Does More Straw Always Sequester More Carbon? Unveiling a Thermal Optimization Window for Soil Carbon Sequestration in Peri-Urban Mollisols

Soil organic carbon (SOC) is a major terrestrial carbon pool, and its maintenance is important for climate change mitigation. Straw retention is widely used in croplands, but the association between crop residue cover (CRC) and soil organic carbon density (SOCD) may be nonlinear and may vary under peri-urban thermal conditions. We integrated multi-source spatial datasets for Mollisol croplands in Northeast China to compare recent mean SOCD in the upper 20 cm between stable rural and rural-to-urban transition zones and to examine the associations among CRC, urban heat island (UHI) intensity, and SOCD. XGBoost combined with SHapley Additive exPlanations (SHAP) was used to quantify predictor importance and characterize the nonlinear contribution of CRC to predicted SOCD. Structural equation modeling and partial least squares path modeling were further used to assess statistical associations and interaction terms among CRC, daytime and nighttime UHIs, and SOCD. Mean SOCD was slightly higher in the transition zone than in the stable zone, whereas mean CRC did not differ significantly between the two zones. The contribution of CRC to predicted SOCD showed nonlinear dependence patterns in both zones, with a similar turning region at approximately 48–50% CRC. The lower-CRC patterns differed between zones, whereas CRC was strongly positively correlated with its SHAP value across the higher-CRC interval in both zones. Daytime and nighttime UHIs were associated with contrasting CRC-SHAP patterns, and the estimated interaction effects differed between the stable and transition zones. These results reveal nonlinear and spatially heterogeneous associations between CRC and SOCD under peri-urban thermal conditions and provide a basis for further temporal and process-based investigation.