Differential Effects of Salt Anion Type and Contents on Aggregate-Associated C and N in Saline-Alkali Soils

Soil salinization adversely affects the structure of soil aggregates, reducing organic carbon (OC) and nitrogen (TN) pools, and ultimately impairing soil fertility. This study explores how saline-alkali barriers impact the soil aggregate composition and the OC and TN distribution in three typical salt-affected soils in China, primarily comprising soda salt, chloride salt, and sulfate salt, under varying salinity levels (non-saline, mild–moderate, and severe). The results indicated that salinity significantly reduced the proportion of macroaggregates (> 0.25 mm) in soda-salt and chloride-salt-affected soils, while sulfate-salt soils showed minimal change across salinity levels. The mean weight diameter (MWD) and geometric mean diameter (GMD) declined with increasing salinity, primarily influenced by the sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP), and Cl⁻, which are critical limiting factors for aggregate stability. In contrast, soil organic carbon and biological factors, including enzyme activity, significantly enhanced aggregate stability. With increasing salinity, the contribution of microaggregates (0.053–0.25 mm) and silt + clay fractions (< 0.053 mm) to OC and TN increased in soda-salt and chloride-salt soils, whereas the sulfate-salt soils exhibited this change only under severe salinity. Negative impacts on aggregate stability and biological activity arise from HCO₃⁻ + CO₃²⁻ and Cl⁻, whereas SO₄²⁻ primarily affected biological factors. The overall findings suggest that sulfate salt-affected soils are less sensitive to saline-alkali barriers than those affected by soda and chloride salts. Targeted interventions to mitigate saline-alkali barriers and enhance the soil biological environment of soil are essential for improving aggregate stability and nutrient storage. These insights provide important theoretical support to develop nutrient management strategies for saline-alkali lands.