[Concentration, Flux, and Emission Factor of N₂O in Rivers with Different Nitrogen Pollution Features]

In this study, 22 rivers in Tieling City were selected to study the concentration, flux, and emission factor (EF_5r) of N₂O. Based on the concentrations and components of nitrogen (N), the 22 rivers can be divided into ammonia nitrogen (NH₄⁺)-polluted rivers (mean NH₄⁺=5.86 mg·L^-1), nitrate nitrogen (NO₃^-)-polluted rivers (mean NO₃^-=3.05 mg·L^-1), and N-limited rivers[mean DIN (NH₄⁺+ NO₃^-)=1.04 mg·L^-1]. Overall, the concentration of N₂O ranges from 17.03 to 9028.60 nmol·L^-1, with a mean value of 546.75 nmol·L^-1 (mean saturation=6256%). The emission fluxes across the water-air interface range from 17.21 to 15655.3 μg·(m²·h)^-1, with a mean value of 949.36 μg·(m²·h)^-1, indicating that those rivers are net sources of atmospheric N₂O. The concentration and flux of N₂O observed in NH₄⁺-polluted rivers are significantly higher than that in the NO₃^--polluted and N-limited rivers. According to the method proposed by the IPCC, EF_5r varies greatly among the three types of rivers and the coefficient of variation of EF_5r is 445%. The EF_5r for NO₃^--polluted rivers is on average 0.0005, which is lower than the recommended value of 0.0025. However, the EF_5r for NH₄⁺-polluted rivers is on average 0.4456, which is 180 times the recommended value and may be caused by the lower NO₃^- concentration of those rivers. The EF_5r of N-limited rivers averages 0.0050 and is two times the recommended value. Thus, it is necessary to assess the pollution status of N before calculating the EF_5r for the riverine system. We suggest that the EF_5r for NH₄⁺-polluted and N-limited rivers should be calculated using[N₂O]/[NH₄⁺] and[N₂O]/[DIN], respectively, without assessing the composition and concentration of N.