site is often scraped off to attain a surface elevation that will allow the site to become flooded or intersect the water table. This soil-like material at depth usually has much different texture, structure, chemistry, and biota than the overlying material that was removed. It is nearly devoid of organic matter content, depauperate in nitrogen, may have been compacted by construction activities, and lacks mychorrizal and microbial populations important to plant establishment and water-quality functions. This material cannot provide the same functions as an intact wetland soil, and the plants that can successfully reproduce in such material may not be the desired ones.
The soil organic matter content of created wetlands has consistently been found to be less than that of reference wetlands (Craft et al. 1988, 1999; Gwin et al. 1990; Moy and Levin 1991; Zedler and Langis 1991, Kentula et al. 1992a; Bishel-Machung et al. 1996; Shaffer and Ernst 1999). In a multiple wetland study (n=95), the average soil organic matter content of newly created wetlands did not change between 1987 and 1993, suggesting that there were no widespread improvements in wetland construction procedures to increase initial soil organic matter content (Shaffer and Ernst 1999). The vertical distribution of organic matter was also different in the profiles of created versus reference wetlands in Pennsylvania, with greater horizonation in the reference wetland soils (soil organic matter content significantly greater at depths of 5 centimeters (cm) than at 20 cm) than in the reference wetland soils (no significant difference in organic matter content at depths of 5 cm versus 20 cm). Although some researchers have reported increases in soil organic matter content in created wetlands over time (Lindau and Hossner 1981; Craft et al. 1988), others have found no significant relationship between soil organic matter content and project age (Bishel-Machung et al. 1996; Simenstad and Thom 1996; Shaffer and Ernst 1999). Low soil organic matter concentrations are associated with reduced levels of function, including poor establishment and growth of vegetation, poor habitat and food chain support for invertebrates and fish, and altered nutrient cycling (Shaffer and Ernst 1999).
Soil bulk density, which is the mass of soil per unit volume, has been found to be greater in created wetlands than in reference wetlands in at least two studies (Craft et al. 1991; Bishel-Machung et al. 1996). Soil texture also differed between created (n=44) and reference wetlands (n=20) in Pennsylvania: soils in created wetlands contained more sand and less clay at 20 cm, and reference wetlands were more silty than created wetlands were throughout the soil profile. Created wetlands may lack the redoximorphic features of low chroma and frequent redox depletions that are diagnostic of hydric soils (Confer and Niering 1992; Bishel-Machung et al. 1996).
Soil nutrient concentrations in created wetlands are generally lower