Straw returning to the field is a pivotal measure for achieving agricultural "carbon neutrality" and sustainable development, yet its carbon sequestration effect is significantly modulated by regional environmental factors. This study employed the Meta analysis method to quantitatively assess the effect of straw return on increasing SOC content in different major rice-growing regions across the country, and deeply analyzed the regulatory mechanism of specific environmental factors in each region on this effect. The results show that the carbon sequestration effect exhibits significant regional differentiation characteristics. Straw returning to paddy fields across the country can significantly increase SOC content (3.76% to 37.64%), but the magnitudes of increase vary greatly. In the northern single-cropping rice area, the increase in SOC is jointly regulated by the amount of straw returned, the amount of fertilizer applied, and soil pH, with the highest increase (8.32% to 68.44%). In the middle and lower reaches of the Yangtze River double-cropping rice area, soil pH, the amount of fertilizer applied, and the average annual temperature are the core influencing factors, with a moderate increase (5.62% to 21.81%). In the southern multiple-cropping rice area, the carbon sequestration effect is deeply influenced by soil type, soil pH, and planting methods, with the lowest increase (1.36% to 11.84%). There is a clear regional critical threshold for nitrogen input. In the northern single-cropping rice area, the threshold for single application of chemical fertilizer to inhibit the increase in SOC is 310 kg/hm2, while the mixed application of organic and inorganic fertilizers still has a significant carbon sequestration advantage at high nitrogen levels. In the middle and lower reaches of the Yangtze River double-cropping rice area, the thresholds for single application and mixed application are 240 kg/hm2 and 160 kg/hm2, respectively; in the southern multiple-cropping rice area, the thresholds are 220 kg/hm2 and 150 kg/hm2, respectively, and more precise nitrogen management is required. This study has identified the key limiting factors for carbon sequestration through straw return and the nitrogen management thresholds in various rice-growing regions, providing a scientific basis for formulating regionalized and precise strategies to enhance agricultural carbon sinks.