When wetland restoration occurs on land previously used for crop production, residual nutrients can cause undesirable plant communities to grow, and increased solubility of excess P may contribute to eutrophication of surface waters. This study assessed how agricultural production in a drained wetland during 15, 20, and 30 yr periods changed morphological and chemical soil properties as compared to natural wetland soils not used for agriculture. The drained wetland, Juniper Bay, is a Carolina bay located in southeastern North Carolina. Three relatively undisturbed Carolina bays with soil types similar to those in Juniper Bay were selected as reference wetlands to compare soil properties. Three general soil types were identified in all the Carolina bays based on thickness of the organic surface layer: 1) organic soils (Histosols), 2) soils with histic epipedons, and 3) mineral soils. The surface horizon of all three soil types at Juniper Bay where crop production had occurred had significantly greater amounts of extractable P, Ca, Mg, Mn, Zn and Cu, along with higher base saturation and pH than soils in the reference bays. Greater length of time in crop production resulted in significant differences in soil chemical properties with depth. For soils farmed for 15 years, significant increases in extractable nutrients occurred only in the topsoil within approximately 20 cm of the soil surface. After 30 years of crop production, significantly increased amounts of extractable nutrients were present to depths of approximately 1 m. Residual nutrients and the higher pH of previously farmed wetland soils are likely to affect restoration of natural plant communities, which consist of plant species adapted to nutrient poor acid soils. Increased solubility of residual P when wetland hydrology and anaerobic soil conditions are restored may degrade water quality. These factors should be considered in planning wetland restoration projects.

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 predrainage water-table elevations could result in a water table that is > 1 m above the existing soil surface.

Successful wetland mitigation is determined by goals and performance standards of a U.S. Army Corp of Engineers approved mitigation plan. This study collected and reviewed historical data for a mitigation site prior to construction to reduce the cost and risk of mitigation failure. Historical records were reviewed to evaluate the hydrology, vegetation, and soils of a drained Carolina bay wetland. Historical data were obtained from courthouse records, aerial photographs, personal interviews, the local Natural Resources Conservation Service, and the National Railroad Historical Society. Photographs and interviews indicated there was open water before drainage, suggesting a potential source of consistent hydrology. Organic soils subside when influenced by land clearance, drainage, and agricultural activities and could result in a water table above the soil surface once hydrology is restored. Aerial photographs show that several drainage systems have existed over the last 90 yr. Soils along the lines of the earlier drainage ditches and a former railroad line still show disturbance to depths of 1m. These areas of disturbance could affect vegetation establishment. Records of agricultural practices suggest higher nutrient levels than those of undisturbed Carolina bays, indicating that undesired vegetation could compete with desired vegetation. Historical information verified that Juniper Bay is a viable restoration site and identified areas where design change could help improve chances for success. Similar historical reviews with other mitigation sites can help reduce cost and risk through evaluation of hydrology, soils, and vegetation.