Salinity threshold of long-term saline water irrigation for winter wheat in Hebei Lowland Plain

Compared with soil salts content measurement, irrigation water salts are much easier to be monitored and regulated under saline water irrigation of crop cultivation. However, the salinity threshold of irrigation water (STIW) is a rather complex and difficult parameter to set. In this study, the STIW value for winter wheat was estimated based on a long-term experiment (2007–2015) at the Dry-land Farming Station in Hengshui, Hebei Lowland Plain. In the study, different salinities of irrigation water 1 g.L^-1 (control), 2 g.L^-1, 4 g.L^-1, 6 g.L^-1 and 8 g.L^-1 were used to determine STIW under long-term saline water irrigation of ‘Shijiazhuang 8’ wheat variety. The relative germination and yield of wheat, soil salinity content and characteristics of wheat growth were measured under different conditions. Yearly STIW was calculated by using the FAO piecewise linear model and the final result decided through comprehensive considerations of the risks of both yield loss and soil salt accumulation. The factors influencing annual fluctuations of STIW were also determined. The results showed that the relative emergence rates of winter wheat under 4 g.L^-1 and 6 g.L^-1 treatment were 93.8% (P > 0.05) and 70.4% (P < 0.05), the relative yield were 86.0% (P < 0.05) and 65.3% (P < 0.05), respectively, compared with the control. Thus the grain yield loss (no more than 15%) and emergence of winter wheat were not limiting factors of STIW of wheat. For the 9-year data, the calculated STIW was within 2.14–3.95 g.L^-1, with an average of 3.19 g.L^-1 and a variation coefficient was 21.1%. The final value of STIW that took into accounts of soil salt accumulation risk and wheat yield was 2.47 g.L^-1. STIW was negatively correlated with soil salt in the 0–100 cm soil profile at pre-sowing stage (coefficient r = 0.587), but positively correlated with fresh water irrigated yield ( r = 0.516). The result of estimated risk of soil salt accumulation indicated that exponential model had the best simulation effect for the relationship between irrigation water salinity and soil salt. After 9 years of consecutive saline water irrigation at STIW, average soil salt was 0.98 g.kg^-1 in the 0–20 cm soil layer and was 1.17 g.kg^-1 in the 0–100 cm soil profile. There was a slight accumulation of soil salt, but no significant effect on winter wheat yield. The risk of soil salinization due to long-term 2.47 g.L^-1 saline water irrigation was weak in Hebei Lowland Plain.