Effects of planting industry agglomeration on water environment: Threshold regression analysis based on panel data

Based on district- and county-level data of Zhangjiakou from 2012 to 2016, this study used the threshold model and other methods to assess the effects of planting industry agglomeration on water environment of the industry. The results are as follows 1) From 2012 to 2016, the chemical oxygen demand (COD) of Zhangjiakou planting industry showed a decreasing trend and the ammonia nitrogen yield showed an increasing trend. 2) Zhuolu, Kangbao, Chicheng, Zhangbei, Huailai, Guyuan, and Chongli represented the clusters of central planting industry, while Yu, Wanquan, and Xiahuayuan represented the clusters of lower planting industry. 3) Before crossing the threshold of agglomeration degree, planting industry agglomeration showed positive effects on COD; however, after crossing the threshold value, it showed negative effects. The degree of planting industry agglomeration showed negative effects on ammonia nitrogen yield. 4) Public finance budget expenditure and funding for research and development showed positive effects on COD and ammonia nitrogen yield. Investment of fixed assets showed negative effects on COD and positive effects on ammonia nitrogen yield. The total output of planting industry showed negative effects on COD and positive effects on ammonia nitrogen yield. Chemical fertilizer application positively affected COD but negatively affected ammonia nitrogen yield.