Livestock manure emission and cultivated land pollution load in the midst upstream of Danjiang River in Shangluo City

Persistent water quality safety and protection have been a hot research in the upstream region of the middle route of China's South-to-North Water Diversion Project. Shangzhou District, Danfeng County and Shangnan County, which are governed by Shangluo City, have already delineated the zone for water source protection in the upstream reach of Danjiang River. There is a need of basic research on scientific management of livestock and efficient implementation of research on water source protection area. It is also necessary to understand development status of livestock, estimate livestock manure emission, analyze absorption capacity of cultivated lands, and evaluate pollution risk caused by the emission. Until now, less data or literature has existed on this field of research. In this study, the emission of livestock manure was estimated using the livestock manure emission coefficient method from 2014 to 2016 in the upstream region of Danjiang River in Shangluo City, including Shangzhou District, Danfeng County and Shangnan County along the river direction. Then the maximum cultivated land load of pig-waste equivalent was confirmed according to cultivated area and pollution risk of cultivated land due to livestock manure in the three districts was evaluated. The results showed that annual pig-waste equivalents of livestock manure in Shangzhou, Danfeng and Shangnan were respectively 5.732×10⁵ t·a^-1, 9.321×10⁵ t·a^-1 and 1.274×10⁶ t·a^-1, with livestock manure emissions from pig and poultry accounting for up to 68.06% of the total. The cultivated land load of pig-waste equivalents for livestock manure were respectively 27.04 t·hm^-2·a^-1, 76.72 t·hm^-2·a^-1 and 90.10 t·hm^-2·a^-1 in Shangzhou, Danfeng and Shangnan. The maximum standard of cultivated land load of pig-waste equivalent of livestock manure was 61.22 t·hm^-2·a^-1 for Shangluo City. Pollution risks were level Ⅱ for Shangzhou (moderate risk), and level Ⅳ for both Danfeng and Shangnan (severe risk). The average cultivated land loads for total nitrogen were respectively 159 kg·hm^-2·a^-1, 450 kg·hm^-2·a^-1 and 530 kg·hm^-2·a^-1 in the three districts; for total phosphorus were 51 kg·hm^-2·a^-1, 179 kg·hm^-2·a^-1 and 199 kg·hm^-2·a^-1; for biochemical oxygen demand (BOD) were 716 kg·hm^-2·a^-1, 2 275 kg·hm^-2·a^-1 and 2 595 kg·hm^-2·a^-1; for chemical oxygen demand (COD) were 768 kg·hm^-2·a^-1, 2 345 kg·hm^-2·a^-1 and 2 667 kg·hm^-2·a^-1; and for NH₃-N were 65 kg·hm^-2·a^-1, 201 kg·hm^-2·a^-1 and 232 kg·hm^-2·a^-1. It was concluded that the number of livestock bred was large and the husbandry structure unreasonable in the three districts of Shangluo City. Because of the limited cultivated land, cultivated land loads and livestock manure pollution were pretty heavy, with increasing risks from upstream to downstream reaches. Excessive pollutants caused loss of nitrogen and phosphorus, which severely threatened water source safety of the South-to-North Water Diversion Project.