2.1. Study site
A field experiment was conducted in 2015-2018 at Yichang Irrigation station (41°07′N, 108°00′E, 1 022 m ASL), in Wuyuan County, Inner Mongolia, China. This region has a semi-arid climate, with annual active accumulated temperature (≥ 10 ℃) of 3 100 °C. The average annual precipitation is ~200 mm, whereas potential evaporation is approximately 2 200 mm, indicating severe water deficit for crop production (Zhang et al., 2019). Precipitation mainly occurs in summer and autumn. Monthly precipitation and potential evaporation during the growing season (i.e., May-October) of the experimental years are shown in Figure 1. The soil is classified as chloride-sulfate saline soil with silty loam. Basic properties in 0-20 cm layer were as below: soil organic matter 11.3 g kg-1, total nitrogen 0.6 g kg-1, total phosphorus 0.7 g kg-1, alkaline hydrolyzed nitrogen 57.3 mg kg-1, available phosphorus 10.2 mg kg-1, and available potassium 139 mg kg-1. Soil nutrient contents in deeper soil layers (20-100 cm) are showed in Table 1. More information of the study site was descripted in Huo et al. (2017).