Background, Short-term and Potential Long-term Denitrification Capacity of Soils in Urbanized Coastal Watersheds on Kiawah Island, South Carolina, Usa

Urbanization is escalating in many coastal areas of the US and is associated with deteriorating water quality. Often the associated changes in land use result in an overabundance of nutrients and other types of pollution entering ground and surface waters. It is important that we understand biogeochemical transformation processes on urbanizing watersheds if we are to develop management practices that can improve nutrient attenuation on the landscape. In this study denitrification capacity was estimated for two watersheds with residential and resort land uses on Kiawah Island, South Carolina. Potential for soils and sediment to reduce nitrogen (N) in runoff was estimated, and ways to improve denitrification capacity were tested. Background denitrification capacities (ambient) were substantially less than the potential short-term capacity (nutrient enriched) that ranged from 0.03 to 1.82 nM nitrous oxide (N2O) gdw h in soil and 0 to 2.48 nM N2O gdw h in pond sediments. Denitrification rates were considerably higher near the soil surface than at the water table, rates were stimulated by adding NO3 to the surface but not by adding it to the subsoil, and added glucose did not affect rates. Potential long-term rates were explored using soil amendments and 340 hour incubations. Denitrification capacity was stimulated with added nitrate (NO3) and with added NO3 plus carbon (C) in the form of glucose and wood fiber, but denitrification rates dropped to zero after addition of only glucose and wood fiber to soil. This suggested that heterotrophic microbes grew on the carbon substrate and reduced N availability through immobilization, and this low N availability was confirmed for C amended soil. These results suggest that 1) immobilization may be an important process in the removal of anthropogenic NO3 from stormwater runoff, 2) the N processing efficiency of a landscape can be sustained with high soil organic matter (SOM) content, as on forested watersheds, and 3) N processing efficiency can be improved on SOM depleted soils by incorporating wood fiber into the soil to provide substrate for microbial activity.