Soil Salinization Characteristics of Artificial Grassland in the Qaidam Basin and Their Impact on Soil Fertility

The Qaidam Basin is situated in the arid region of northwestern China, characterized by extensive areas of saline–alkali land and significant soil salinization, which severely impedes the sustainable development of local agriculture. This study focuses on the saline soils of the Qaidam Basin Artificial grassland, where 109 surface soil samples (0–20 cm depth) were collected to analyze the salinity characteristics and soil fertility across varying degrees of salinization. The relationship between soil salinity ions and soil fertility indicators is investigated. The results indicate: (1) All samples were categorized into five salinization levels based on total salt content: non-salinized, lightly salinized, moderately salinized, heavily salinized, and saline soil, with average total salt contents of 0.75, 2.30, 3.45, 8.90, and 14.95 g/kg, respectively. Among the soil salt ions, the anion SO42− constituted the highest proportion. The primary anionic components in the study area were SO42− and Cl−, while the predominant cations were Na+ and Ca2+. As salinity increased, the proportion of Na+ and Cl− within the total ions gradually increased, whereas the proportions of Ca2+, Mg2+, and HCO3− ions progressively decreased. (2) As salinity increased, soil total nitrogen, total phosphorus, total potassium, alkali-hydrolysable nitrogen, and available potassium content all showed an upward trend, while soil pH exhibited a downward trend. Soil organic matter content fluctuated across different salinity levels, but no significant differences were observed in organic matter content between soils of varying salinity degrees. (3) The correlation analysis revealed that the contents of total nitrogen, total phosphorus, total potassium, alkali-hydrolysable nitrogen, and readily available potassium were highly positively correlated with total salinity. Among these, the strongest correlation was observed between total salinity and readily available potassium, with a correlation coefficient of 0.828, followed by total nitrogen at 0.767. The pH level exhibited a highly significant negative correlation with total salinity, while demonstrating a highly significant positive correlation with HCO3−. Soil organic matter demonstrated highly significant positive correlations with K+ and SO42−, along with a significant positive correlation with Ca2+. (4) Redundancy analysis indicated a negative correlation between soil organic matter and HCO3−, while total nitrogen, total potassium, alkali-hydrolysable nitrogen, available phosphorus, and available potassium were positively correlated with K+. In both non-salinized and saline soils, the pH level showed a positive correlation with HCO3−. K+ and Na+ exerted substantial influence on soil fertility, and as salinization increased, the impact of total salinity on soil fertility progressively intensified. This study aims to analyze the relationship between soil salinity and soil fertility, facilitating subsequent in-depth investigations into the spatial variation characteristics of soil fertility and salinity. It endeavors to provide a theoretical basis for the remediation of saline–alkali soils in the Qaidam Basin.