Soil Carbon Pools In Forested Riverine Landscapes.

Riverine forest soils are influenced by both fluvial and pedogenic processes. The combination of these processes result in additions, redistribution, and burial of soil organic carbon (SOC). As a result, alluvial soils tend to have complex morphologies with buried carbon-rich A and O horizons at variable depths from the soil surface. Although these aggrading floodplain soils may be a significant watershed carbon sink, they are difficult to assess at a landscape-scale because of inherent variability in SOC distribution. Thus, improvements are needed in the estimation of alluvial SOC pools. In this study, we quantified SOC to a depth of 1 m in an old-growth bottomland forest at Congaree National Park (CNP), South Carolina and 1st through 3rd order riparian zones of southern New England (SNE). In both regions, hydric mineral soils that contained buried surface horizons had significantly greater SOC pools (mean 291 Mg C ha^-1 in CNP and 277 Mg C ha^-1 in SNE) than similarly mapped soils without these features (mean 108 Mg C ha^-1 in CNP and 188 Mg C ha^-1 in SNE). Separation of soils into those with buried horizons and those without decreased the overall variability in mean SOC pools (coefficient of variation decreased by 23-35% in CNP and by 3-12% in SNE). Results from our studies suggest that variations in riverine forest SOC pools are related to differences in sedimentation patterns and carbon burial rates. Thus, recognition and differentiation of soil mapping units based on the presence or absence of buried surfaces may be an important consideration for future soil carbon accounting efforts.