Characteristics of Nitrogen and Phosphorus Migration in Red Soil Sloping Farmland after Fertilization under Intermittent Rainfall

In order to explore the sustained migration characteristics of nitrogen and phosphorus in fertilized red soil slope farmland under intermittent rainfall, eight events of indoor artificial rainfall simulation experiments were conducted intermittently from July to August 2022 to observe the changes in concentration and flux of nitrogen and phosphorus, and to analyze the first flush effect (FFE) of different forms of nitrogen and phosphorus in runoff. The results show that: (1) If the whole intermittent rainfall experiment is regarded as a complete rainfall cycle, the cumulative loss of nitrogen in interflow is greater than that in surface runoff, while the loss of phosphorus is mainly due to surface runoff. There is a significant correlation between the cumulative loss of nitrogen and phosphorus from surface runoff and interflow. The exponential or logarithmic function can be used to fit the relationship between the cumulative nitrogen/phosphorus loss and the cumulative production flow. (2) Under the condition of intermittent rainfall, the first flush phenomenon of nitrogen and phosphorus in fertilized red soil slope farmland occurred. The intensity of FFE varied as TDP>TP>TPP>NH₄⁺-N>TN>NO₃^--N. (3) The emigration of NH₄⁺-N, TP, TDP and TPP was mainly concentrated in the first three rainfall events, and their concentrations in surface runoff were always greater than that in interflow, and their intensity of FFE in interflow was slightly higher than that in surface runoff. In contrast, the migration of NO₃^--N and TN with runoff exhibits a high concentration in surface runoff during early intermittent rainfall and a high concentration in interflow during later stages, and the first flush phenomenon of NO₃^--N and TN in surface runoff is more significant. The above results indicate that, the effective ways to prevent and control non-point source pollution of fertilized red soil slope farmland in South China are: (1) to control the loss of nitrogen and phosphorus in surface runoff and interflow in early rainfall events and (2) to pay attention to the long-term and sustained loss of nitrogen in interflow.