Spatial patterns of soil carbon stocks, vertical fluxes, and lateral exchange associated with hydrological connectivity in China's major estuarine wetlands

Estuarine wetlands are critical global carbon sinks, yet their carbon budgets, encompassing vertical and lateral fluxes, remain poorly constrained at macro-scales. Specifically, how different dimensions of hydrological connectivity are associated with these budget components across estuarine wetlands remains unclear. Integrating 1089 field measurements from 114 studies with remotely sensed hydrological variables, we used random forest regression to upscale and quantify soil organic carbon (SOC) stocks, vertical fluxes, and lateral exchange across China's ten major estuaries. Model R 2 values ranged from 0.54 to 0.75. SOC stocks exhibited localized landward or seaward accumulation gradients, whereas contemporary carbon burial and plant inputs clustered predominantly in low-latitude estuaries. These elevated inputs were partially offset by gaseous emissions with contrasting patterns: CO 2 dominated carbon loss and formed southern hotspots, whereas CH 4 fluxes were lower and more fragmented. This spatial mismatch between carbon inputs and losses generated pronounced regional heterogeneity in lateral carbon exchange ( LCE ), whose absolute magnitude was equivalent to 50.10% of the summed absolute vertical carbon-cycling fluxes, with a Monte Carlo 95% uncertainty interval of 44.82–54.85%. These dynamics were closely linked to hydrological connectivity, but different carbon components showed distinct hydrological associations. SOC was mainly associated with water persistence and hydrological stability, plant carbon fixation with vegetation and soil moisture, whereas carbon burial, CH 4 , CO 2 , and LCE were more strongly associated with runoff. These findings highlight lateral transport as a critical spatial compensatory mechanism and emphasize the need to integrate lateral fluxes into blue carbon accounting and tailor hydrological interventions to maintain estuarine carbon balances.