Denitrification in an estuarine headwater creek within an agricultural watershed

Nonpoint agricultural surface water runoff has been identified as a signifi cant contributor to watershed nitrogen (N) loading. Denitrification is thou ght to be a substantial sink for N inputs to estuaries. Denitrification rat es and inorganic N concentrations were measured in Culvert Creek, NC for a 3-yr period with an adaptation of the acetylene block technique using satur ation kinetic incubations that provided estimated in situ and potential rat es. Estimated in situ denitrification and inorganic N concentrations sere e levated in portions of Culvert Creek, NC following fertilizer application a t adjacent Open Grounds Farm Statistical correlations indicated that denitr ification in Culvert Creek was regulated by inorganic N, paticularly at hea dwater sites. Regulation was complex at estuarine sites, where dynamic hydr ological conditions affected microbial processing of N through coupled nitr ification-denitrification. Potential denitrification rates were often an or der of magnitude higher than estimated in situ rates. Potential denitrifica tion showed a similar seasonal-spatial pattern to that of nitrification at the most estuarine site, dth lower rates at the mid-creek sites where nitri fication was inhibited during periods of anoxia. Annual N removal in Culver t Creek was estimated at 1.27% (in situ) or 68.70% (potential rates) of inp uts. An estimated 11% of N inputs, were removed via the nitrification-denit rification pathway in Culvert Creek assuming that rates were closely couple d. 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 r ange of rates measured in various freshwater wetland systems receiving agri cultural drainage. In Culvert Creek? denitrification was a significant medi ator of estuarine headwater N cycling.