Effects of saline water mulched drip irrigation on cotton yield and soil quality in the North China Plain

The shortage of freshwater resources is a considerable challenge for agricultural production in the North China Plain (NCP). Safe and efficient use of saline water resources is thus urgently required. To reveal the effects of different salinities of irrigation water on the yield and soil quality of mulched drip-irrigated cotton, a field experiment was conducted from 2017 to 2019. Five salinity levels of irrigation water were included: 1.3 (T1, control), 5.4 (T2), 8.8 (T3), 12.4 (T4) and 15.9 (T5) dS·m⁻¹. The results showed that the harvesting density and seed cotton yield increased first and then decreased with increasing salinity of irrigation water. Saline water irrigation with salinity ≤ 8.8 dS·m⁻¹ did not lead to salt accumulation in the main root zone (0–60 cm) with each passing year but a decrease of salt in 2018. In the third year of saline water irrigation, with the increase in irrigation water salinity, the electrical conductivity of the saturated soil extract (ECₑ), soil sodium adsorption ratio (SAR), pH and bulk density (BD) in the plow layer gradually increased. However, the soil saturated hydraulic conductivity (Kₛ), water stable macroaggregate (> 0.25 mm) content, catalase (CAT) and urease (URE) activity decreased with increasing salinity. Moreover, the soil organic carbon (SOC) content and alkaline phosphatase (ALP) activity increased first and then decreased. Irrigation water salinity ≤ 5.4 dS·m⁻¹ had no significant effect on most physicochemical properties, such as pH, SOC content, BD, Kₛ, water stable aggregate content and activities of CAT, URE and ALP. Compared with the T1 treatment, the soil quality index (SQI) of T2, T3, T4 and T5 treatments decreased by 1.2%, 10.5%, 16.5% and 23.7%, respectively. Considering cotton yield, soil salt accumulation and SQI, mulched drip irrigation is conducive to the sustainability of cotton with saline water levels below 5.4 dS·m⁻¹.