Abstract: The agricultural non-point source pollution in the upper reaches of Yellow River is becoming the main pollution source of Yellow River, which threatens the safe use of water resources in the lower reaches of Yellow River basin. Through the investigation on the soil and water characteristics of cropland drainage ditches in the Yellow river irrigation regions of Ningxia and Inner Mongolia, it was found that there existed significant differences in pH and TN, TDS, and NO₃^--N concentrations of drained water among different drainage ditches and within different levels of drainage ditches. The TN, TP, NH₄⁺-N, and COD concentrations of drained water were significantly affected by the drainage time, levels of drainage ditches, and their interactions. The total salt content in the bottom soil of the drainage ditches was obviously lower than that in the side slope soil, whereas the TN and TC contents in the side slope soil of the drainage ditches were 1.2 times higher than those in the bottom soil. The increased proportion of TN, TP, and TC contents in the bottom soil of the ditches relative to that in the side slope soil in Inner Mongolia irrigation region was 1.6, 3.8, and 6.3 times higher than the increased proportion in Ningxia irrigation region, respectively. The TN, TP, NH₄⁺-N, and COD concentrations of drained water in Inner Mongolia in August were 3.2, 2.5, 2.8, 10.0, and 1.5 times more than those in Ningxia irrigation region, respectively. The nitrogen concentration in drained water was higher than the State Surface Water Standard of China, especially in the subfeeder and main drainage ditches in Ningxia and Inner Mongolia irrigation regions. The phosphorus concentration in drained water was relatively low, which hardly cause any pollution. During the main drainage periods in the irrigation region of Inner Mongolia, the nitrogen pollution of subfeeder and the nitrogen and phosphorus pollution of main drainage ditches were most serious in August.

Key words: winter wheat, summer maize, nitrate nitrogen, yield, nitrogen balance.