Tree-Ring Nitrogen Isotopes Reflect Anthropogenic NOx Emissions and Climatic Effects

Anthropogenic emissions of atmospheric nitrogen have increased over the last century, but the monitoring of nitrous oxide concentrations is only recent. Can trees from temperate regions be used to infer past changes in nitrogen cycles? To consider this question, we investigate nitrogen isotope (delta N-15) ring series from pine and beech trees near Montreal, and beech specimens of Georgian Bay Islands. The delta N-15 values show coherent intertree and interspecies trends, independent of the sapwood-heartwood transition zones, implying that these results reflect local environmental conditions. At both sites, short-term isotopic fluctuations correlate directly with precipitation and inversely with temperature. Long-term isotope decreases of 1.5 to 2 parts per thousand suggest progressive changes in soil nitrogen after 1951. In Georgian Bay, an additional important change is inferred on the basis of a 15 parts per thousand increase initiated after 1971. At both sites, long-term series correlate with a proxy for NOx emissions. We propose that the contrasted long-term delta N-15 changes of Montreal and Georgian Bay reflect deposition of NOx emissions from cars and coal-power plants, with higher proportions from coal burning in Georgian Bay. This research suggests that tree-ring delta N-15 series may record both, regional climatic conditions and anthropogenic perturbations of N cycles.