脱甲河水系N2O关键产生过程及氮素来源探讨

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

开展氮素迁移转化研究有助于深入了解农业小流域氮循环过程,也可为小流域氮素流失溯源提供典型案例.为深入了解和识别脱甲河水系N_2O关键产生过程和流域氮素主要来源,采用稳定同位素方法,于2016年11月至次年10月分析了脱甲河4级(S1~S4)河段水体硝态氮的氮氧双同位素(δ^(15)N-NO_3^-、δ^(18)O-NO_3^-)和沉积物有机质氮同位素(δ^(15)N_(org))、碳氮比值(C/N)特征;探讨了流域氮素的迁移转化过程及其来源.结果表明,水体δ^(15)N-NO_3^-、δ^(18)O-NO_3^-分别在-19.87‰~8.11‰和-3.03‰~5.81‰范围内变化,氮素来源具有多元化特征且各河段存在差异.S1~S4河段δ^(15)N-NO_3^-均值分别为1.72‰、2.62‰、4.10‰和-1.28‰,而δ^(18)O-NO_3^-均值依次为2.60‰、-0.06‰、0.85‰和-0.62‰.S1河段硝态氮来源于土壤流失氮,而S2和S3来源为土壤流失氮、铵态氮肥和人畜粪便,S4则来源于铵态氮肥的硝化反应;硝态氮来源受生产生活影响显著.沉积物有机质δ^(15)N(δ^(15)N_(org))和C/N值波动范围分别是-0.69‰~11.21‰和7.30~12.02,S1~S4河段δ^(15)N_(org)均值分别为1.91‰、2.96‰、4.72‰和3.23‰,C/N均值分别是10.62、8.63、9.05和9.22.S1河段沉积物氮素来源于土壤有机质,而S2~S4河段δ^(15)N_(org)虽存在差异,但其来源均主要为流域内的污水.而硝化过程中δ^(18)O-NO_3^-分别是-7.01‰、-0.17‰、-0.28‰和-0.60‰;δ^(15)N-NO_3^-与δ^(18)O-NO_3^-的比值分别为0.66、-41.01、-30.23和9.39;S1~S4河段NO_3^--N质量浓度为1.08、1.46、1.54和1.50 mg·L-1,δ^(15)N-NO_3^-与NO_3^--N浓度呈正相关.因此,脱甲河水系中氮素转化可能以硝化过程为主体,硝化过程对N_2O的贡献可能占据优势.

The nitrogen（ N） pollution of water is a common global problem. To understand the key production process of N2O 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（ δ^15N-NO3^-,δ^18O-NO3^-） in water,and N isotope（ δ^15N（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 δ^15N-NO3^- in reaches S1,S2,S3,and S4 were 1. 72‰,2. 62‰,4. 10‰,and-1. 28‰,respectively,while the average δ^18O-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 δ^15N（org）in sediment organic matter ranged from-0. 69‰ to 11. 21‰,and C/N was between 7. 30 and 12. 02. The mean δ^15N（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 δ^15N（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 δ^18O-NO3^- in reaches S1-S4 were-7. 01‰,-0. 17‰,-0. 28‰,and-0. 60‰,respectively,indicating that nitrification was the key N2O production process in these reaches. The ratios of δ^15N-NO3^- and δ^18O-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 δ^15NNO3^-. To summarize,the N transformation and N2O production could be dominated by the nitrification process in Tuojia River.