Spatial Variability in Sedimentation, Carbon Sequestration, and Nutrient Accumulation in an Alluvial Floodplain Forest

Abstract

We measured soil properties, vertical accretion, and nutrient (organic C, N, and P) accumulation across a range of habitats to evaluate spatial variability of soil properties and processes of alluvial floodplain wetlands of the Altamaha River, Georgia, USA. The habitats vary in elevation and distance from the river channel, creating differences in the depth and duration of inundation. Habitats closer to the river had lower bulk density and higher total P than habitats further removed. 137Cs and 210Pb accretion rates were also greater at sites closer to the channel. Mineral sediment deposition and nutrient accumulation were greater in sloughs closer to the channel and lower in elevation relative to other habitats. We found distance to be a significant predictor of mineral soil properties across the floodplain. Bulk density increased whereas TP and silt content decreased with distance from the river channel. 137Cs accretion, P accumulation, and mineral sediment deposition also decreased with distance from the main channel. Elevation was not a significant predictor of soil properties or processes measured. Long-term (100 year) sediment accumulation rates based on 210Pb were significantly higher than 50-year rate of sedimentation based on 137Cs, perhaps as the result of greater land clearing for agriculture and lack of best management practices in the southeastern USA prior to 1950. Distance from the main channel is the driving force behind the spatial variability of soil properties and processes measured; however, slough habitats closest to the channel and lowest in elevation relative to other habitats maintain distinct vegetation patterns and are hotspots for N, P, and sediment accumulation. Characterization of soil properties and processes of alluvial floodplain forests and other wetlands should take into consideration microtopographic and spatial variation across the wetland.