SCROLL BAR TOPOGRAPHY INFLUENCES SOIL CARBON STORAGE ON FLOODPLAINS: EVIDENCE FROM THE MISSISSIPPI FLOODPLAIN, CENTRAL LOUISIANA

Vegetated floodplains are critical landscapes for accumulation and storage of soil organic carbon (SOC), but alluvial floodplains have complex micro-topography that can impact soil properties and carbon storage. Scroll bars, curvilinear ridges of sediment deposited on the inside of meander bends, are a common topographic feature on floodplains, but the extent to which these small-scale (<2m high ridges and adjacent swales) geomorphic features impact soil carbon storage has not been quantified. Here, we evaluate the vegetative and geomorphic controls on soil development and carbon storage within the paleo-Mississippi River scroll bars at Bayou Cocodrie National Wildlife Refuge, Ferriday, LA. We collected 1m deep soil cores and measured basal area and palmetto (Serenoa repens) density at eight scroll ridge-swale transects. Using a Shimadzu CHN analyzer, SOC concentration (%) was analyzed from 0-32cm, 32-64cm, and 64-96cm depth intervals for each soil core collected at ridge and swale sites.

Scroll bar ridge sites comprise silty clay and somewhat poorly drained soils with higher palmetto densities (mean = 1.26 P/m²) relative to swales, which comprise very poorly drained clay soils with lower palmetto densities (mean = 0.38 P/m²). The top 32cm of swale soil cores contained significantly higher SOC% compared to the ridges (swale mean: 2.88% vs 1.89% on ridges) at all transect sites, indicating that the scroll bar topography does impact soil carbon storage despite the opposite vegetation trends. Ridge-to-swale slope (ranging from -0.177 to 0.000) had no correlation with SOC% but had a strong positive correlation (r = .75) with swale palmetto density, likely due to inhibited growth in very poorly drained swale soils.

These results suggest that in the Mississippi River floodplain, scroll bar topography exerts a stronger control on SOC than vegetative density and distribution, possibly due to the ridge-swale slope, swale drainage, and grain size. Understanding soil carbon storage across complex floodplain topography can assist conservation efforts to address emissions and ecosystem health.