Wetland hydrology can be restored to soils that have been drained by plugging ditches to return the water table to its original elevation. Organic soils subside after drainage, and when ditches are plugged the restored water table may rise above the soil surface, killing newly planted vegetation. This study developed a method to estimate amounts of primary (settling) and secondary (oxidation) subsidence that could be applied to any organic soil. Primary subsidence was estimated from differences in bulk density between the drained and representative undrained sites. Secondary subsidence was estimated from accumulation of sand in the surface (Oap) horizons and changes in bulk density between oxidized and unoxidized organic horizons. Total subsidence was the sum of primary and secondary subsidence. Bulk density, particle size, and organic carbon data were gathered from one drained (Juniper Bay) and three undrained Carolina bay wetlands. Juniper Bay was drained with a network of ditches in three stages, 15, 20, and 30 years ago. Mean total subsidence was not significantly different (0.10 level) over time and averaged 121 cm for the three drainage periods. The mean rate of primary subsidence across the three drainage periods was 4 cm yr−1, while secondary subsidence was approximately 2 cm yr−1. Subsidence values were variable across Juniper Bay and were not related to distance from a field ditch. Restoration of the hydrology in Juniper Bay to pre-drainage water-table elevations could result in a water table that is >1 m above the existing soil surface.
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Vol. 26 • No. 1