农业流域非点源磷污染的遗留效应及其模拟模型研究

Legacy phosphorus (P) that is the surplus P derived from past human activities and is stored in soils, sediments, biomass and even groundwater can be re-released and contributed a considerable P pollution load to receiving waters. The effect of legacy P has been one of major causes of failing to yield the expected improvements in river water quality after implementing P pollution control measures for decade or several decades in many watersheds. However, little quantitative knowledge is available concerning watershed legacy P quantity and its contribution to current nonpoint source pollution (NPS) load. To address the legacy effect, monitoring works for agricultural soils, meteorology, hydrology, and water quality will be conducted synergistically for a small agricultural watershed in eastern China. Phosphorus dynamics across anthropogenic inputs, soils, groundwater, surface runoff (agricultural drainage and overflow across riparian zones), sediments and river sections will be fully examined. Phosphorus uptake, retention, re-release and transport processes and their influencing factors will be then determined. Referring to ideals of relevant watershed models, law of mass conservation, and equal replacement method, watershed legacy P quantity and its yielded pollution load will be both skillfully described as the functions of several watershed attribute variables and parameters. This project then will develop a lagged variable model of watershed NPS P pollution for qualifying the legacy effect. Each of model parameters will be expressed as a function of several watershed attribution variables and then be calibrated by Bayesian statistic method. Calibrated model will be well validated by comparing modeled and measured or estimated P fates (i.e., biomass uptake, riverine export, and storage in soils) as well as by comparing modeled results between seasonal and annual scales and between sub-catchments and entire watershed. Based on the former efforts, finally, watershed legacy P quantity and its yielded pollution load, lag time between anthropogenic P inputs and riverine export, and P balance will be fully explored, guiding the development of efficient P management strategies.

土壤、沉积物、生物、地下水中遗留的部分历史输入磷所产生的污染负荷是一些流域经过多年污染控制努力后水质仍未见成效的重要原因。然而，现有的模型大多未考虑这一遗留效应，对流域遗留磷问题尚缺乏定量认识，阻碍了磷污染控制进程。本项目将基于农业流域气象、水文、水质、作物等协同监测分析，揭示人为输入−土壤−地下/地表径流−沉积物−河流磷动态特征，明确磷吸收、累积、释放、输移规律及影响因素；借鉴相关模型建模思路、质量守恒原理、等量代换方法，将流域的遗留磷量及其产生的污染负荷表达为相关影响因素和参数的函数，从而创建流域非点源磷污染过程的滞后变量模型。利用以上监测分析结果及贝叶斯统计等方法，并通过与相关测算结果及多时空尺度模拟结果间的相互比较，实现模型有效校验；最终，从过程分析和模型模拟两方面，解析流域遗留磷量及其产生的污染负荷、人为磷输入与河流输出之间的滞后时长、磷收支平衡，为农业磷有效管理提供科学依据。

土壤、底泥等中累积的历史遗留磷产生的污染负荷是一些流域经过多年非点源污染控制努力后水质仍未见成效的重要原因。由于缺乏对流域非点源磷污染遗留效应形成机制、污染贡献和作用时长的科学认识，阻碍了非点源磷污染的有效控制。本项目重点探索了流域非点源磷污染的滞后效应形成机制，估算了遗留磷的污染贡献及作用时长，研究了农田土壤遗留磷的农学效益，分析了水文过程调控等对遗留磷释放形态的影响。研究结果表明，流域非点源磷污染遗留效应的形成机制包括：（1）由于过量的人为磷输入，造成大量磷在流域土壤、底泥、地下水中累积；（2）土壤、底泥、地下水等中磷形态循环转化过程经历较长时间（<10年~100年）；（3）驱动磷从土壤、沉积物等向水体输移的水文过程存在显著时间滞后性（可达数天到数十年）。对永安溪流域的实例研究表明，1980-2010年流域的累积人为磷输入量约为60000吨，其中95%遗留在土壤、沉积物等中。基于净人为磷输入、降雨量等建立的统计模型估算结果显示，永安溪流域遗留磷库贡献了8-58%的河流磷输出通量。永安溪流域的基流贡献达65-86%，且径流汇流时长可达3.2-6.3年，意味着水文过程的时间滞后性显著。在无人为磷输入情节下，永安溪流域过去60年累积形成的遗留磷库通过河流输出和作物吸收过程消耗，需经过150年以上才能使河流总磷浓度达标（0.05 mg P L-1）。但是，永安溪流域农地土壤遗留磷对每年作物吸收磷的贡献由1980年的3%持续增加到了2010年的31%，其农学效益不容忽视。对长江流域的实例研究表明，1980-2015年长江流域累积净人为磷输入量为61050 kg P km-2，其中94%遗留在流域土壤、沉积物等。由于植被覆盖和坝/水库数量增加，导致颗粒态磷在土壤和沉积物中截留累积；但形成的遗留磷有利于可溶磷释放，成为长江干流可溶性磷浓度增加的重要原因。以上研究结果较为系统地阐释了流域非点源磷污染遗留效应，为突破非点源磷污染控制困境提供了关键科学依据。