VALLEY BOTTOM POSITION AND THE OCCURRENCE OF PALEOSOLS AFFECT SOC DYNAMICS

Soil organic carbon (SOC) storage in depositional zones has been a growing topic of interest in recent years as these areas may be sinks of SOC. However, SOC dynamics greater than 1 m in depth in river valley bottom soils are not well understood. This study examines the soil organic carbon along three alluvial landscape positions in the forested, humid-subtropical setting of the Clarks River in the western Kentucky portion of the Mississippi River basin. These soil and depositional profiles range in age from ~8,000 years ago to modern, 21 cal yr. BP, (BP = AD 2010). The mean surface soil SOC stocks (kg/m2) decrease from Floodplains (2.62 ± 0.3), Terraces (2.31 ± 0.21) to Bars (1.32 ± 0.24); whereas, the mean stocks of buried layers (buried soils and lithologic discontinuities) decrease from Terraces (4.13 ± 0.24), Bars (3.07 ± 0.54) to Floodplains (2.68 ± 0.24). Total SOC estimates in the buried layers make up over half of the SOC inventory for all landforms. The isotopic composition of SOC (δ13C) at all sites is consistent with C3 vegetation. The depth profiles show that δ13C becomes less negative with depth, likely due to a combination of the Suess effect and microbial decomposition. A classification and regression tree analysis shows that soil horizon, pH, landscape position, and magnetic susceptibility are significant predictors of mean SOC content. Notably, the tree shows that alkaline pH (\u3e7.9) is an important predictor in higher mean SOC. These alkaline soil pH values are found in buried calcareous soils with pedogenic carbonate in the Clarks River Terraces, where acidified samples were found to have higher mean SOC. A 14C age from SIC shows that this carbonate may have formed under drier conditions of the mid-Holocene hypsithermal. This age association suggests that the legacy of buried soils in valley bottoms plays a role in deep SOC storage today. This study showed that buried soils and sediments contain the majority of the SOC in the Clarks River, while landform position and a calcareous paleosol played an important role in the storage of that SOC