| Nitrate (NO3) enrichment of the shallow aquifer represents a potentially significant water quality problem in the Coastal Plain of North Carolina. Ultimately, discharge of nitrate-enriched groundwater into surface waters may stimulate eutrophication of N-limited coastal waters or proliferation of parasitic microorganisms. Riparian buffers, vegetated streamside zones uncultivated due to relief or high water table, have demonstrated a clear ability to remove nitrate from the shallow groundwater. Perhaps the most significant mechanism expediting this loss of nitrate from the groundwater is denitrification, the microbial reduction of nitrate to nitrogen gas (N 2). This research was conducted to determine nitrate depletion below a swine lagoon effluent-irrigated Coastal Bermuda grass pasture ( Cynodon dactylon) and adjacent riparian zone, and denitrification activity in the riparian zone above and below flow-control structures. Shallow groundwater monitoring wells (1--5 m below surface) indicated that although substantial nitrate losses may have occurred in flatter bottomland pastures and riparian zones, nitrate depletion was limited in many pasture and riparian zones. Significant increases in nitrate concentrations in low order receiving streams suggested that the riparian buffers were not capable of depleting all of the nitrate from the shallow groundwater. Denitrification activity was substantial (> 1,200 kg N ha-1 yr-1) in perennially saturated areas, and significantly higher above flow-control structures installed in lower-grade stream channels (p < 0.01). Because nitrate and dissolved organic carbon concentrations were optimum for denitrifiers in most areas, soil moisture was the most important factor in determining the magnitude of denitrification losses. |
