Carbon sequestration service flow assessment and threshold-based key factors controlling supply–demand balance in the Middle and Lower Yangtze River Plain

Carbon sequestration services are crucial for climate regulation, yet current assessments predominantly focus on static carbon storage while overlooking dynamic spatial flows and management thresholds in rapidly urbanizing regions. This study developed an integrated framework combining multi-scale spatial analysis with XGBoost-SHAP modeling to evaluate CSS flows and identify thresholds across the Middle and Lower Yangtze River Plain from 2002 to 2022. The findings reveal: (1) A widening supply-demand gap, with the supply-demand ratio declining from 1.18:1 to 0.40:1 as carbon supply decreased from 8.33 × 108 t to 8.25 × 108 t while demand surged to 20.48 × 108 t; (2) Intensifying spatial heterogeneity from provincial to grid scales, with carbon deficits concentrated in northeastern industrialized areas; (3) A fundamental shift from natural to anthropogenic control, with land use intensity becoming the dominant predictive factor by 2012 (SHAP = 0.137, intensifying to 0.180 by 2022), revealing critical thresholds of population density (109.17 persons/km2), land use intensity (2.44), elevation (187.72 m), and temperature (17.72 °C); (4) Eight priority management zones covering 24,297.77 km2 (2.64% of study area), concentrated in topographically complex regions and vital for regional carbon balance. The irreversibility of threshold transitions renders immediate protection of these areas an urgent policy imperative for achieving carbon neutrality goals.