Reinforced Soil Salinization with Distance Along the River: A Case Study of the Yellow River Basin

Clarifying the relationship between salt-affected soils and their adjacent river systems is critical to address the challenges posed by soil salinization on agricultural production. Among the various biophysical and land management factors linked to soil salinization, few studies have investigated the importance of the distance along the river. Based on the Hetao Irrigation District of the Yellow River Basin, we collected 5314 soil samples and analyzed 17 soil parameters to explore the relationship between salt-affected soils and their distance along the river. Soil salinization was reinforced by the distance along the river, as a trade-off between soil ion accumulation and nutrient regulation. Soil total water-soluble salt content increased by 16.4 mg kg−1 every kilometer, likely due to ions leaching from the soils upstream and partially accumulating in the soils downstream, especially for Cl-, SO42-, and Na+. Greater input and less solubility of Ca2+ based soil amendments, phosphorus fertilizers, and organic materials on the less salt-affected soils upstream may explain the declines in soil Ca2+, available phosphorus, and soil organic matter with distance along the river. With every kilometer along the river, soil exchangeable sodium and cation exchange capacity increased by 8.89 × 10−4 and 1.26 × 10−2 cmol kg−1, respectively, which led to an increase in soil exchangeable sodium percentage by 9.59 × 10−5. The increase in soil exchangeable sodium along the river was mainly due to Na+ accumulation, while soil cation exchange capacity was regulated by soil organic matter and total nitrogen. Soil pH increased by 1.21 × 10−3 per kilometer along the river, associated with the decrease of soil organic matter. Future saline soil amelioration and reutilization initiatives at regional scales may be improved by accounting for the changes in soil physicochemical properties resulting from the distance along the river.