亚热带丘陵小流域土壤有效磷空间变异与淋失风险研究

肥料过施导致的土壤磷素累积和淋失是农业面源污染的重要方面.以湖南省长沙县金井镇脱甲河小流域(52 km2)为研究区,采用高密度布点采样、Arc GIS软件和属性相似反距离加权插值法研究了亚热带丘陵小流域表层(0~20 cm)土壤有效磷(Olsen-P)含量(以P计,下同)的空间分布特征与磷素的淋失风险.结果表明,菜地、果园、稻田和茶园土壤Olsen-P平均含量为62.0、16.1、14.4和13.7 mg·kg-1,是林地(平均含量为2.36 mg·kg-1)的5.8~26.3倍.5个土地利用类型土壤Olsen-P含量均具有高等变异水平和中等程度的空间自相关性(块基比C0/(C0+C)=50%),这与区内地形地貌、土壤母质、人工施肥等具有密切关系.根据土壤0.01 mol·L-1Ca Cl2浸提态P和Olsen-P的非线性关系可确定区内红壤和水稻土P的淋失风险临界值分别为69.97和98.40 mg·kg-1,并据此对脱甲河小流域土壤磷素淋失的风险进行了定量评价,结果表明旱地土壤具有明显较高的淋失风险,其中中等以上的比例占36.4%,而稻田土壤仅有0.2%,为中等以上淋失风险.因此,控制旱地(尤其是菜地)磷肥的投入是降低亚热带丘陵小流域土壤P淋失风险和减轻农业面源污染的关键.

Spatial variation and leaching risk of soil phosphorus in a small hilly watershed of the subtropical China

Soil phosphorus (P) accumulation and its corresponding leaching risk associated with excessive fertilizer application are concerned as one of the major sources of non-point agricultural pollutants. Spatial distribution of soil Olsen-P, and P leaching risk from the topsoil (0~20 cm) downward were assessed in a representative small hilly watershed for the subtropical region of China. The watershed is located in Jinjing town, Changsha County, Hunan Province and covers an area of about 52 km². Data for soil Olsen-P were obtained from the analyses of soil samples as 10 sites per km. The spatial assessments were carried out by inverse distance weighting of similar attributes using ArcGIS software and interpolation method. The means of Olsen-P content accumulated in soils under vegetable, orchard, paddy rice and tea cultivation were 62.0, 16.1, 14.4, and 13.7 mg·kg^-1, respectively, which was 5.8~26.3 times larger (p<0.05) than that (2.36 mg·kg^-1) for woodland soils. High extents of spatial variations and moderate degree of spatial autocorrelation (C₀/(C₀ +C) = 50%) were found in the Olsen-P contents of five land use patterns, and were also associated with topography, soil parent materials, and artificial fertilization. The analysis of the nonlinear relationship between soil 0.01 mol·L^-1 CaCl₂-extracted P and Olsen-P showed that the thresholds of soil P leaching risk for red earth and paddy soils in the watershed were 69.97 and 98.40 mg·kg^-1, respectively. Based on these two thresholds, the land proportion with P leaching risk above the medium degree accounted for 36.4% of the total for the upland area, while only 0.2% in the paddy area. Thereby, to mitigate agricultural non-point source P-pollution in subtropical hilly watershed and reduce P leaching risk, it is important to reduce P fertilizer application in upland areas, particularly for vegetable cultivation.