太湖流域不同植被类型氮排放时空变化模拟与分异机制解析

Accompanied by the intensive agricultural production in the Taihu Lake Basin, large amounts of nitrogen fertilizer were entering the watershed ecosystem, raising the issues of water eutrophication and algal blooms. Non-point source nitrogen pollution can be influenced by many factors, such as meteorological conditions, soil physical and chemical properties, vegetation types, fertilization intensity and terrain conditions, and associated with many processes such as biological growth and nutrient utilization, soil nitrification and denitrification, mineral decomposition, soil leakage and surface runoff loss, etc. However, due to the lack of quantitative understanding of spatial and temporal heterogeneity of surface nitrogen emissions in the regional scale, local and static emissions parameters were usually adopted to estimate the loads of regional non-point source pollution. This would reduce the accuracy of the estimates, and also bring blindness to the policy-making. In this proposal, the distribution characteristics of meteorological, terrain, soil, vegetation and farming systems for the whole basin will be analysed, and the characteristics influential factors and their combination will also be revealed. The rainfall-runoff monitoring for five different vegetation types including farmland, tea garden, vegetable land, bamboo and pine will be carried out in this basin. Soil samples, surface runoff and soil flow, soil moisture and soil respiration were monitored in the plots, and the surface runoff and soil samples will also be collected periodically. Based on the collection of above data, the biogeochemical model for different vegetation types will be constructed and validated to simulate the photosynthesis and respiration process, biomass change and nitrogen use, soil NO3-N leakage, ammonia volatilization, N2O emission and nitrogen loss in surface runoff. Then, the zones with different meteorological and soil assemblages will be partitioned, following by the interpretation of the distribution characteristics of the controlled variables affecting nitrogen emission in Taihu Lake Basin. The model will be simulated for each zone individually using zone-specific combined-parameters of the meteorological-soil-vegetation-tillage after validating in the fields. The above work would be expected to form a set of dynamic parameters that can describe the spatial heterogeneity of surface nitrogen emission, to improve the estimation accuracy of nitrogen emission in regional scale, and to evaluate the present situation of surface nitrogen emission under current land use types in Taihu Lake Basin. Based on this, the spatial heterogeneity characteristics can be revealed, and the spatial and temporal differentiation rules and mechanism of nitrogen emission in Taihu Lake Basin can be clearly interpreted. Finally, the implementation of this proposal can further improve spatial structure of planting patterns in the Taihu Lake Basin.

太湖流域面源氮污染极其严重，是太湖富营养化和藻类水华暴发的重要因素之一。因对面源氮污染在区域尺度上时空分异规律缺乏深入理解，采用局地、静态参数进行区域评估，导致估算精度低，给减排方案制定带来盲目性。本研究系统分析太湖流域气象条件、植被类型、土壤理化性质和耕作制度分布格局，明确相关表征变量的取值范围和统计分布。建立农田、茶园、菜地、竹林和马尾松五种类型模拟试验小区，开展气象、地表径流和壤中流、土壤水分与土壤气体排放等高频连续定位监测，揭示不同种植类型不同氮排放途径、分量及影响因素，构建并验证不同植被类型生物地球化学模型。通过模型辅助模拟太湖流域不同气象-植被-土壤-耕作制度组合类型的氮排放序列，解析其时空分异的发生机制，建立定量表达太湖流域面源氮排放时空异质性的动态参数，提升区域尺度面源氮污染估算的精度，提出太湖流域氮减排优化调控策略。

太湖流域的富营养化问题自20世纪80年代以来开始显现并且迅速发展，至今仍然未得到有效解决。目前点源的监测手段和治理技术已经比较成熟，面源污染也受到越来越多的关注。面源来源广泛、影响因子众多、时空变化快速的特征，为面源监测和治理研究带了重大困难。面源污染的时空量化研究对面源污染治理尤为重要。本项目重点以太湖流域的农田为研究对象，采用农小区试验与地球生物化学模型相结合的方法，通过多种影响因子的敏感性分析，筛选出氮流失的主控因子，然后建立氮流失与主控因子的定量化关系，在此基础上进行农田氮流失的精细化计算，为流域面源污染减排控制提供依据。本项目通过资料的调查、收集和分析， 结合径流小区实验，建立了DNDC模型对不同施肥、不同种植类型的氮径流和渗漏流失的变化具有较好的模拟能力。在空间上，由于氮素流失对降雨因子的敏感性，流失强度呈现出从南向北、从西向东降低的趋势，从高到低依次为湖州市、嘉兴市、杭州市、镇江市、无锡市、上海市、常州市和苏州市。对太湖流域农田种植业氮流失进行综合评估，结果显示，氮素流失总量为9.79万吨，稻麦农田氮流失占到了70.8%，菜地占29.2%。相比20世纪80年代，虽然稻麦种植面积削减了53%，但是施肥变化使太湖流域稻麦轮作的氮流失量增加了50%。在菜地种植面积比例最高的上海市和杭州市，旱地种植的污染贡献率超过50%，成为区域最主要的农田种植氮污染源。在稻麦农田中恢复90年代的施肥模式，整个太湖流域水田氮素流失可削减33.8%。太湖流域氮素流失强度等级和影响因子风险等级划分的结果显示，湖州市和杭州市地区的高强度和极高强度氮流失等级主要是由于高施肥强度和高降雨量引起的，太湖周边和流域西北部分的高强度和极高强度氮流失主要是高施肥强度和低黏的土壤性质引起的，上海周边的高强度和极高强度氮流失是由于低黏的易于流失的土壤引起的。高施肥强度是所有极高和高强度氮流失等级分区氮污染的重要原因，而在太湖流域，局部地区的高施肥强度主要原因是高投入、高产出的菜地的集中分布。该项目成果可为水环境高效治理提供方向和建议，为太湖流域面源污染控制的科学决策提供重要支撑。