农业流域非点源氮污染的水文滞后效应及其定量源解析研究

Due to long residence times (ranging from months to decades) of watershed lateral flow and groundwater runoff, nitrogen loading delivered by them may be mainly derived from historical nitrogen inputs and would not be significantly decreased in response to decline of current anthropogenic nitrogen inputs to terrestrial surface. Such hydrologic lag effect has been one of major causes of failing to yield the expected improvements in river water quality after implementing nitrogen nonpoint source (NPS) pollution control measures for decade or several decades in many watersheds. However, little quantitative knowledge is available concerning watershed hydrologic lag effect of nitrogen NPS pollution, resulting in considerable uncertainties in riverine N sources apportioned results in previous studies. To address the hydrologic lag effect, monitoring works for hydrologic and nitrogen delivery processes will be conducted synergistically for a typical agricultural drainage watershed in eastern China. By adopting and integrating various methods and technologies (e.g., the watershed hydrologic model, hydrologic diagram approach, digital filtering method, end–number model, water stable isotopes based sine wave method, and 3H and chlorofluorocarbon analyses), the research will provide new and reliable insights on the confluence principles of watershed surface runoff, lateral flow and groundwater runoff, their individual contributions to river water discharge, and the age constructions of each type of runoff and river water. Combining the conventional statistical methods, Bayesian stable isotope mixture model, 15N and 18O dynamics analyses, and the N2:Ar ratio method with membrane inlet mass spectrometry, the research will further explore the nitrogen dynamics of different types of runoff and river, quantify denitrification rates for different hydrologic pathways, and identify nitrogen sources in each type of runoff and river. Coupled with these two aspects of achievements, riverine nitrogen sources, their main hydrologic delivery pathways, and their associated hydrologic lag times will finally quantified and determined, guiding the development of efficient watershed nitrogen NPS pollution control strategies.

由较缓慢的非地表径流汇流过程引起的非点源氮污染水文滞后效应是不少流域经过多年的氮污染控制努力后河流水质未见成效的主因之一。然而，已有研究大多忽略了水文滞后效应的影响，氮源解析结果可能“张冠李戴”了，阻碍了氮污染控制进程。本项目将通过对农业流域水文过程和氮输移过程的协同监测分析，联合应用流域水文模型、水文图解、数字滤波、端元模型、基于氢氧同位素的正弦波分析、氚和氯氟烃分析等，揭示流域地表径流、侧向流、地下径流汇流规律，动态分割不同类型径流对河流流量的贡献，解析不同类型径流和河水的年龄构成；以此为基础，进一步联合应用常规统计方法、贝叶斯稳定同位素混合模型、氮氧同位素分析、N2:Ar法等，阐明不同类型径流和河流氮输移动态特征，揭示反硝化速率动态规律及其对氮输移负荷的影响，解析不同类型径流和河流氮的来源，从而定量阐释河流氮源构成、主要输移途径及其水文滞后时长，为突破氮污染控制困境提供关键科学依据。

水文滞后效应是一些流域经过多年农业非点源污染控制努力后水质仍未见成效的重要原因。然而，对流域氮非点源污染水文滞后效应尚缺乏定量认识，阻碍了氮污染高效防治。本项目以浙江永安溪流域、长江流域等为研究对象，应用氮收支平衡、同位素分析、模型模拟等方法，探究了研究流域不同类型径流对河川径流的贡献及其年龄构成，定量识别了研究流域不同类型径流和不同污染源对河流氮污染的贡献，探究了滞后效应对河流氮磷污染的重要影响。结果表明，永安溪流域河川径流主要来源于地下径流（65-86%），且水文输移时长为3.2-6.3年；河流氮污染以NO3––N为主（67 ± 18%），地下水氮（~50%）和土壤氮（> 30％）是河流NO3––N的主要来源；流域地表径流和地下径流氮输移过程中硝化作用强、反硝化作用较弱，促进了NO3––N在地下水系统的累积；地下径流贡献了66%的河流氮输出通量，并贡献了河流N2O输出通量的38-88%，意味着地下径流是非点源氮污染作用于河流的主要途径，具有显著的时间滞后性；尽管2000-2019年流域化肥氮输入量下降了49%、畜禽养殖数量减少了73%以及点源污染排放降低，但由于土壤和地下水的遗留氮污染贡献，使得河流TN和NO3––N输出通量仍持续增加。1980-2015年，长江流域遗留氮对大通站河流DIN、 DON、 PN 和TN输出通量的贡献分别为~ 49%（26-82%）、51%（<1-78%）、95%（87-98%）和56%（36-86%）；由于植被覆盖和大坝建设的增加，促进了遗留磷形成和转化，使得大通站河流PP输出通量降低、DP输出通量增加，增加了水体藻类爆发风险；土壤氮含量和径流深是长江流域农田总氮径流流失的主要影响因子，使得加强土壤和径流管理能更加有效地削减农田氮流失负荷。因此，为应对非点源污染的水文滞后性，在落实化肥输入等源控制的同时，应加强与径流和农田土壤的协同管理，并重视地下径流过程截留。本项目较为系统地阐释了流域氮非点源污染水文滞后效应形成机制及其对河流氮污染的重要影响，为突破非点源污染控制困境提供了关键科学依据。