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

Abstract

In this study, 22 rivers in Tieling City were selected to study the concentration, flux, and emission factor (EF5r) 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, EF5r varies greatly among the three types of rivers and the coefficient of variation of EF5r is 445%. The EF5r for NO3--polluted rivers is on average 0.0005, which is lower than the recommended value of 0.0025. However, the EF5r 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 EF5r 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 EF5r for the riverine system. We suggest that the EF5r 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.