Spatial Variability of Sedimentary Organic Carbon Stocks and Sources Across Tidal Zones in an Estuarine Mangrove Forest

Mangrove sediments represent vital reservoirs for carbon storage in coastal wetland ecosystems, and characterizing their organic carbon stocks and sources is critical for elucidating carbon cycling dynamics and accurately quantifying blue carbon sinks strength. However, spatial heterogeneity of sediment organic carbon stocks and their sources across tidal elevations within estuarine mangroves remains limited. Sediment cores (100 cm in length) were retrieved across high, middle, and low tidal flats in the Nanliu River Estuary, southwestern China. Integrating geochemical proxies including total organic carbon, total nitrogen, C/N ratios, stable carbon isotopes, and lignin phenols, with a three-end-member mixing model and Markov chain Monte Carlo simulations, we revealed the spatial differentiation characteristics of sediment organic carbon stocks and their sources across tidal elevations. The results showed that sediment organic carbon stocks in the high and middle tidal flats (143.40 and 148.65 Mg C ha−1, respectively) were significantly higher than that in the low-tidal flat (74.24 Mg C ha−1), which was attributed to the thriving mangrove vegetation in the high and middle tidal zones compared to its sparse distribution in the low tidal zone. Across all three tidal flats, the influence of mangrove litterfall and root systems diminishes with depth, leading to an overall vertical decline in carbon stocks. Sediment organic carbon is consistently dominated by Terrestrial organic carbon across all three tidal flats, reflecting substantial allochthonous inputs delivered by estuarine runoff. In terms of vertical variations, mangrove-derived organic carbon driven by seaward tidal flat progradation and mangrove colonization expansion, in the high and middle tidal flats exhibits a high in shallow layers and low in deep layers pattern. The intensities of hydrodynamics and mangrove vegetation on the low tidal flat are irregular, and both terrigenous and marine organic carbon exhibit a multi-stage variation pattern. Overall, coastal blue carbon assessments must integrate tidal elevation-driven spatial distribution characteristics to reduce sink evaluation uncertainties.