± 2.1 nM (mean ± 1σ, n=4) in the headwaters in a preserved hill forest to 153±102 nM (n=8) in the lower reach in Tokyo suburb. High N<SUB>2</SUB>0 concentrations in sewage waters (227±187 nM, n=11) suggested a direct influx of dissolved N20 from the sewers as a cause of the N20 elevation downstream. A mass balance evaluation in the lower reach suggested that the amount of N<SUB>2</SUB>0 carried by sewers was roughly equal to that produced in the river course where an active oxidation of NH<SUB>4</SUB>+ to NO<SUB>2</SUB>- and N0<SUB>3</SUB>- occurred. The molar ratio of N<SUB>2</SUB>0 produced to NH<SUB>4</SUB>+ oxidized was 0.04%, in agreement with the N<SUB>2</SUB>0 yield in ammonium oxidation. The δ<SUB>15</SUB>N of dissolved N<SUB>2</SUB>0 in the middle reach was a little higher than the value under atmospheric equilibrium (+ 8 '/o, ), whereas it was down to -39.8 %o in the lowermost site. As δ<SUB>15</SUB>N of dissolved N<SUB>2</SUB>0 in the sewers ranged from -0.5 to -9.2 %o, the N<SUB>2</SUB>O accumulation with a remarkably low s15N could only be attributed to the in situ production of <SUP>15</SUP>N-depleted N<SUB>2</SUB>O, most probably through an oxidation of NH<SUB>4</SUB>+ derived from sewage waters.
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