不同土地利用单元汇水径流中氮的同位素特征及其转化途径解析

Analyses of isotopic characteristics and transformations of nitrogen in the runoff of sub-catchments with various land uses

不同来源氮的δ^(15)N值域存在重叠,导致δ^(15)N溯源结果存在不确定性,因此对δ^(15)N溯源结果的校验具有重要意义.淮河入海水道具有明确的汇水分区及差异的土地利用方式,为δ^(15)N溯源结果验证提供了良好的研究场所.基于2017年枯、丰水期淮河入海水道水体δ^(15)N,δ^(18)O值域特征,结合汇水片区土地利用方式,甄别并校验了氮的主要来源;综合^(15)N,^(18)O分馏特征,探讨了氮的转化途径.结果表明:(1)运西高片和通榆低片农业区,农田退水分别通过古盐河及沿程汇流入河,其δ^(15)N-NO^-_3分别为6.4‰±0.4‰和7.9‰±0.8‰,验证了土壤氮δ^(15)N与理论值(0～8‰)的一致性;δ^(15)N-NH^+_4分别为8.4‰±9.0‰和23.5‰±7.6‰,表明NH^+_4来源于沿河生活污水.而两运夹城镇区,片区污水排放通道清安河δ^(15)N-NO^-_3和δ^(15)N-NH^+_4分别高达25.9‰和40.3‰,基本位于城镇污水δ^(15)N理论值(4‰～35‰).(2)运东高片城镇和农业混合区,区域汇水沿程入河,其δ^(15)N-NO^-_3为7.7‰～11.6‰,位于土壤氮和城镇污水δ^(15)N重叠区;而δ^(15)N-NH^+_4为17.9‰±7.3‰,显示了城镇污水δ^(15)N特征.(3)沿程取样点δ^(18)O-NO^-_3为3.5‰±5.1‰,位于硝化作用理论范围(-5‰～15‰),表明发生了持续的硝化过程;沿程δ^(15)N-NO^-_3和δ^(18)O-NO^-_3无同步富集,表明反硝化过程不显著.

Due to the overlap of δ^15N range among various nitrogen sources,it is of great significance to assess the uncertainty of nitrogen sources identification based on isotope approach. The Huaihe River Channel(HRC)watershed,which consists of distinct sub-catchments and typical land use types,provides an ideal study area to verify nitrogen sources identification based on δ^15N. In the present study,water samples were collected during the dry and wet seasons in 2017. The values of δ^15N and δ^18O,and characteristics of the catchment runoff and land uses were analyzed to elucidate the sources of nitrogen.Furthermore,15N and18O fractionation characteristics were analyzed to reveal the dominant nitrogen transformation pathways. The results showed that agricultural drainage area within Yunxi and Tongyu flowing into the HRC by Guyan River or through the runoff along the HRC has mean δ^15N-NO3^- values 6.4‰±0.4‰ and 7.9‰±0.8‰,respectively,which were consistent with the previously reported δ^15N end member of soil organic nitrogen(0~8‰),while the mean δ^15N-NH4^+ values were 8.4‰±9.0‰,23.5‰±7.6‰,respectively,indicating that NH+4 mainly derived from rural sewage along the river. In the urban area of Liangyunjia,domestic sewage is discharged through Qing’an river. Measured δ^15N-NO3^- and δ^15N-NH4^+ were as high as 25.9‰ and 40.3‰,which were higher than most of the reported δ^15N values of sewage(4‰~35‰). In the mixing area of Yundong between agricultural and urban,catchment runoff flows into the HRC along the river. Measured δ^15N-NO3^- value(7.7‰~11.6‰)was located in the overlap δ^15N range of soil organic nitrogen and sewage,while δ^15N-NH4^+ value(17.9‰±7.3‰)suggested that NH4^+ mainly derived from urban sewage. All δ^18O-NO3^- values were located in theoretical δ^18O range of nitrate derived from nitrification,suggesting that nitrification was the dominant transformation pathway. Denitrification may have not predominantly occurred in the overlying water according to the absence of synchronous enrichment of15N and18O of nitrate.