亚热带流域氮磷排放与养殖业环境承载力实例研究

畜禽养殖业粪便排放已经成为我国农村地区主要的农业面源污染源之一,也是制约养殖业良性发展的主要瓶颈.本文以湖南省长沙县典型亚热带流域为研究单元,基于流域水环境定位观测、耕地氮(N)磷(P)消纳能力以及养殖业调查和土壤分析资料,初步分析了亚热带丘陵区的面源污染现状及畜禽养殖业的环境承载力.结果表明,研究区金井河流域134.4 km2范围内N、P年负荷分别为N 2.72 t·km-2和P0.11t·km-2,其中养殖粪便对水体总氮(TN)、总磷(TP)负荷的贡献率分别约为42.2％和62.0％.区内平均畜禽养殖密度为3.46 AU·hm-2(相当于流域内年出栏生猪24.39万头),显著高于现有化肥用量条件下流域的实际承载力1.13 AU·hm-2(相当于流域内年出栏生猪6.35万头),因此养殖密度过高是导致研究区水体NP负荷较高的主要原因.区内N、P盈余量分别为N 35.8 kg·hm-2、P 18.61 kg·hm-2.研究区基本不施用化肥条件下畜禽养殖业的最大环境承载力为7.26 AU·hm-2,在有机肥占合理施肥量30％条件下,当地畜禽养殖业的环境承载力为2.74AU·hm-2(相当于流域内年出栏生猪19.50万头).降低养殖密度、调整养殖业空间布局以及提高养殖废弃物的资源化利用率是防治当前面源污染的有效途径.

A case study on non-point source pollution and environmental carrying capacity of animal raising industry in subtropical watershed

The discharge of faeces from animal production is a major source of nitrogen (N) and phosphorus (P) pollutants in the subtropical region of China. To quantify the N and P load from animal production to hydro-systems in the region, surveys on pig density and N,P inputs to farmlands, soil N and P contents, and annual observations of the N and P flows in the streams and the terminal river were carried out in a hilly watershed (134.4 km²) in Changsha County, Hunan Province. It was showed that the total NP loads from the watershed to the terminal river was 2.72 t and 0.11 t·km^-2·a^-1, respectively; within which 42.2% of the N and 62.0% of the P were loaded from animal production sites. The mean animal density in the watershed was currently 3.46 AU·hm^-2. The current NP inputs to farmlands was 35.8 kg and 18.6 kg·hm^-2 in the watershed. Based on an estimate for the environmental adaptation capacity (EAC) of N and P in the watershed, the current animal density was 2.4 times larger than the critical density (1.13 AU·hm^-2) of the EAC for the watershed under the current N and P fertilizer inputs levels. Assuming that 30% of N discharge for animal production is re-used by farmlands, the animal density under the EAC can be set at 2.74 AU·hm^-2 (equivalent to 195,000 pigs for slaughter every year). Thus, we propose to reconstruct the agricultural systems by reducing animal production and establishing an integrating crop and animal farming systems.