Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils

Soil salinization is closely related to land degradation and is presumed to exert a significant effect on the preservation of soil organic carbon (SOC). However, the salinization-induced changes in SOC accumulation over the application gradient of amendments remain unclear. To evaluate the potential for salinization elimination and C sequestration, incubation experiments with four straw addition levels and six aluminum sulfate (Al3+) gradients were conducted in a soda saline–alkali soil, followed by the analysis of partial least squares path models (PLS-PM). The results showed that combined applications significantly reduced soil salinity and sodicity. The C sequestration performance under coapplications was greater than that under individual applications. The SOC and heavy fraction OC (HFOC) contents under the coapplication of 1.6% Al3+ and 10% straw were greater than those under the individual applications of either 1.6% Al3+ or 10% straw by 231.08% and 149.86%, and 9.70% and 18.78%, respectively. Coapplications significantly increased macroaggregates and aggregate-associated SOC levels. PLS-PM demonstrated that Na+, Ca2+ and HCO3− were important environmental factors associated with C sequestration. Overall, our results suggest that Al3+ and straw enhanced C sequestration by regulating salt ions and increasing soil aggregates and that 10% straw combined with 1.6% Al3+ had a greater effect on soda saline–alkali soil. Our study is highly important for the utilization of saline–alkali land and C sequestration in western Jilin Province.