A case study on non-point source pollution and environmental carrying capacity of animal raising industry in subtropical watershed
亚热带流域氮磷排放与养殖业环境承载力实例研究

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.