Annual soil salt balance and crop performance under brackish water irrigation during the winter wheat season

The annual double cropping system of winter wheat-summer maize is a stable cropping system in the low plain of the Bohai Sea. Owing to the shortage of fresh water and the relative abundance of shallow saline water in this region, saline water is used to irrigate winter wheat. The accumulated salt in the soil after the winter wheat season affects the following summer maize season, and the annual soil salt balance is affected by saline irrigation. To address these issues, a continuous 4-year field study was conducted at the Nanpi Eco-agriculture Experimental Station of Chinese Academy of Sciences from 2015 to 2019. Four treatments were set up to irrigate winter wheat once using irrigation water (70 mm) with different total salt contents at the jointing stage, i.e., freshwater 1 g·L^-1 (F), 3 g·L^-1 brackish water (S3), 4 g·L^-1 brackish water (S4), and 5 g·L^-1 brackish water (S5). There was also a treatment without irrigation (rain-fed, CK). The results showed that there was no significant difference in winter wheat yield among the four irrigation treatments, and the average yield with irrigation was 31.6% greater than that of CK. Irrigation using brackish water increased the soil salt content of the top 1 m soil profile during winter wheat harvesting. The soil salt content increase was proportional to the salt content of irrigation water. There were no significant differences in the soil salt contents below 1 m. The necessary irrigation using 70 mm freshwater for the seedling establishment of summer maize after winter wheat harvest reduced the salt content of the top 20 cm soil layer, which created favorable soil conditions for maize growth. No significant differences in summer maize yield were observed among different treatments. With concentrated rainfall during the summer monsoon season, the salt content in the top soil layer (0-40 cm) for the S3, S4, and S5 treatments decreased by more than 30% due to leaching, but the salt content of deep soil did not change. The results showed that the high salt tolerance of winter wheat maintained a stable yield with saline water irrigation. With irrigation at maize sowing and the subsequent summer rainfall, the accumulated salt is leached out of the major root zone to enable the continued use of saline water irrigation for winter wheat in this region.