Modeling Reactive Nitrate Cycling on a Forested Hillslope Using 15 N and D2 O Tracers.

Headwater forests are important sources of drinking water because they remove reactive nutrients, e.g., nitrate (NO₃), that enter the terrestrial ecosystem through atmospheric deposition. In non-forested ecosystems the deposited nutrients are exported downstream to drinking water reservoirs, where high rates of export can degrade water quality, leading to algal production and impacting aquatic food webs. Nutrient export from forests can be impacted by many factors, including nutrient loading rates, biological uptake, sorptive interactions in soil, denitrification, climate variability, and subsurface flow processes. The dynamics and relative importance of such factors are not well understood. We propose a nutrient (¹⁵NO₃) and water (D₂O) tracer addition experiment on a unique experimental hillslope (15 m long) to elucidate the biogeochemical reactions in soil at hydrologic steady-state. Transport of the ¹⁵NO₃ tracer relative to the D₂O tracer will provide a better understanding of how soil processes can retain nitrate and impact export rates. Soil water chemistry and flow data will be incorporated into a numerical model to further clarify future effects of nutrient loading and climate variability on water resources. Current work is being done to prepare the historic (ca. 1967) soil model. This study can yield high-impact results for land use and watershed managers.