Effect of underground brackish water depth on soil water-salt distribution and water consumption of winter wheat

Production of winter wheat in the low plains around the Bohai Sea faces the constraint of freshwater resource shortage, and the efficient and safe use of the relatively abundant shallow brackish water resources is of great importance for sustainable agricultural development. Soil column simulation experiments were conducted at the Nanpi Ecological Agricultural Experiment Station of the Chinese Academy of Sciences in 2021–2022. Four treatments, including no groundwater but freshwater (487.5 mm) irrigation treatment (CK) and underground brackish water depths of 0.5 m (GW1), 1.0 m (GW2), and 1.5 m (GW3) with 20 mm freshwater irrigation were applied, with three replications for each treatment. This experiment investigated the characteristics of soil water, salinity content, and water use in winter wheat. The results showed that the distribution of soil water and salt in the surface soil (0–10 cm) gradually decreased with increasing groundwater depth. Compared with the CK treatment, the surface soil water content of the GW1 treatment significantly increased by 30.9% and that of the GW3 treatment significantly decreased by 79.3%, whereas there was no significant difference for the GW2 treatment. Compared with the CK treatment, the salinity of surface soil in the GW1 and GW2 treatments significantly increased by 3.4 g·kg⁻¹ and 2.0 g·kg⁻¹, respectively, whereas there was no significant difference in the GW3 treatment. Salt in the GW1 and GW2 treatments mainly accumulated in the surface soil, whereas that in the GW3 treatment was low and mainly accumulated at a depth of 30–50 cm. The evapotranspiration of winter wheat significantly decreased with increasing groundwater depth. The evapotranspiration of winter wheat significantly increased by 50.2% and 20.3% under the GW1 and GW2 treatments, respectively, compared to CK, and there was no significant difference between the GW3 and CK treatments. The grain yield of the GW3 treatment was the highest, which was significantly increased by 38.04% compared with that of the CK treatment. The highest values for water use efficiency at the biomass and yield levels in the GW3 treatment were significantly higher than those in the CK treatment by 26.7% and 40.1%, respectively. The above results show that 1.5 m is the upper limit of the suitable groundwater depth for winter wheat growth in underground brackish water shallow burial areas when the mass concentration of brackish water is 3 g·L⁻¹ and groundwater depth is 0.5–1.5 m. Under these conditions, the surface salinity and crop evapotranspiration were the lowest, and the yield and water use efficiency were the best.