The mechanisms that cause the rapid and abundant release of nitrogen from hill slope farmlands into streams are still poorly understood. In hilly areas composed of purple soils high in nitrogen pollution content, TN, NO3-N, NH4-N, NO2-N, PN, and D/H, isotopic ratios loss processes were monitored in two sloping farmland plots over the course of two storms. The results show that the dominant transportation pathway of nitrogen loss involved NO3-N movement was through the subsurface flow both vertically and laterally. Subsurface flow can also contribute to significant degrees of PN loss. TN and PN peaks occurred before the subsurface flow peak was reached in every case. However, NO3-N peaks occurred both before and after flow peaks occurred. Hydrograph separation using delta D shows that preferential flows dominated the generation of subsurface flows with low NO3-N concentrations. The delta D profile response to rainwater shows that piston flows caused the accumulation of high soil water quantities in the outflow. Most of the samples demonstrated that dissolved inorganic nitrogen concentrations are positively correlated with outflow soil water proportions. Consequently, the piston flow type is assumed to have caused high degrees of dissolved nitrogen loss in the examined sloping farmlands. After being dispelled by the piston flow, preferential flows acted as the main transportation carrier of nitrogen in the runoff. These results suggest that the soil water proportions in flows significantly influence levels of dissolved nitrogen loss.