Key Production Process of Nitrous Oxide and Nitrogen Sources in Tuojia River

: The nitrogen (N) pollution of water is a common global problem. To understand the key production process of N2 O and identify the dominant N sources, Tuojia River, a typical agricultural watershed in a subtropical area, was investigated. To analyze the characteristics of dual nitrate isotopes (δ15N-NO3-,δ18O-NO3-) in water, and N isotope (δ15Norg) and carbon-nitrogen ratio (C/N) in sediment organic matter from four reaches(S1-S4), the stable isotopes method was used. The results showed that the sources of nitrate varied significantly among river segments and were affected by agricultural production and human habitation on the land surface. The average δ15N-NO3- in reaches S1, S2, S3, and S4 were 1.72‰, 2.62‰, 4.10‰, and -1.28‰, respectively, while the average δ18O-NO3- were 2.60‰,-0.06‰, 0.85‰, and -0.62‰. The N in terrestrial soil made a large contribution to nitrate sources in reach S1, while soil N, ammonium N fertilizer, and manure played a main role in reaches S2 and S3. Most of the nitrate came from ammonium N fertilizer in reach S4. We also found that δ15Norg in sediment organic matter ranged from -0.69‰ to 11.21‰, and C/N was between 7.30 and 12.02. The mean δ15Norg in reaches S1-S4 were 1.91‰, 2.96‰, 4.72‰, and 3.23‰, respectively, and the mean C/N values were 10.62, 8.63, 9.05, and 9.22, respectively. Although there were some differences in δ15Norg among reaches S2-S4, the dominant N source was sewage in those reaches. However, soil organic matter was the main N source in the sediments of reach S1. The mean δ18O-NO3- in reaches S1-S4 were -7.01‰,-0.17‰,-0.28‰, and -0.60‰, respectively, indicating that nitrification was the key N2 O production process in these reaches. The ratios of δ15N-NO3- and δ18O-NO3- were 0.66,-41.01,-30.23, and 9.39 in reaches S1-S4, respectively. Finally, we found that there was a positive correlation between NO3--N and δ15N-NO3-. To summarize, the N transformation and N2 O production could be dominated by the nitrification process in Tuojia River.