Elevated emissions of nitrogen oxides (NOx) in the Athabasca Oil Sands Region, Alberta and higher foliar nitrogen (N) concentrations in jack pine (Pinus banksiana) needles close to major emission sources has led to concerns that the surrounding boreal forest may become N-saturated. Despite these concerns, N deposition and impacts on upland forests in the region is poorly quantified. The objective of this study was to characterize N cycling in five plots representing the two dominant upland forest types (jack pine and trembling aspen, Populus tremuloides) close (<30 km) to the largest mining operations in the region, during a 2-year period. Despite the high level of NOx emissions, bulk throughfall and deposition measured at both study sites were surprisingly very low (<2 kg N ha−1 year−1). Internal N cycling was much greater in aspen stands; annual N input in litterfall was ten times greater, and net N mineralization rates were two to five times greater than in jack pine stands. Nitrogen use efficiency (NUE) was much greater in jack pine when calculated based on N litterfall indices, but not when N pools in biomass were considered. Despite differences in internal cycling among forest types, nitrate leaching from mineral soil in both forest types was negligible (<0.1 kg N ha−1 year−1) and patterns of 15N in roots, foliage, and mineral soil were typical of N-limited ecosystems, and both sites show no evidence of N saturation.
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This research was undertaken, in part, thanks to funding from the Canada Research Chairs Program and a Natural Sciences and Engineering Research Council (NSERC) Discovery grant. Financial support for this study was also provided by the Cumulative Environmental Management Association (CEMA) and an NSERC Collaborative Research and Development Grant awarded to J. Aherne and S.A. Watmough. The authors gratefully acknowledge M. Maloney (Trent University) for his GIS expertise, J. Owen (Trent University) for the natural abundance analysis and everyone who helped with sample collection and processing. The authors would also like to acknowledge M. Fenn (USDA) and colleagues for their generous donation of IER column supplies and analysis.
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Laxton, D.L., Watmough, S.A. & Aherne, J. Nitrogen Cycling in Pinus banksiana and Populus tremuloides Stands in the Athabasca Oil Sands Region, Alberta, Canada. Water Air Soil Pollut 223, 1–13 (2012). https://doi.org/10.1007/s11270-011-0833-6
- Pinus banksiana
- Populus tremuloides
- Nitrogen deposition
- Nitrogen saturation