HOW DO ALLUVIAL LANDFORMS AFFECT THE STORAGE OF SOIL ORGANIC CARBON? A CASE STUDY FROM HUMID-TEMPERATE WESTERN KENTUCKY

Soil organic carbon (SOC) storage in river valley bottoms at depths greater than one meter and the processes that operate on that SOC are not well understood. These depths may house large stocks of SOC that so far have been underestimated. It is also unclear how different alluvial landforms and landform properties may affect SOC storage. This study examines the stock and isotopic composition of SOC along two alluvial landforms, a floodplain and terrace, in the Clarks River National Wildlife Refuge (CRNWR). We argue that landform-dependent soil texture and structure are a primary control on SOC storage at depths greater than one meter in valley bottoms.

Preliminary SOC and texture data collected from western Kentucky floodplain soils show a direct correlation between SOC and clay %; whereas terraces show an inverse correlation between SOC and clay %. We tested this further by compiling characterization data from 15 western Kentucky pedons using the National Cooperative Soil Survey Soil Characterization Database. The SOC and clay % were plotted against each other for upland, terrace, and floodplain soils. The slope of the trend lines, i.e., ΔSOC/ΔClay, was determined and then compared by group. Upland soils had a mean ±SD slope of -11 ± 10, terrace soils had a mean ±SD slope of -9 ± 7, and floodplain soils had a mean ±SD slope of -2 ± 17. With the exception of one upland soil, only floodplain pedons were observed to have a + ΔSOC/ΔClay correlation. The common inverse correlation between SOC and clay % observed in terrace soils is more similar to upland soils than floodplain soils in the CRNWR. Floodplains have a low mean slope possibly due to a lack of soil development and frequent deposition from flooding. The inverse correlation found in terraces could be due to longer duration of soil development due to infrequent deposition from flooding. These preliminary data suggest that texture-dependent storage in floodplains behaves differently than storage in terraces. Deposition of flood sediment and soil development play an important role in affecting the ΔSOC/ΔClay on floodplains.