Beyond universal application: dissecting the differential amelioration and carbon sequestration performance of biochar in chloride-, sulfate-, and soda saline-alkali soils

• Not all saline-alkali soils suit biochar amelioration and carbon sequestration. • Biochar showed better performance in chloride and sulfate saline-alkali soils. • Biochar drove labile-to-stable matter shift under Cl − stress to sequester carbon. • Biochar acted as electron donor to mitigate C mineralization under SO 4 2− stress. • Biochar enhanced C storage in soda saline-alkali soil via CaCO 3 precipitation. Biochar is widely recommended for ameliorating saline-alkali soil and enhancing soil organic carbon. However, its efficacy across different saline-alkali soils (chloride, sulfate, and soda-types) remains unclear. This study evaluated biochar’s amelioration and carbon sequestration performances on distinct saline-alkali soils. Results showed biochar addition significantly reduced electrical conductivity in chloride and sulfate saline-alkali soils from 4.55 and 4.54 mS cm −1 to 2.97 and 2.74 mS cm −1 , respectively, accompanied by a marked promotion of plant growth. In contrast, it failed to improve soda saline-alkali soil properties or plant growth, indicating strong type-dependent efficacy. After the plant growth, biochar addition increased organic carbon in chloride, sulfate, and soda saline-alkali soils by an additional 27.23, 25.61, and 8.40 g kg −1 soil, respectively, which was 59.67%, 52.03%, and 14.67% higher than that in the controls without biochar. Biochar’s carbon sequestration capacity was strongest in chloride saline-alkali soil, intermediate in sulfate saline-alkali soil, and weakest in soda saline-alkali soil. Mechanistic analysis revealed that in chloride saline-alkali soil, biochar alleviated Cl − stress to halophytes, shifting metabolites from decomposable low-molecular-weight substances to stable high-molecular-weight substances. In sulfate saline-alkali soil, biochar-derived electrons were utilized by sulfate-reducing bacteria, reducing organic acid mineralization and increasing their proportion to 18.34%. In soda saline-alkali soil, Ca 2+ from biochar promoted CaCO 3 precipitation, enhancing physical protection of organic carbon. This study challenges biochar’s universal applicability, showing not all saline‑alkali soils suit biochar amendment, thereby providing a critical scientific basis for formulating differentiated biochar remediation and carbon sequestration strategies tailored to specific saline‑alkali soil types.