Temporal and spatial variability of nitrogen in soil in the Wulate irrigation district of the Hetao irrigation area in Inner Mongolia
河套灌区不同灌季土壤氮素时空分布特征分析

The spatial variability of nitrogenous pollutants in soil has a great significance for the balance of field fertilization and the decrease in the loss of nitrogen. After analysis of soil samples collected from the Wulate irrigation district, a case study area in the east of the Hetao irrigation area, traditional statistics and the ordinary Kriging method of geostatistics were applied to determine the spatial distribution of nitrogen in soil (total nitrogen, ammonia nitrogen and nitrate nitrogen) within a depth of 0 to 20 cm. The correlations between nitrogenous pollutants and groundwater level, irrigation quantity, drainage and return water were also investigated. The results indicated that the theoretical variogram model of Total Nitrogen (TN), Ammonium Nitrogen (AN) and Nitrate Nitrogen (NN) in different periods of irrigation (April, July and November) were presented well of the spherical, Gaussian and exponential forms. The residual sums of squares (RSS) of the theoretical variogram between the actual variogram of TN, AN, NN were in the range from 0.009 to 0.1238. The nitrogen in soil had a strong spatial relevancy except the ammonium nitrogen in November. The nitrogen content in soil was generally high in the northeast and low in the southwest in April and July. In the whole area, the total nitrogen and nitrate nitrogen in soil reached the highest content in April and the lowest in November, while the ammonium nitrogen in soil had the highest content in July and the lowest in November. The temporal and spatial variability of nitrogen in soil was strongly correlated with the groundwater level, irrigation quantity, drainage and return water.