磷素（P）在环境中的过量累积是导致农业面源污染的主要因素。论文以湖南省长沙县脱甲河农区小流域（52 km²）和涧山河森林-农区小流域（50 km²）为研究单元,基于入户调查资料和连续4 a的流域把口站水文水质定位观测数据,采用物质流分析法,对比研究了亚热带丘陵区典型小流域P的收支平衡及流失特征。结果表明,农区小流域出口地表水总磷（TP）含量变化为0.03~0.68 mg·L^-1,平均含量为0.21 mg·L^-1,整体达到Ⅲ类水质标准;森林-农区小流域TP含量变化范围为0.01~0.35 mg·L^-1,平均为0.08 mg·L^-1,整体为Ⅱ类水质,表明森林-农区小流域地表水水质明显优于农区小流域。农区小流域P的环境滞留强度（32.0 kg·hm^-2·a^-1）显著高于森林-农区小流域（20.6 kg·hm^-2·a^-1）,对环境影响更大。以2013年为例,农区小流域P的主要输入项为饲料,占53.1%,森林-农区小流域则以肥料为主,占53.0%;两个流域的P输出项都以植物和畜禽产品输出为主,均占总输出量的94.0%左右。控制小流域肥料和饲料投入、增加循环利用途径以及提高P利用率是当前减轻水体富营养化的有效途径。

Surplus phosphorus is a main source of non-point source pollution. Tuojia, a small agricultural watershed (52 km²) and Jianshan, a small forest-agricultural watershed (50 km²), located in Changsha County, Hunan Province, are taken as the case study. Based on the household surveys and 4-successive-year of observations of the phosphorus (P) flows at the terminal of river, substance flow analysis was used to quantify the budgets of P. Results showed that the concentrations of P in the agricultural ranged 0.03-0.68 mg·L^-1, with the average 0.21 mg·L^-1, while the corresponding one in the forest-agricultural ranged 0.01-0.35 mg·L^-1 with the average 0.08 mg·L^-1. The surface water quality in the forest-agricultural watershed was obviously better than that in the agricultural watershed. The phosphorus surplus intensity in the agricultural watershed was apparently higher (32.0 kg·hm^-2·a^-1) than that in the forest-agricul-tural watershed (20.6 kg·hm^-2·a^-1), and thus had a greater effect on environment. In 2013, livestock feeding accounted for 53.1% of total P inputs in the agricultural watershed, while fertilizer contributed 53.0% in the forest-agricultural. Generally, the phosphorus from plant and animal products dominated in both watersheds, approximately accounting for 94.0% of the total. Thus, controlling the amount of P fertilizer and improving its utilization efficiency are good ways to mitigate the eutrophication in the study area.