In the tidal salt marshes, the spatial variation of soil carbon (C) quality along the vegetation zones is still unclear. Here, a study is conducted in the mudflat, open shrubland, and dense shrubland in the Liaohe Delta, Northeast China, and soil organic C (OC) stock, particulate organic matter C (POMC), and aggregate‐associated OC in the 0–15 and 15–30 cm depths are investigated along these vegetation zones. Among the three vegetation zones, dense shrubland had the greatest soil OC stock, open shrubland had the lowest value, and mudflat had an intermediate value. In each soil depth, dense shrubland generally had higher soil OC and POMC concentrations, and POMC/OC ratio compared to mudflat and open shrubland, respectively. Moreover, dense shrubland had a greater proportion of macro‐aggregates (>0.25 mm in diameter) and OC content in the macro‐aggregates than mudflat and open shrubland, respectively. In addition, the OC content in the soil macro‐aggregates positively correlated with soil POMC concentration. These results suggest that the high spatial variability in soil OC quantity and quality along the vegetation zones should be incorporated to accurately develop regional C budget in coastal wetlands in the Liaohe Delta, and also imply that a shift in vegetation zones induced by sea level rise could potentially cause substantial changes in the quantity and quality of soil OC in the tidal salt marshes.

BACKGROUND Since the 1990s, drylands have been extensively converted to rice paddy fields on the former wetlands in the Sanjiang Plain of northeast China. However, the influence of this successiveland‐use change from native wetlands to drylands to rice paddy fields on soil organic carbon (C) dynamics remains unexplored. Here, we compared the difference in soil organic C stock among native wetlands, drylands, and paddy fields, and then used a two‐step acid hydrolysis approach to examine the effect of this land‐use change on labile C I (LPI‐C), labile C II (LPII‐C), and recalcitrant C (RP‐C) fractions at depths of 0–15 cm, 15–30 cm, and 30–50 cm. RESULTS Soil organic C stock at a depth of 0–50 cm was reduced by 79% after the conversion of wetlands to drylands but increased by 24% when drylands were converted to paddy fields. Compared with wetlands, paddy fields had 74% lower soil organic C stock at a depth of 0–50 cm. The conversion of wetlands to drylands reduced the concentrations of LPI‐C, LPII‐C, and RP‐C fractions at each soil depth. However, land‐use change from drylands to paddy fields only increased the concentrations of LPI‐C and LPII‐C fractions at the 0–15 cm and 30–50 cm depths. CONCLUSION The conversion of drylands to paddy lands on former wetlands enhances the soil organic C stock by promoting labile C fraction accumulation, and labile C fractions are more sensitive to this successive land‐use change than recalcitrant C fractions in the Sanjiang Plain of northeast China. © 2022 Society of Chemical Industry.