Wetlands serve as crucial carbon reservoirs within terrestrial ecosystems, with plant-derived and microbial-derived organic carbon being essential components for the stable accumulation of soil organic carbon (SOC) in these environments. Lake wetlands represent complex ecological systems that integrate hydrological, topographical, and vegetational elements, and are characterized by notable elevation variations due to their position at the interface between water and land. This study examined the surface (0–20 cm) and subsurface (20–40 cm) soil layers of the Poyang Lake wetland. By integrating data on aboveground and belowground plant communities with fundamental soil physicochemical properties, the research aimed to elucidate the distribution patterns and determining factors of plant-derived and microbial-derived organic carbon along elevation gradients. The findings indicated that SOC content in the surface soil was markedly higher than in the subsurface soil at corresponding elevations, and displayed a gradual increasing trend with elevation. In contrast, SOC content in the subsurface soil did not exhibit significant variation along the elevation gradient (P > 0.05). Furthermore, the contributions of microbial-derived organic carbon (ranging from 28.21% to 62.66% in surface soil and from 23.44% to 54.10% in subsurface soil) to total SOC were significantly greater than those of plant-derived organic carbon (ranging from 15.81% to 25.85% in surface soil and from 16.73% to 28.35% in subsurface soil) at all corresponding elevations (P < 0.05). Both the absolute content of microbial-derived organic carbon and its proportional contribution to SOC increased progressively along the elevation gradient in both surface and subsurface soils, whereas plant-derived organic carbon did not show any significant trend in variation. Analysis using a partial least squares path model further revealed that plants and soil properties primarily influenced SOC content in the surface soil through their effects on microbial-derived organic carbon, whereas their impact on SOC in the subsurface soil was relatively minimal.