Soil N and salinity leaching after the autumn irrigation and its impact on groundwater in Hetao Irrigation District, China

Soil water and salinity are crucial factors influencing crop production in arid regions. An autumn irrigation system employing the application of a large volume of water (2200-2600 m^3 ha^-1) is being developed in the Hetao Irrigation District of China, since the 1980s with the goal to reduce salinity levels in the root zone and increase the water availability for the following spring crops. However, the autumn irrigation can cause significant quantities of NO3- to leach from the plant root zone into the groundwater. In this study, we investigated the changes in soil water content, NO3-N and salinity within a 150 cm deep soil profile in four different types of farmlands: spring wheat (FW), maize (FM), spring wheat-maize inter-planting (FW-M) and sunflower (FS). Our results showed that (1) salt losses mainly occurred in the upper 60 cm of the soil and in the upper 40 cm for NO3-N; (2) the highest losses of salt and NO3-N could be observed in FW, whereas the lowest losses were found in FW-M. NO3-N concentration, pH and electrical conductivity (EC) in the groundwater were also monitored before and after the autumn irrigation. We found that the autumn irrigation caused the groundwater concentration of NO3-N to increase from 1.73 to 21.6 mg L^-1, thereby, exceeding the standards of the World Health Organization (WHO). Our results suggest that extensive development of inter-planting tillage might be a viable measure to reduce groundwater pollution, and that the application of optimized minimum amounts of water and nitrogen to meet realistic yield goals, as well as the timely application of N fertilizers and the use of slow release fertilizers can be viable measures to minimize nitrate leaching.