Denitrification in an Estuarine Headwater Creek within an Agricultural Watershed

Nonpoint agricultural surface water runoff has been identified as a significant contributor to watershed nitrogen (N) loading. Denitrification is thought to be a substantial sink for N inputs to estuaries. Denitrification rates and inorganic N concentrations were measured in Culvert Creek, NC for a 3-yr period with an adaptation of the acetylene block technique using saturation kinetic incubations that provided estimated in situ and potential rates. Estimated in situ denitrification and inorganic N concentrations were elevated in portions of Culvert Creek, NC following fertilizer application at adjacent Open Grounds Farm. Statistical correlations indicated that denitrification in Culvert Creek was regulated by inorganic N, paticularly at headwater sites. Regulation was complex at estuarine sites, where dynamic hydrological conditions affected microbial processing of N through coupled nitrification-denitrification. Potential denitrification rates were often an order of magnitude higher than estimated in situ rates. Potential denitrification showed a similar seasonal-spatial pattern to that of nitrification at the most estuarine site, with lower rates at the mid-creek sites where nitrification was inhibited during periods of anoxia. Annual N removal in Culvert Creek was estimated at 1.27% (in situ) or 68.70% (potential rates) of inputs. An estimated 11% of N inputs were removed via the nitrification-denitrification pathway in Culvert Creek assuming that rates were closely coupled. Mean annual (1995–1997) creek denitrification rates of 2.17 kg N ha⁻¹ yr⁻¹ (in situ) and 90.52 kg N ha⁻¹ yr⁻¹ (potential) were within the range of rates measured in various freshwater wetland systems receiving agricultural drainage. In Culvert Creek, denitrification was a significant mediator of estuarine headwater N cycling.