Threshold delineation of in-situ and ex-situ carbon ecological compensation in mountain-water-forest-farmland-lake-grassland composite ecosystems

Precisely characterizing the spatial transfer processes of carbon sink services (CSSs) and defining compensation thresholds are essential for improving ecological compensation (EC) mechanisms. Addressing the deficiencies of existing studies that overemphasize static supply and demand while neglecting spatial radiation processes and cross-scale transmission mechanisms, this study constructs an analytical framework of “CSS flow – multi-scale radiation – multi-scale compensation threshold”. Based on the breaking point, HYSPLIT model and other quantitative methods, this study thoroughly reveals the evolutionary characteristics, spatial radiation effects and compensation threshold characteristics of CSSs within the mountain-water-forest-farmland-lake-grassland composite ecosystems. The results reveal that: (1) During 2000–2020, the total flux of CSS flow decreased by 36.79%, and the supply–demand mismatch continued to intensify. (2) In-situ radiation was primarily dominated by the forest subsystem with a contribution of 61.75%, and the grassland subsystem presented a diffusion characteristic of high frequency but low intensity. In terms of ex-situ radiation, the forest subsystem of Chongqing formed obvious radiation corridors along the middle and upper reaches of the Yangtze River. (3) In-situ carbon EC exhibited a spatial pattern of centralized payment in metropolitan areas and dominant compensation reception in ecological functional zones. The forest subsystem occupied the largest proportion of EC (62.98%–74.29%). Under the ex-situ carbon EC, the number of compensation subjects at the municipal scale was 5.5 times that at the provincial scale. The research findings of this study can offer a theoretical basis for establishing a trans-regional horizontal carbon EC mechanism and optimizing regional coordinated development in Chongqing.