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Hales, Heidi

Plant and Soil Science

2005

PhD

We studied NO₃⁻ transformations and movement in two catchments located in Camels Hump State Forest, Vermont. Increased amounts of nitrogen deposition have been hypothesized to increase NO₃⁻ leaching losses in forested ecosystems. However, the source (atmospheric vs. soil processes) of NO₃⁻ in streamwater remains unclear. We studied nitrogen movement and transformation in two contrasting watersheds with elevated nitrogen deposition levels in the Green Mountains of VT. Two tributaries of Brush Brook, with different pH (4.8 vs. 6.8), were found to have dramatically different NO₃⁻ export. These catchments drained areas containing soils with similar chemistry. One of our objectives was to determine if the (delta)¹⁵N and (delta)¹⁸0 of NO₃⁻ in precipitation, soil solution and streamwater could be used to differentiate between NO₃⁻ sources. We found that the isotopic composition of NO₃⁻ can be used to distinguish between precipitation and soil solution NO₃⁻. Soil solution (delta)¹⁵N was variable, but soil solution and streamwater (delta)¹⁸0 were similar. The (delta)¹⁵N and (delta)¹⁸0 of streamwater and soil solution NO₃⁻ was clearly different than atmospheric NO₃⁻, even during snowmelt periods. The (delta)¹⁵N and (delta)¹⁸0 of NO₃⁻ suggests that although the two tributaries have different NO₃⁻concentrations, they have a similar NO₃⁻ source. Disturbance of forest soils, both research-induced and natural, has been found to promote net nitrification after disturbance. In soils sensitive to disturbance-induced nitrification, it can be difficult to obtain accurate measurements of nitrification rates and the mechanism of disturbance-induced nitrification is not clear. Nitrification in forest soils is of special interest in ecosystems considered to be approaching nitrogen saturation. We analyzed the (delta)¹⁵N and (delta)¹⁸0 of soil NO₃⁻ in soils subjected to sampling disturbance approximately 30 minutes after sampling and after a 2-3 day incubation period at 10 °C. The isotopic composition of soil NO₃⁻3 changed dramatically between the first sampling time and after incubation. Enriched NH₄CI (111.9%) was added to soils. As expected, added NH₄⁺ did not increase net nitrification rates after disturbance, but the enriched NH₄⁺ was rapidly incorporated into the soil NO₃⁻ pool. Acetylene inhibited net nitrification, indicating an autotrophic pathway. The magnitude and rapidity of NO₃⁻ release presents problems for monitoring and research efforts.