Field experiments were conducted to investigate the effects of four fertilization treatments, that is, controlled-release fertilizer (CRF), biochar combined with controlled-release fertilizer (BC+ CRF), biochar combined with stabilized fertilizer (BC+ SF), and biochar combined with controlled-release fertilizer and stabilized fertilizer (BC+ CRF+ SF), on the dynamics of the pH and nitrogen mass concentration and runoff losses of nitrogen in paddy field surface water in the Taihu Lake Basin. The results show that the average pH of the surface water decreases by 3.16%-4.48% for BC+ CRF+ SF and is in the range of 5.64-8.15. The average total nitrogen (TN) mass concentration of surface water ranges from 19.05 to 25.23 mg·L-1. A significant decrease of 4.75%-6.58% in the TN mass concentration of surface water was observed for BC+ CRF+ SF. The average ammonium (NH4+-N) and nitrate (NO3--N) mass concentration of the surface water vary from 0.01-17.26 mg·L-1 and from 0.24-3.11 mg·L-1, respectively. Inorganic nitrogen is dominated by NH4+-N in surface water. Compared with individual CRF, other treatments significantly reduce the NH4+-N mass concentration of surface water by 35.89%-48.78% and the NO3--N mass concentration of surface water by 20.54%-37.01%. The BC+ SF combination shows a significant reduction in the NH4+-N and NO3--N mass concentration of the surface water, which greatly lowers the risk of inorganic nitrogen loss via runoff. The runoff losses of TN, NH4+-N, and NO3--N are in the range of 16.24-18.09, 1.76-2.22 and 0.76-1.38 kg·hm-2, respectively. Compared with the individual CRF, the runoff loss of TN, NH4+-N, and NO3--N of other treatments is reduced. The BC+ CRF+ SF combination shows a significant reduction in the runoff losses of nitrogen, which greatly lowers the risk of nitrogen nonpoint source pollution from paddy fields.
Keywords: biochar; controlled-release fertilizer; nitrogen; paddy field surface water; runoff; stabilized fertilizer.