浅层地下水升降对洱海周边菜地土壤氮素迁移的影响

Vegetable field around Erhai Lake is very important for regional and local. Excess nitrogen inputs in the growing of vegetables cause the excessive nitrogen accumulation in the subsoil of vegetable field for a long time. Moreover, shallow groundwater level around Erhai Lake has large fluctuations. These factors accelerate the process that nitrogen of the subsoil transfers into the shallow groundwater, which will be serious threats to the water quality of Erhai Lake and groundwater of its surroundings. The observation instrument of migration of soil nitrogen under the condition of groundwater level fluctuation was designed according to the movement way of shallow groundwater that is main lateral flow along with some vertical flow in slope soil. This study will focus on the migration of soil nitrogen in vegetable field by the simulated tests and the situ monitoring. The behavioral variation characteristics of NO3--N, NH4+-N and NO2--N of soil will be studied and the nitrogen migration amount will be determined in the fluctuation zone of groundwater level, and their change laws in soil and groundwater will be clarified. The migration models about NO3--N, NH4+-N and NO2--N under fluctuation of shallow groundwater level also will be built. The results of the study will provide a theoretical basis to the accurate prediction of the water pollution caused by the nitrogen of farmland soil for Erhai Lake and its surroundings.

洱海周边菜地对区域和地方都很重要，长期以来蔬菜种植过程中氮肥的过量使用，造成了氮素在蔬菜地中过度富集，加之洱海周边浅层地下水埋深较浅且波动大，加速了土壤中的氮素快速向地下水中运移，严重威胁着洱海周边地下水及洱海的水质安全。本研究根据坡面浅层地下水流的运动方式（主要是侧向流动，伴随着一定的垂直流动），设计了一种基于浅层地下水升降过程中土壤氮素迁移的观测装置，并采用模拟试验和原位监测相结合的方法，研究水位升降区土壤中不同形态氮（NO3--N、NH4+-N 和NO2--N，称之为三氮）的行为变化特征，及土壤中氮素的迁移量；揭示浅层地下水升降过程中土壤和地下水中三氮的变化规律，建立浅层地下水升降过程中三氮的迁移模型。本项研究可为农田土壤中氮素对洱海周边地下水及洱海水污染的准确预测提供一定的理论基础。

洱海近岸菜地剖面土壤中氮过量富集，加之该区浅层地下水埋深浅且波动大，加速了土壤中氮素快速向地下水中运移，严重威胁着该区地下水及洱海的水质安全。本项目通过原位监测和室内模拟试验相结合的方法，研究了浅层地下水升降对洱海周边菜地氮素迁移的影响，结果表明：洱海近岸菜地浅层地下水埋深旱季深、雨季浅，且距洱海越远，海拔越高，浅层地下水埋深越浅。洱海水位、降雨、灌溉、潜水蒸发和土壤物理特性的空间变异是引起浅层地下水埋深时空变化的主要因素。浅层地下水中TN、NO3--N和NO2--N 随着高程和距洱海距离的增加而增加，但是NH4+-N浓度有相反的变化，且雨季各氮形态浓度大于旱季。NO3--N是浅层地下水中主要的氮形态，研究区超过80%的浅层地下水样硝态氮浓度大于10 mg/L。冗余度分析表明超过70%的浅层地下水中氮的时空变化可以被浅层地下水埋深解释，仅大约10%的氮的时空变化可以被土壤孔隙度、粉黏粒含量、土壤NH4+-N和NO3--N浓度解释。随着浅层地下水埋深的增加，浅层地下水中TN和NO3--N呈指数函数递减，NH4+-N呈指数函数递增，而剖面土壤中的DTN、NO3--N和NH4+-N呈线性增加。浅层地下水中的TN、NO3--N和NH4+-N浓度随水位波动区剖面土壤DTN、NO3--N和NH4+-N浓度的增加呈指数函数增加。建立了浅层地下水在非均质饱和土壤迁移的三维模型及浅层地下水中溶质运移的三维模型。通过模拟试验实测数据对溶质迁移模型进行验证，NH4+-N和NO3--N浓度预测值和实测值的平均误差ME、均方根误差RMSE、相关系数R、误差的方差VAR分别为-0.01和0.006、0.26和0.829、0.997和0.997、0.07和0.69，表明所建立的溶质迁移模型预测精度较高，对NH4+-N的模拟效果好于NO3--N。本项研究可为调控洱海近岸菜地浅层地下水位，预测氮对浅层地下水的污染，减少氮素对洱海周边地下水及洱海水质污染提供科学依据。