Loss of nitrogen and phosphorus from farmland runoff and the interception effect of an ecological drainage ditch in the North China Plain—A field study in a modern agricultural park

Agricultural non-point source pollution caused by excessive fertilization is a problem in the North China Plain. The experimental site was at a modern high-tech agricultural park in Dezhou City of Shandong Province that is located within the lower Yellow River impact plain. The fertilization, irrigation, and crop management in the modern agricultural park were investigated to determine the proportion of nitrogen and phosphorus loss of the total input. An ecological drainage ditch was constructed in the modern agricultural park in 2016. At the same time, a Doppler flowmeter and a runoff automatic acquisition instrument were installed in the inlet and outlet of the ecological drainage ditch for monitoring the flow of runoff and collecting runoff samples under natural rainfall in 2018 and 2019. The nitrogen, phosphorus, and sediment runoff contents were determined in the laboratory in order to calculate the loss of nitrogen and phosphorus from farmland runoff and the nitrogen and phosphorus interception rate of the ecological drainage ditch. The results showed that there were a total of 13 runoff events in 2018 and 2019 and that the average concentration of total nitrogen and phosphorus in farmland runoff was 2.63–18.84 mg·L−1and 0.81–13.14 mg·L−1 in both years combined, respectively. The main forms of nitrogen found in runoff were NO3−-N, and the main form of phosphorus found in runoff was particulate phosphorus. The amount of nitrogen and phosphorus lost through runoff was 2.90% and 5.07%, respectively, of the average nitrogen and phosphorus input from both years combined. Runoff was generated when daily precipitation was above 17.4 mm and daily maximum precipitation intensity was above 5.1 mm·h−1. There was a linear relationship between daily precipitation, daily maximum precipitation intensity, and the amount of runoff after natural rainfall. A linear regression equation was established to predict the amount of runoff in the study area. The results indicated that ecological ditches in the watersheds of modern agricultural parks could be used to intercept nitrogen and phosphorus from farmland runoff effectively.