Particle composition, nutrient content, and alkalinity determine organic carbon variations in saline-alkali soils...

Land-use changes alter soil nutrient content, salinity and alkalinity, particle composition, and soil organic carbon (SOC) content. However, although SOC has been extensively studied, its dynamics in saline-alkali soils, particularly in the context of land-use change, remain underexplored. This study investigated variations in soil total nitrogen (TN), total phosphorus (TP), carbon-to-nitrogen ratio (C/N), pH, electrical conductivity (EC), particle composition, and SOC content across paddy fields, dry fields, and grasslands in the western Songnen Plain to explore the mechanisms driving SOC dynamics. The results showed that SOC content, TN, TP, EC, and silt content were higher in paddy fields than in grasslands. The soil properties of dry fields exhibited minimal variations compared to those of paddy fields and grasslands. Correlation analysis revealed negative and significant associations between SOC content and soil pH and sand content. Conversely, SOC content was positively and significantly correlated with TN, TP, clay content, and silt content. Random forest analysis identified soil TN (12.83 %), TP (8.46 %), silt content (7.96 %), sand content (7.55 %), and pH (5.66 %) as the most influential predictors of SOC content. Furthermore, variation partitioning analysis demonstrated that the interaction between soil nutrient content and particle composition, along with nutrient content alone, exerted the strongest influence on SOC variability. The interaction among particle composition, salinity and alkalinity (particularly alkalinity), and nutrient content accounted for 19.34 % of the variance in SOC content. Our research suggests that reducing soil alkalinity, increasing soil nutrient and fine particle contents can increase the C sink function of saline-alkali soils.