基于系统动力学的湖滨湿地水-沉积物界面无机氮迁移转化研究

Lake eutrophication is one of the key aquatic environmental problem worldwide and ecological restoration has grown over the last decades. Erhai is the second large plateau lake in Yunnan province, southwestern China, the lake has great water pollution potential with the eutrophication promotion factors of phosphorus and nitrogen. Luoshijiang, the key freshwater providing river for Erhai, locates upper flow of its basin; and the proposed function of its riparian wetland was effective reduction of nutrients and remediation of ecosystem. More and more professional scientists have been keeping sustainable focus on nitrogen reduction mechanism for lake water environment protection, and the fundamental theory has been achieved for lake Eutrophication control. However, the key transporting processes and driving factors of nitrogen in wetland has still not been systematically analyzed; the accurate simulation and quantitative evaluation for nitrogen is in limited situation. Based on these scientific issues, The Luoshijiang riparian wetland of Lake Erhai is taken as a case study. Temporal and spatial variability of nitrogen retention effect were revealed based on the field monitoring, lab experiments, isotopic tracer, in situ test and system dynamic model developing. Two key biogeochemical processes in the wetland will be focused, including (a) nitrogen transporting in water, sediment and plant; and (b) identification for the key process of wetland nitrogen retention, driving factors and dynamic sediment source and sink mechanism. In order to realize quantitative assessment of nitrogen retention effect, the model integrated each nitrogen transportation process will be developed based on system dynamics (SD), which applied widely in multivariable, nonlinear, multi- feedback systems. With the systematic study accomplishment, several results will be formed as (a) the nitrogen transporting mechanism and its key driving factors in Luoshijiang Lake riparian wetland will be identified; and (b) the hydrodynamic, water quality and ecological model platform will be established, which could be utilized to accurately evaluation the nitrogen transporting dynamics. It is believed that the findings and the approach proposed in this study will provide more interesting and useful hints to support effective riparian wetland management and designing.

湖滨湿地是湖泊生态修复重要手段之一，氮在水-沉积物中的迁移转化机理及其驱动因子是目前研究热点。但存在两方面问题需深入研究：无机氮在湿地水-沉积物界面迁移转化关键驱动因子识别；精准度高的无机氮截留定量化评估方法。本项目以云南洱海罗时江湖滨湿地为研究对象，通过现场监测与室内模拟试验获取基础数据并拟合参数；揭示湿地对无机氮截留效应时空分异特性，识别沉积物中无机氮“源-汇”的转换关键过程及驱动因子；探究无机氮在水-沉积物界面过程与驱动因子间的耦合效应，拟合无机氮迁移转化的动力学方程，量化转化系数，利用适于处理多变量、非线性、复杂时变、多重反馈的系统动力学方法，构建耦合了单个无机氮迁移转化过程的系统模型，实现湿地对氮截留效应的定量评估。本项目旨在通过典型研究，探索基于系统动力学模型的湖滨湿地氮截留的关键过程与驱动因子作用机制，为制定科学的湿地管理及优化设计提供支持。

入湖河口湿地是高原湖泊生态修复重要手段之一，沉积物常被认为营养盐的蓄积库，但水-沉积物界面是湿地系统中氮等物质交换频繁且复杂的界面，且氮价态从-3~+5变化很大，不同形态间氮的转化过程复杂，目前对氮素硝化、反硝化、矿化等单一过程及机理开展深入研究，但缺乏考虑各单个转化过程间的相互影响。本研究针对人工湿地水-沉积物界面，运用系统动力学原理，将前述单个独立的氮迁移转化过程进行耦合，构建高湿地水-沉积物界面无机氮迁移转化过程的系统动力学模型。.以云南洱海北部罗时江表流湿地为研究对象，开展为期两年（2017年8月-2019年7月）的逐月现场监测及模拟试验，结果表明：①湿地年平均水深、水温分别为0.78 m和18.35 ℃；上覆水TN、NO3--N和NH4+-N的平均浓度分别为：3.06、0.67和0.61 mg/L；②湿地表层（0-5 cm）、中层（5-10 cm）、底层（10-15 cm）沉积物的有机质（OM）平均值分别为：65.55±1.95、56.86±1.77和50.05±1.59 g/kg，TN平均值分别为：1407.35±35.49、1247.73±31.77和1134.23±37.93 mg/kg；NO3--N平均值分别为：2.83±0.12、2.64±0.14和2.32±0.13 mg/kg，NH4+-N平均值分别为：4.69±0.46、3.60±0.18和3.92±0.20 mg/kg；③湿地N2O通量的平均值为12.85±1.52 μg·m-2·h-1，总体表现为N2O排放源；④沉积物中微生物主要包括：变形菌门占比在45.43%~63.24%，平均为53.14%，是门水平上相对丰度最大的菌门。其次为拟杆菌门和浮霉菌门，平均丰度为14.08%和11.22%，疣微菌门（4.71%），酸杆菌门（4.60%），绿弯菌门（1.65%），硝化刺菌门（1.30%），硝化螺旋菌门（1.16%）；⑤连续流动培养试验表明沉积物为氨氮的“源”，其释放通量在270.60~747.58 mg·m-2·d-1；PVC顶盖原位培育试验表明净氮矿化速率和硝化速率分别为0.048和0.015 mg·kg-1·d-1；⑥基于野外调研数据及模拟试验数据，拟合氮迁移转化的动力学方程，量化转化系数，构建了湿地水-沉积物界面氮迁移转化的系统动力学模型，为制定科学的湿地管理及优化设计提供支持。