THE EFFECTS OF ALLUVIAL LANDFORM VARIABILITY ON SOIL ORGANIC CARBON IN THE CLARKS RIVER VALLEY, WESTERN KY, USA

Soil organic carbon (SOC) dynamics at depths greater than 1 m in river valley bottom soils are not well understood. This study examines the stock and isotopic composition of SOC along three alluvial landforms: floodplains, terraces, and bars in the Clarks River valley, Kentucky. Soil samples from four terrace profiles, three floodplain profiles and three bars, ranging in age from 8,036 to 21 cal yr BP, (BP = AD 2010), were collected to depths of refusal or water table. Bulk density, SOC and thickness were used to estimate stocks in each landform type. Mean SOC stocks varied by landform for surface soil and buried layers. Bars had the greatest surface and buried stocks of all landforms with mean stocks of 0.93±0.15 kg/m² and 0.98±0.08 kg/m². Floodplains had the second highest surface stock and the lowest buried stock with mean values of 0.80±0.05 kg/m² and 0.45±0.03 kg/m². Terraces had the lowest surface stock and the second highest buried stock with mean values of 0.76±0.05 kg/m² and 0.81±0.05 kg/m². The isotopic composition at all sites was consistent with C₃ vegetation and δ¹³C became less negative with depth, likely due to decomposition and microbial C contributions. A classification and regression tree analysis (CART) was used to explore what effects landform type and physicochemical properties of the soil had on predicting SOC. CART showed that bulk density (≤1.0 g/cm³), pH (>7.8), landform position (floodplain = yes/no), and magnetic susceptibility (X_lf <2.37x10^-7 kg/m³) were the best predictors of SOC content, with a prediction error of ±0.4 wt %. CART also showed that SOC was highest for samples with bulk density ≤1.0 g/cm³, which was frequently found in the near-surface samples. Deep calcareous layers in the terrace, where pH was >7.8, were found to have the next highest mean SOC, where exchangeable Ca binds to organic matter and clay, leading to stabilization. Lower mean SOC values were more common in floodplains with X_lf <2.37x10^-7 kg/m³. Although buried vs. surface soil had no significance in the pruned-tree results, landform type did. For this Clarks River valley study area, the terrace and bar positions appeared to store more SOC due to greater soil development and clay accumulation (terrace) or the deposition of organic matter (bars), while floodplains appeared to oxidize SOC more readily.