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Groundwater Nitrate and Denitrification in a Coastal Plain Riparian Forest

USDA-ARS, Southeast Watershed Res. Lab., P.O. Box 946, Tifton, GA 31793.

^* Corresponding author.

ABSTRACT

Mechanisms of nitrate (NO₃) removal from groundwater in riparian forests are poorly understood. This study was conducted in the Georgia coastal plain to: (i) determine changes in NO₃ and Cl concentrations within shallow groundwater moving from a row-crop field to a stream; (ii) determine the spatial and temporal distribution of denitrifiration potential relative to changes in NO₃ concentrations; and (iii) determine whether NO₃ or C supply was limiting denitrification potential. Nitrate and Cl concentrations in groundwater were measured biweekly or monthly for October 1988 through May 1990. Denitrification potentials, indicated by the denitrification enzyme assay, were measured bimonthly from October 1988 through October 1989. Modified potential measurements, lacking either NO₃, C, or both, were also performed bimonthly. Both NO₃ and NO₃/Cl ratios in groundwater decreased by a factor of 7 to 9 in the first 10 m of forest. Within the next 40 m of forest, mean NO₃ concentration decreased from 1.80 to 0.81 mg NO₃-N L^–1. Denitrifiration potential was more than two orders of magnitude higher in the top 10 cm of soil than in the top 10 cm of the shallow aquifer. Denitrification potential was consistently highest in surface soil nearest the field and nearest the stream and was limited by NO₃ availability in all surface soil samples. Denitrification potential was highest in October and August. Although NO₃ is definitely being removed from shallow groundwater, it is apparently not due to direct denitrification from the saturated zone. High denitrification potential in surface soils, especially near the field/forest interface, may contribute to NO₃ disappearance from shallow groundwater. Processes associated with intact riparian vegetation appear to play the primary role in N removal.

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