Contrasting patterns and controls of soil carbon and nitrogen isotope compositions in coastal wetlands of China

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.