Depth-Independent Soil Organic Carbon Persistence across Climate Gradients in Coastal Wetlands

Coastal wetland soils constitute a major global blue carbon pool. Yet, how soil organic carbon (SOC) persistence varies with soil depth and responds to climate change remains poorly understood. Here, we assessed depth-related patterns and climatic controls on SOC persistence using radiocarbon (Δ14C) measurements from 82 samples across 41 sites spanning temperate to tropical coastal wetlands. Radiocarbon-inferred SOC persistence did not differ significantly between topsoil (0–10 cm) and subsoil (40–60 cm) across climate zones and wetland types, with mean Δ14C values of −179.0 ± 155.5‰ and −200.9 ± 161.8‰, respectively (p > 0.05). Interactions among climate, vegetation, and soil biogeochemistry explained about 55% of the variation in SOC persistence in both topsoil and subsoil. Both layers exhibited comparable sensitivity to mean annual temperature and precipitation. However, the inferred regulatory pathways differed between soil layers, with stronger direct climatic effects in topsoil and greater indirect mediation through soil physicochemical and mineral-associated processes in subsoil. Across spatial climatic gradients, higher mean annual temperature and precipitation were associated with greater SOC accumulation but lower persistence, suggesting a potential trade-off between SOC quantity and persistence. These findings demonstrate limited vertical differentiation in SOC persistence between the 0–10 cm and 40–60 cm layers and highlight the need to better represent depth-specific regulatory processes in carbon-climate feedback frameworks.