Effects of applying manure on soil physical properties and crop yield under long-term saline water irrigation

To maintain soil quality under long-term saline water irrigation, the influence of manure on soil physical properties was examined. Long-term saline irrigation has been conducted from 2015 to 2024 at the Nanpi Eco-Agricultural Experimental Station of Chinese Academy Sciences in the Low Plain of the North China Plain, comprising four irrigation treatments: irrigation once at the jointing stage for winter wheat with irrigation water containing salt at fresh water, 3, 4 and 5 g·L^–1, and maize irrigation at sowing using fresh water. Manure application was conducted under all irrigation treatments, with treatments without manure application used as controls. The results showed that under long-term irrigation with saline water, the application of manure increased the soil organic matter content, exchangeable potassium, available phosphorus, and total nitrogen content in the 0–20 cm soil layer by 46.8%, 117.0%, 75.7%, and 45.5%, respectively, compared to treatments without manure application. The application of manure reduced soil bulk density. It also increased the proportion of water-stable aggregates and the abundance of bacteria, fungi, and actinomycetes in the tillage soil layer compared to the controls. Because of the salt contained in the manure, the application of manure had dual effects on soil salt content. During the winter wheat season, manure application increased soil salt content. The salt content was significantly reduced during the summer maize season, owing to the strong salt-leaching effects under manure application, resulting in a smaller difference in salt content between the manure and non-manure treatments. During the summer rainfall season, improvements in soil structure under manure application increased the soil desalination rate for the 1 m top soil layer. The desalination rate for 0-40 cm and 40-100 cm was averagely by 39.1% and 18.9% higher, respectively, under manure application as compared with that under the no-manure treatments. The yield of winter wheat under manure application was 0.12% lower than that of the control, owing to the higher salt content during the winter wheat season. In contrast, the yield of summer maize improved by 3.9% under manure application, owing to the increased soil nutrient content and effective salt leaching. The results of this study indicated that manure application helped maintain the soil physical structure, which is important for the long-term use of saline water. In practice, using manure with a low salt content is suggested to reduce the adverse effects of saline water irrigation on soil properties and achieve sustainable saline water use.