Concentration Characteristics of Nitrogen in the Paddy-Wheat Rotation Soil of the Yellow River Irrigation Area in Jinan, China

Abstract

In view of the lack of research on non-point source pollution in the lower reaches of the Yellow River Irrigation Area in China, an experiment was conducted in typical paddy-wheat rotation fields in Jinan Yellow River Irrigation Area to study the distribution characteristics of nitrogen accumulation in soil and shallow groundwater, and the influencing factors. The results showed that total nitrogen (TN) concentration varied considerably at 0∼80 cm soil depth, while there was less variation at the depth of 80∼120 cm. The nitrate (NO3−-N) concentration showed a similar distribution profile in soil. TN and NO3−-N were greatly accumulated in the 0-100 cm layer in both paddy and wheat soils. The TN and NO3−-N contents in paddy soil increased at the rates of 96.48% and 300.61%, respectively. Consequently, nitrogen accumulation occurred at deeper levels of soil. NO3−-N content in groundwater during paddy growth exceeded the maximum contamination level of 20 mg/L, which is worse than class III of Quality Standard for Groundwater. There was significant correlation between NO3−-N in shallow groundwater and the depth of shallow groundwater as well as NO3−-N content in deeper soil. Moreover, nitrogen content in soil and shallow groundwater was affected by irrigation. Irrigation after topdressing during paddy growth caused leaching of NO3−-N from soils into groundwater. It can be concluded that irrigation caused strong leaching of residual NO3−-N in soil without input of nitrogen, which poses a potential threat to the shallow groundwater quality.