天津排污河溶存氧化亚氮产生机制模拟研究及其排放通量估算

Nitrous oxide (N2O) is an important greenhouse gas that depletes stratospheric ozone and attributes to global warming. An increase of active nitrogen （N）loading resulting from human activities to watersheds makes the seriously polluted river systems a potentially important source of N2O. So it was really important and meaningful to focus on the production mechanism and diffusion flux of nitrous oxide. Here, sewage discharge channels of Tianjin in the Hai River Basin were selected as the research key object, combined with the whole river network. Multi-disciplinary synthetic study methods will be used in the research, including environmental geochemistry, biogeochemistry, physical geography and so on. Based on field samplings, laboratory analysis, in situ measurements and incubation experiments, this research is aimed to systematically study the spatial and temporal variabilities of solube N2O concentrations and saturations, the effects of primary environmental factors especially external nitrogen input on gas concentrations, the production mechanism of solube N2O, and the calculation of nitrous oxide total emissions across air-water interface of Tianjin sewage discharge channels and its share in the whole river network. The research will provide the scientific foundation for understanding the biogeochemical cycling characteristics of nitrous oxide and evaluating impact of natural and human factors on nitrous oxide emissions in different types of rivers.

氧化亚氮（N2O）是一种重要的温室气体，既能损耗臭氧又能促进全球变暖。人为来源氮负荷逐年上升使污染严重的河流成为大气N2O的潜在排放源。因此开展污染河流水体溶存 N2O 排放通量及产生机制研究具有重要的现实意义。本研究以海河流域天津市排污河流水体为重点研究对象，结合全市河网布点监测，对排污河流水体溶存N2O浓度、饱和度时空变化特征以及水-气界面通量进行分析和讨论；分析温带季风和城市化作用下环境因子的控制作用和影响，探讨外部氮源输入对河流溶存N2O浓度和饱和度的影响；通过现场试验与室内培养试验，探究城市河网水体N2O的产生机制；使用GIS空间分析和双层模型估算天津市排污河流水体水-气界面N2O排放通量及其在天津市河网水体排放总量中的份额，为认识不同类型的河流水体中N2O的生物地球化学循环特征提供科学基础，同时为评价自然因素和人为因素对水体释放N2O的影响提供科学依据。

本项目以天津市排污河为主要研究对象，在野外采样实测和室内培养实验的基础上，运用静态顶空气相色谱法和双层模型分析法计算河流水体溶存温室气体浓度、饱和度及水-气界面和沉积物-水界面排放通量，通过Pearson相关分析探究人类活动干扰下环境因子对温室气体溶存浓度及排放的影响，选取乙炔抑制培养法测定反硝化速率。结果表明：天津排污河水体溶存CO2、CH4和N2O浓度均值分别为38.14±8.57 μmol/L、1.35±1.16 μmol/L和47.88±24.36 nmol/L，依次为自然河的1.17倍、5.64倍和1.30倍；各自季节变化特征表现为春季＞冬季＞秋季＞夏季、春季＞夏季＞冬季＞秋季和夏季＞春季＞冬季＞秋季；空间变化特征皆表现为综合污染的河流点位较高、轻度污染河流点位较低。排污河水体三种温室气体排放通量均值分别为830.92±418.08 μmol·m-2·h-1、70.48±67.36 μmol·m-2·h-1和1860.32±1635.91 nmol·m-2·h-1，分别是自然河的1.57倍、6.12倍和1.58倍；时间变化特征分别表现为春季＞夏季＞秋季＞冬季、春季＞夏季＞冬季＞秋季和夏季＞春季＞秋季＞冬季；空间变化特征均表现为排污河排放通量大于自然河，其中CH4空间差异最大。天津河流水-气界面CO2排放通量与NH4+-N、水温和气温呈显著正相关，CH4与NH4+-N呈显著正相关，N2O与水温和气温呈显著正相关；三者均与DO呈显著负相关。研究期间天津河网水体三种温室气体年排放总量分别约为219.33kt、4.31kt和493.63t。河流成为大气中温室气体的重要来源，其中排污河流为主要排放源。天津排污河沉积物反硝化速率均值为2.09±0.98mg-N·m-2·h-1，自然河为1.98±1.29mg-N·m-2·h-1；季节变化特征表现为夏季＞春季＞冬季＞秋季；空间分布规律表现为周边环境较恶劣、排污类型复杂的河流样点反硝化速率较高，周边环境条件较理想、河道宽阔、水流流速较快的河流样点反硝化速率均值较低。天津排污河沉积物-水界面三种温室气体排放通量均值分别为自然河的1.22倍、4.37倍和134.5倍。天津河流沉积物-水界面CO2和N2O排放通量均与水温呈显著正相关关系，而CH4与NH4+-N呈显著正相关关系。总体上沉积物是河流CO2的汇，是CH4和N2O的源。