Soil accretion, sediment deposition, and nutrient (N, P, organic C) accumulation were compared in floodplain and depressional freshwater wetlands of southwestern Georgia, USA to evaluate the role of riverine (2600 km2 catchment) versus depressional (<10 km2 catchment) wetlands as sinks for sediment and nutrients. Soil cores were collected from three floodplain (cypress-gum) and nine depressional (three each from cypress-gum forest, cypress-savannah, and herbaceous marsh) wetlands and analyzed for radionuclides (137Cs, 210Pb), bulk density, N, P, and organic C to quantify recent (30-year) and long-term (100-year) rates of sediment and nutrient accumulation. There was no significant difference in organic C, N, or sediment accumulation between depressional and floodplain wetlands. However, P accumulation was 1.5 to three times higher in the floodplain (0.12–0.75 g/m2/yr) than in the depressional wetlands (0.08–0.25 g/m2/yr). Sediment and nutrient accumulations were highly variable among depressional wetland types, more so than between depressional and floodplain wetlands. This variability likely is the result of differences in historical land use, hydrology, vegetation type, NPP, and perhaps fire frequency. Mean (n = 12) one-hundred-year rates of sediment deposition (1036 g/m2/yr), organic C (79 g/m2/yr), N (6.0 g/m2/yr), and P accumulation (0.38 g/m2/yr) were much higher than 30-year rates (sediment = 118 g/m2/yr, C = 20 g/m2/yr, N = 1.5 g/m2/yr, P = 0.09 g/m2/yr). Higher 100-year (210Pb) sediment and nutrient accumulations likely reflect the greater numbers of farms, greater grazing by livestock, and the absence of environmentally sound agricultural practices in southwestern Georgia at the turn of the century. Our findings suggest that the degree of anthropogenic disturbance within the surrounding watershed regulates wetland sediment, organic C, and N accumulation. Phosphorus accumulation also is greater in floodplain wetlands that have large catchments containing fine textured (clay) sediments that are co-deposited with P.
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Vol. 20 • No. 2