Characteristics of runoff processes and nitrogen loss via surface flow and interflow from weathered granite slopes of Southeast China

Rainfall intensity and slope gradient are two of the most important factors affecting the variations of runoff nitrogen (N). However, the effects of slope gradient and rainfall intensity on N loss via surface flow and interflow on weathered granite slopes are poorly understood. In this study, 12 artificial rainfalls (three rainfall intensities and four slope gradients) were simulated to investigate the coupling loss characteristics of surface flow-interflow-total nitrogen (TN), nitrate nitrogen (\({\rm{N}}{{\rm{O}}_{{3^ -}}} - {\rm{N}}\)) and ammonia nitrogen (\({\rm{N}}{{\rm{H}}_{{4^ +}}} - {\rm{N}}\)) on weathered granite slopes. The results show that slope gradient has a greater impact on the surface flow when the rainfall intensity is relatively large. The effect gradually weakens with the decrement of rainfall intensity. The interflow yield increases firstly with the prolongation of rainfall duration, then tends to be stable and finally decreases. The total surfaceflow percentage increases with rainfall intensity while it decreases with increasing slope gradient with a range of 10.88%–71.47%. The TN loss concentration of the surface flow continually decreases with rainfall duration while that of the interflow shows different fluctuations. However, the TN loss loads of both surface flow and interflow increase with increasing rainfall intensity and slope gradient. The \({\rm{N}}{{\rm{O}}_{{3^ -}}} - {\rm{N}}\) concentration of interflow is much higher than that of the surface flow. The \({\rm{N}}{{\rm{H}}_{{4^ +}}} - {\rm{N}}\) concentration is always less than that of \({\rm{N}}{{\rm{O}}_{{3^ -}}} - {\rm{N}}\) with no significant difference between surface flow and interflow. The percentages of the TN, \({\rm{N}}{{\rm{O}}_{{3^ -}}} - {\rm{N}}\), and \({\rm{N}}{{\rm{H}}_{{4^ +}}} - {\rm{N}}\) total loss load and concentration of surface flow and interflow were analyzed. The results show that N loss via both surface flow and interflow occurs mainly in the form of \({\rm{N}}{{\rm{O}}_{{3^ -}}} - {\rm{N}}\). Most of the N loss is caused by interflow which is the preferential path of runoff nutrient loss. These findings provide data support and underlying insights for the control of runoff and N loss on the weathered granite slopes.