Salt effects on soil organic carbon in irrigated farmland in arid region

• The effect of salt on soil organic carbon depends on its concentration level. • High salt not only reduces SOC content, but also the share of recalcitrant organic carbon. • The dominant salt ions have the greatest impact on soil organic carbon. Salinization is one of the major challenges facing irrigated agriculture in arid regions, and it has substantial influences on soil carbon pools. Understanding the response pattern of soil organic carbon (SOC) to salt is paramount for carbon balance under the intensification salinization. This study investigated the concentration of soil salt, its major ions, as well as SOC and its chemical compositions across 106 irrigated farmlands in the arid region of northwest China, and analyzed the relationship between soil salt and SOC. We showed that the recalcitrant organic carbon (ROC) accounted for 75.03 % of SOC, while the labile organic carbon (LOC) only accounts for 7.89 %. SOC had a significant quadratic function relationship with salt, where it responded positively when the salt concentration was below 3.95g kg −1 , and began to decline after exceeding this level. The high salt significantly increased the proportion of LOC and decreased the share of ROC. In addition, soil C:N ratio decreased linearly with increasing salt concentration. Among the salt ions, the dominant ions like SO 4 2- and Ca 2+ had the greatest impact on SOC and the fractions. Redundancy analysis, variance partitioning analysis and random forest model all further confirmed the importance of salt and its ions on the SOC components of irrigated farmland in the arid area. Our study highlights that salinity alters both SOC content and chemical composition, with high salt reducing stable carbon fractions. This study also emphasizes the potential application of dominant salt ions in enhancing the accuracy of SOC sequestration-related prediction models.