TERRACE SOIL VARIABILITY AND EFFECTS ON BOTTOMLAND OAK FOREST DEVELOPMENT IN THE MISSISSIPPI RIVER VALLEY

Bottomland forests are of ecological importance as they provide travel corridors and food for wildlife. More recent work shows that these forest soils also store carbon on millennial timescales and thus are an important, yet understudied, component of the C cycle. Few have rigorously tested links between bottomland forest type and associated soil. This study explores differences in bottomland oak distribution and soil formation along alluvial terraces in humid subtropical western Kentucky, USA. We examine 13 terrace sites that have ~5 m relief and no statistically significant difference in surface slope or curvature. Parent material and ¹⁴C data suggest that these soils weather fine-grained mixed Holocene alluvium overlying Late Pleistocene clayey lacustrine deposits, and have done so for 500 to 8000 years. The 13 sites differ by oak-stand type: post oak (PO) dominated (n=7) versus cherry bark oak (CBO) dominated (n=6). Although both PO and CBO sites are mapped as the same soil series (Natalbany Series – Vertic Epiaqualf), the preliminary soil moisture time-series data differ between the two sites. Late spring soil-moisture data for both PO and CBO show a distinct breakpoint in soil moisture variability between 25 and 45 cm below the surface, which coincides with the top of the clay-rich subsoil. Soil moisture above this breakpoint differs by site. Above the breakpoint, the mean late-spring PO soil moisture increases with depth, whereas, the mean late-spring CBO soil moisture decreases with depth. These different moisture trends could be due to differences in uptake, which will be explored further by comparing extractable soil chemistry (proxy for bioavailable) with tree chemistry. Preliminary soil chemistry suggests that there is a reservoir of bioavailable nutrients at depths >85 cm, below a lithologic discontinuity in a hardpan layer. Because hardpans are thought to play a role in maintaining bottomland forest ecosystems, we infer this hardpan to be an important nutrient reservoir for the bottomland hardwoods when flooding is reduced along terraces like PO and CBO sites.