Nitrate and Aluminum Transport Through Soil Layers in a Clear-Cut Watershed

The 24-ha Dry Creek watershed in the Catskill Mountains of New York State was clear-cut during 1997 to evaluate nutrient release to New York City reservoirs due to forest harvesting. The Dry Creek watershed is in the headwaters of the Neversink watershed, which is part of the New York City Reservoir system that supplies drinking water to over 20 million people. Soil water, groundwater seeps, and stream water chemistry were monitored to trace the transport of solutes before and after the timber harvest. Automated sequential zero-tension lysimeters and standard zero-tension lysimeters were installed at depths of 70, 300, and 500 mm to sample soil water in the O, B, and C-horizons, respectively. Pre-cut (water years 1993-1996) mean soil water concentrations from zero tension lysimeters indicate that O-horizon soil water (70 mm depth) had the highest nitrate (NO₃^-) and monomeric aluminum (Al_m) concentrations (73 and 18 μmoles l^-1, respectively). During that same time period water from ground-water seeps had lower NO₃^- and Al_m concentrations (22 and 0.88 μmoles l^-1, respectively) than any soil waters sampled. During the two years following the clear-cut, groundwater seep NO₃^- concentrations were 138-123 μmoles l^-1 and Al_m concentrations were 50-30 μmoles l^-1 lower than that measured in soil water. Throughout the same time period, B-horizon soil water had the highest mean NO₃^- concentration (345 μmoles l^-1) while C-horizon soil water had the highest mean Al_m concentrations (51 μmoles l^-1). But during storms in the first year after the clear-cut O-horizon soil water NO₃^- and Al_m concentrations often peaked at more than twice those measured in the B-horizon. During the second year after the clear-cut, B-horizon storm NO₃^- concentrations were consistently greater than O-horizon concentrations. During the fourth and fifth years following the clear-cut, soil water NO₃^- concentrations had dropped below pre-cut concentrations however NO₃^- in groundwater seeps remained elevated. The NO₃^- concentration at the watershed outlet also remained above pre-cut levels. During the first years following the clear-cut, in the absence of watershed vegetation, soil NO₃^- was leached to watershed streams and to deeper groundwater. As the forest has regenerated soil NO₃^- has been immobilized while groundwater continues as a source of NO₃^- to watershed streams 4-5 years after the cut. Four to five years after the clear-cut Al_m concentrations were below pre-cut levels for all waters sampled. The elevated stream water NO₃^- concentrations that continue to be measured at the stream outlet, are not accompanied by elevated Al_m concentrations since the groundwater seeps that are the source of the NO₃^- have never been a significant source of Al_m.