Rapid denitrification of N-contaminated groundwater in a low-gradient blackwater stream

Leaching of excess N to groundwater in fertilized landscapes can overwhelm natural biogeochemical processes and cause long-term eutrophication of aquatic systems. We investigate nitrogen fate and transport from a heavily-fertilized short-rotation woody crop plantation through the riparian zone of an intermittent low-gradient blackwater stream. Following two years of pretreatment monitoring of water quality, 50% of a managed forest watershed was clearcut using modern forestry BMPs and the following year converted to short-rotation pine production. This watershed was heavily instrumented with wells, riparian piezometers, soil moisture probes, stream flumes, interflow interception trenches, and an eddy flux tower. Groundwater nitrate concentrations jumped to 0.5 - 1.5 mg/L within six months after the first fertilization and persisted in most wells in the seven years since. While the groundwater nitrate has remained elevated, a corresponding increase in riparian and stream water total nitrogen and nitrate concentrations has not occurred. Groundwater travel time modeling shows that N from near-stream plantation areas should have already reached streams. Two years of measuring N in a series of landscape-based well networks (within the plantation, swale, riparian edge, forested hillslope, and valley), show rapid transformation and denitrification within the forested wetland valleys. Based on analysis of the first year of measurements, denitrification in shallow groundwater system within toe slopes and the riparian zone is removing >80% of N.