Spatial variability and drivers of soil organic carbon burial in the tidal flats of the Yellow River Delta

Coastal tidal flats play a critical role in the global carbon cycle, yet the spatial patterns and controlling mechanisms of soil organic carbon (SOC) burial within estuarine wetlands remain poorly understood. Here, we investigated the lateral (seaward-landward) and vertical variability of SOC burial across low, middle, and high tidal flats in the Yellow River Delta. Based on 210 Pb chronology and coupled with δ 13 C and biomarker (lignin phenols, amino sugars) analyses, soil cores were collected to quantify sediment accumulation rates (SAR), organic carbon burial rates (OCBR), SOC stocks (SOC stock ), and sources. Results showed that SAR decreased progressively from the seaward to the landward flats, while OCBR showed no significant lateral variation. The mean OCBR was 37.44 ± 7.57, 32.71 ± 9.16, and 24.62 ± 4.09 g m −2 yr −1 at the low, middle, and high tidal flat, respectively. Both SAR and OCBR reached their lowest values at 60–100 cm layer. SOC stock were significantly higher in the high tidal flats, especially within the 0–30 cm layer. The ISOsource model revealed that marine phytoplankton-derived organic carbon decreased landward across all soil depths, whereas riverine particulate organic matter and local plant-derived contributions increased correspondingly. Although lignin phenols generally decreased from sea to land, their vertical variation remained limited. In contrast, microbial necromass carbon exhibited no significant lateral or vertical trends. Random forest analysis indicated that the C/N ratio played a significant role in controlling SOC stock throughout the soil profile. These results highlight pronounced spatial heterogeneity in SOC burial across tidal flats, necessitating the integration of both lateral and vertical variations in assessing carbon sequestration potential.