Aims: Natural stable isotope compositions of carbon (δ13C) and nitrogen (δ15N) can reveal biogeochemical mechanisms that control ecosystem carbon (C) and nitrogen (N) processes. However, little is known about the latitudinal patterns and controlling mechanisms for soil δ13C and δ15N in coastal wetlands based on a large spatial scale. Methods: A total of 76 sites of coastal wetlands were sampled along a 5000 km transect across temperate-subtropical-tropical zones to explore biological and environmental controls on soil stable C and N isotopic compositions. Results: The results showed that soil δ13C (ranging from -27.5‰ to -18.3‰) and δ15N (from 2.66‰ to 9.97‰) varied over a broad geographic scale. The C4-plant (Spartina alterniflora) dominated sites have 2–6‰ higher δ13C values than those of other vegetation types, while mangrove soils have lower δ13C values compared to those of marshes; and soils with vegetated C4-plants and mangroves have 1–3‰ higher δ15N values relative to native grass marshes. There were no significant relationships between mean annual temperature (MAT) or precipitation (MAP) and δ13C, but positive correlations between MAT and δ15N, as well as MAP and δ15N. Conclusions: Vegetation composition and plant C inputs directly control the spatial variability of δ13C patterns. Simultaneously, climate and edaphic variables (e.g., soil water content, pH, and C availability) are the predominant factors influencing δ15N patterns. These findings provide new insights into soil organic matter turnover and response to climate and environmental changes and improve the prediction of C stability and burial in coastal wetlands.