[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 (δ¹⁵N_org) 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 δ¹⁵N_org in sediment organic matter ranged from -0.69‰ to 11.21‰, and C/N was between 7.30 and 12.02. The mean δ¹⁵N_org 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 δ¹⁵N_org 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.