南水北调中线水源区丹江口市域景观格局变化及氮磷净化能力

丹江口库区是南水北调中线工程重要水源地,通过探究库区景观格局变化与氮磷净化能力,对未来景观规划和生态系统功能提升提供科学依据。在县域尺度上以丹江口市为研究区,运用景观格局指数和InVEST模型研究2003—2018年库区景观格局和氮磷净化能力,并利用Pearson相关分析和RDA分析法探讨二者之间的关系,分析引起氮磷净化能力变化的因素。结果表明:在斑块类型上,水域面积和林地面积不断增加,耕地和园地面积逐渐减少,建设用地面积整体上呈增长趋势;在景观水平上,景观形状指数不断减少,蔓延度指数和聚集度指数整体呈增长趋势,形状复杂程度降低,景观聚集程度提升,散布与并列指数增加了5.58,香农多样性指数变幅较小,各斑块类型趋于规则,呈均衡趋势分布。2003年、2008年、2013年和2018年氮磷净化能力呈持续增强趋势,其中TN输出总量分别为899.224、801.481、776.979、672.149 t, TP输出总量分别为77.308、69.921、68.163、60.802 t, 15年间TN、TP的净化能力分别增强了25.3%和21.4%;对氮磷净化能力重要性等级划分表明,极重要、高度重要区主要分布于库区、河流两岸和林地区域。利用Pearson相关分析和RDA分析法表明,在斑块类型上林地与氮磷的输出呈显著负相关,耕地、园地和建设用地与氮磷输出量呈正相关;在景观水平上氮磷输出量与景观形状指数呈显著正相关,和散布与并列指数表现出显著负相关,与蔓延度指数、聚集度指数呈负相关。因此,可在库区氮磷输出量较高的地区种植喜氮喜磷植物,通过调节景观格局结构,合理规划土地利用,增加林地面积,建设河岸植被缓冲带充分滞缓径流等一系列措施,充分保障库区水源水质安全。

Change of landscape pattern and nitrogen and phosphorus removal in Danjiangkou City, the Middle Route of the South-to-North Water Diversion Project

The Danjiangkou Reservoir is an important water source for the South-to-North Water Diversion Middle Route Project. Exploring changes in the landscape pattern and nitrogen and phosphorus removal capabilities of the reservoir area provides a scientific basis for future landscape planning and ecosystem function enhancement. Taking Danjiangkou City as the research area, using the landscape pattern index and the InVEST model to study the landscape pattern and nitrogen and phosphorus removal capacity of the reservoir area from 2003 to 2018. Pearson correlation analysis and RDA analysis were used to analyze the relationship between them and analyze the factors causing the change of nitrogen and phosphorus removal capacity. The results show that in terms of patch types, the water area and woodland continue to increase, the area of cropland and orangery land gradually decreases, and the area of urbanized land shows an overall growth trend. At the landscape level, the landscape shape index continues to decrease, the contagion index and the aggregation index indicate an overall increasing trend, the shape complexity decreases, the landscape agglomeration degree increases, the interspersion and juxtaposition index increases by 5.58, and the Shannon diversity index has a smaller change. Each patch type tends to be regular, showing a balanced trend distribution. In 2003, 2008, 2013 and 2018, the nitrogen and phosphorus removal capacity continued to increase. The total output of TN was 899.224 t, 801.481 t, 776.979 t, and 672.149 t, respectively, and the total output of TP was 77.308 t and 69.921 t, 68.163 t, 60.802 t, respectively. The water removal capacity of TN and TP has increased by 25.3% and 21.4% respectively in 15 years. The classification of the importance of nitrogen and phosphorus removal capacity shows that extremely important and highly important areas are mainly distributed in reservoir areas, river banks and woodland areas. Pearson correlation analysis and RDA analysis show that the woodland area and the output of nitrogen and phosphorus are significantly negatively correlated on the patch type, while the output of cropland land, orangery land, and urbanized land is positively correlated with the output of nitrogen and phosphorus. At the landscape level, there is a significantly positive correlation between the output of nitrogen and phosphorus and the landscape shape index, and a significantly negative correlation between the interspersion and juxtaposition index, and the negative correlation with the contagion index and the aggregation index. Therefore, plants that removing nitrogen and phosphorus can be planted in areas with high nitrogen and phosphorus output in the reservoir area. A series of measures, such as adjusting landscape structure, rationally planning land use, increasing forestland area, constructing riparian vegetation buffer zone to fully slow down runoff, can fully guarantee the water quality safety of reservoir area.