强蒸发驱动下井灌区包气带氮素迁移与累积过程及其环境效应

The accumulation of nitrate in deeper unsaturated zone and groundwater environmental degradation mainly led by the excessive input of agricultural nitrogen. After rainfall and irrigation, the hydrodynamic process of nitrogen in saturated zone is affected by stronger evaporation in semi-arid area. Therefore, from the perspective of the hydrodynamic condition impacted on nitrogen vertical transport capacity in saturated zone, taken the agriculture and pasturage interlaced zone of DAMAO County in inland river basin in Inner Mongolia as typical area, one dimensional laboratory experimentation and situ test zone are employed to identify the main controlling factor of nitrogen accumulation with environmental change in saturated zone in this research project, and the statistics of water and nitrogen variable are calculated. Secondly, the temporal and spatial distribution principle of nitrogen transport and accumulation affected by precipitation, irrigation and evaporation is analyzed in the different thickness of saturated zone, then the transport principle of nitrogen from deeper to more shallow soil layer and the re-accumulation process of nitrogen controlled by stronger evaporation below the root zone are studied. In the end, the correlation relationship between the change of nitrogen contamination concentration in deeper saturated zone and the alternative of soil and water environment is show a response to evaporation process. The research can provide practical guiding significance for the higher water and nitrogen efficiency based on the couple controlled by total amounts and contamination, and promoting the groundwater environmental protection.

农业氮素过量输入是导致包气带深层硝酸盐累积和地下水环境恶化的主因。在干旱区，强蒸发作用改变了降水和灌溉后包气带氮累积区氮素的垂向分布。针对这一问题，以内蒙古中部内陆区达茂旗农牧交错带为典型研究区，从影响包气带氮素垂向迁移能力的水动力学条件出发，通过室内一维土柱实验与原位试验场观测，辨识变化环境下包气带氮累积的主控因子，分析不同土壤深度水氮要素的统计特征，揭示降水、灌溉和蒸发多变量控制下不同包气带厚度氮素迁移与累积的时空分布规律，深入研究强蒸发驱动下浅埋区氮素由深层向浅层的迁移规律及根系区以下氮素再累积过程，模拟和预测根系区至潜水面间含氮污染物浓度变化与地下水环境劣变间的关系及其对蒸发过程的响应，构建氮素投入量—深层累积量—潜水面输入量之间的量化关系模型，对中国北方干旱区农灌带基于含氮污染物总量控制和浓度控制的水-氮高效利用及地下水环境保护具有重要的实践指导意义。

农牧业氮素过量输入是导致包气带深层硝酸盐累积和地下水环境恶化的主因。在干旱区，强蒸发作用改变了降水和灌溉后包气带氮累积区氮素的垂向分布。针对这一问题，以内蒙古中部内陆区达茂旗农牧交错带为典型研究区，研究强蒸发驱动下浅埋区氮素向上迁移规律 及再累积过程，建立氮素浓度变化与水环境演变之间的相互关系，最终依据地下水环境容 量控制包气带氮素输入量，取得了以下主要结论：1、阴山北麓农牧交错带包气带氮素迁移与累积受到多因素的共同影响，其中外部驱动力是降水和蒸发条件变化，内部影响因素是不同土地利用方式；2、乌克镇灌溉饲草料地包气带以硝酸盐为主，其主要来源为铵态氮或尿素，而石宝镇牧区包气带氮素主要来源为牲畜粪便，在强降水或灌溉（>10mm）之后蒸发是导致包气带硝酸盐含量增高的主要原因；3、不同介质类型氮素的累积深度不同，氨氮主要积累在60cm以上表层土中，硝态氮在黏土介质主要积累在表层0~20cm和60~100cm，在砂土介质中主要积累在0~20cm和60~120cm；4、硝态氮与氯离子基本落在1:1斜率趋势性附近，说明可能是蒸发作用导致包气带中的硝态氮浓缩和富集，降水或灌溉48h后强蒸发作用对包气带硝酸盐再分布起到了决定性作用；5、在包气带中硝酸盐背景浓度较低（0.22mg/L）时，蒸发作用将显著降低包气带对地下水硝酸盐污染物的稀释作用；在包气带硝酸盐浓度较高（大于0.5mg/L）时，蒸发作用将显著降低包气带硝酸盐污染物对地下水的排泄强度，有效延缓地下水污染时间。研究成果对中国北方干旱区农灌带基于含氮污染物总量控制和浓度控制的水-氮高效利用及地下水环境保护具有重要的实践指导意义。