Regulation of Water‐Soluble Salt Ions by Plantations to Enhance Carbon Sequestration in Coastal Saline‐Alkali Soils

ABSTRACT Soil carbon stability is critical for global carbon balance and ecosystem sustainability. Coastal saline‐alkali lands have great potential for carbon sequestration, yet the mechanisms by which water‐soluble salt ions regulate soil carbon dynamics remain unclear. To elucidate this relationship, this study systematically evaluated the co‐variations among water‐soluble salt ion distribution, soil chemical properties, and carbon fractions within the 0–100 cm soil profile under different plantation types ( Taxodium hybrid “Zhongshanshan” , Carya cathayensis, and Ulmus parvifolia ) in coastal saline‐alkali land. The objective was to reveal the regulatory mechanisms of salt ions on soil carbon processes. The results showed that afforestation significantly reduced soil pH and electrical conductivity, thereby alleviating soil salinization, particularly within the 40–100 cm layer where the total salt content was markedly lower in C. cathayensis plantations. Afforestation also reshaped the composition of water‐soluble salt ions: Na + and Mg 2+ concentrations decreased in surface soils, while Cl − and SO 4 2− concentrations increased, reflecting the regulatory role of the rhizosphere microenvironment on ion migration. Regarding carbon accumulation, both T . hybrid and C. cathayensis significantly enhanced total carbon and organic carbon contents in the 0–40 cm soil layer. Redundancy analysis and correlation tests indicated that Na + was the primary driver of soil carbon variation, showing a significant negative correlation with total and organic carbon but a positive correlation with inorganic carbon. Other influential factors included pH, HCO 3 − , K + , and SO 4 2− . This study elucidates how water‐soluble salt ions affect carbon dynamics in coastal saline‐alkali soils, providing theoretical and practical guidance for enhancing carbon sequestration and optimizing plantation selection.