【Objective】Saline-alkali soils have enormous potential for carbon sequestration, and straw inputs in these soils strongly influence the microorganism-mediated sequestration of soil organic carbon (SOC). Nevertheless, the mechanisms and effects of different salinization degrees on the sequestration of straw-derived carbon in soil aggregates remain unexplored. 【Method】A 90-day laboratory experiment using continuous 13C labeling combined with amino sugars microbial biomarkers technology was conducted to investigate the effects of saline-alkali degrees on the content and distribution of straw-derived carbon and its contribution rate to soil organic carbon. Also, the content of microbial necromass carbon and its contribution rate to 13C-SOC at aggregate sizes in different saline-alkali soils was evaluated.【Result】(1) From the perspective of aggregate size fractions, the straw-derived carbon was primarily distributed in the 2~0.25 mm aggregate, and its content was higher than that in other aggregate size fractions. In contrast, the >2 mm aggregate exhibited the lowest content of straw-derived carbon. With the increase of soil salinization degrees, significant increase in the distribution content of straw-derived carbon was observed in the >2 mm and < 0.25 mm aggregate, but the distribution content of straw-derived carbon in the 2~0.25 mm aggregate significantly decreased. (2) At the aggregate scale, in saline-alkali soils, fungal necromass carbon dominated within microbial necromass carbon, accounting for approximately 84.74% to 95.29% of the microbial necromass carbon. The content of 13C-fungal and 13C-bacterial necromass carbon was the highest in the <0.25 mm aggregate, while the highest ratio of 13C-fungal necromass carbon and 13C-bacterial necromass carbon was in the >2 mm aggregate. The content of 13C-fungal necromass carbon and the ratio of 13C-fungal necromass carbon and 13C-bacterial necromass carbon significantly increased with increasing salinization degrees, but the content of 13C-bacterial necromass carbon showed a opposite trend. (3) Contribution rate of straw-derived carbon to SOC and 13C-microbial necromass carbon to 13C-SOC increased gradually with decreasing aggregate sizes in soil aggregates. In addition, soil salinization degrees significantly increased the contribution rate of straw-derived carbon to SOC in each aggregate size and the contribution rate of 13C-fungal necromass carbon and 13C-microbial necromass carbon to 13C-SOC in the 2~0.25 mm and <0.25 mm aggregate. However, a significant decreased was observed for the contribution rate of 13C-microbial necromass carbon to 13C-SOC in the >2 mm aggregate and the contribution rate of 13C-bacterial necromass carbon to 13C-SOC in each aggregate size.【Conclusion】This study clarifies the microbial mechanism of straw-derived carbon sequestration processes at aggregate sizes in different saline-alkali soils, which provides important theoretical guidance for regulating soil organic caron sequestration processes in saline-alkali soils ecosystems through straw-returning practices.