Nitrogen stable isotope ratio of groundwater N₂O

δ 15 N of N 2 O and NO 3 − were measured to determine the mechanisms producing N 2 O in nitrate contaminated groundwaters. These groundwaters contained N 2 O at concentrations between thirty to one hundred times higher than the atmospheric equilibrium. The N 2 O/NO 3 − molar ratios in groundwaters were from 0.2 × 10 −3 to 4.4 × 10 −3 . The δ 15 N values of N 2 O, −5.8 to −21.3 permil, were much lower than those of coexistent NO 3 − , +3.6 to +7.3 permil. In the light of evidence that 15 N in N 2 O from nitrification is highly depleted, the present findings suggest nitrification as a dominant mechanism for the production of N 2 O in the groundwaters. However, unlike other aquatic systems where nitrification is a major route of N 2 O production, no clear correlation was observed between ΔN 2 O and −ΔO 2 in the groundwater. This poor correlation strongly suggests that N 2 O in the groundwater was predominantly produced in the soil column and/or unsaturated zone. Based on two‐year observation of N 2 O flux from a spring, a possible charging rate of N 2 O to the aquifer was estimated as 1.2–2.3 mg N 2 O‐N·m −2 ·y −1 . The rate is much smaller than the reported soil emission rates, suggesting the groundwater N 2 O to be an insignificant source for atmospheric N 2 O. Large discrepancies in δ 15 N of N 2 O between the groundwaters and the surface airs support this view.