Eco-hydrological processes in controlling vertical and lateral carbon fluxes in a subtropical salt marsh: Insights from modeling and scale-resolved analysis

Carbon exchange in coastal salt marshes is shaped by complex interactions among vegetation-soil system, climate, and tidal regimes. In this study, the spatiotemporal heterogeneity and environmental driving of the vertical and lateral carbon cycle in a subtropical salt marsh were investigated by utilizing a process-based model (SMM-YE). Following the simulation of high-frequency carbon fluxes, a joint diagnostic analysis (XGBoost-SHAP) was used to quantify the scale-dependent seasonal variability and regulatory patterns of environmental factors. Due to high integration of species-specific phenology, hydrodynamic processes, and carbon biogeochemistry, the model effectively captures the seasonal course of gross primary productivity (GPP), ecosystem respiration (ER), net ecosystem exchange (NEE), methane flux (F_CH4), dissolved organic and inorganic carbon fluxes (F_DOC, F_DIC). The diagnostic analysis indicated that NEE and F_DOC were sensitive to air temperature and tides, respectively. F_CH4 showed a seasonal variability in its sensitivity to thermal and hydrodynamic factors, while F_DIC tended to respond jointly to both drivers. The wavelet-based spectral analysis displayed the divergences in spectral characteristics between the vertical and lateral carbon fluxes, as that NEE and F_CH4 displayed smooth energy distributions that aligned closely with temperature, whereas F_DOC and F_DIC exhibited more variable energy profiles synchronized with tide. The wavelet coherence analyses further illustrated the scale-dependent controls in vertical and lateral carbon fluxes synchronized with diurnal and tidal cycles, respectively. As a result, the dynamics of net ecosystem carbon balance exhibited marked spatiotemporal variability. Our findings highlight the importance of high-resolution mechanistic studies for rigorous carbon accounting and prediction of long-term sequestration potential in coastal ecosystems.