Tree-ring δ15N values to infer air quality changes at regional scale

Abstract

In this study, tree-ring concentrations and stable isotope ratios of nitrogen in red spruce trees growing near Québec City (Canada) were used as records of environmental changes for the period between 1880–81 and 2007. On one hand, the trends in nitrogen concentrations of red spruce trees are strongly influenced by the heartwood–sapwood transition zone. Such physiologically-influenced pattern suggests that tree-ring nitrogen concentrations are not representative of soil or air nitrogen variations. On the other hand, the δ15N trends of the investigated trees do not show any sign of influence from the heartwood-sapwood boundary, meaning that these isotopic values can be representative of environmental conditions. The interannual variations of the δ15N values were correlated with the total annual precipitation and the measured amounts of regional nitrogen oxide emissions for the 1990–2007 period. The long-term δ15N values show a decrease of 1.8‰ between 1956–57 and 2007. This decreasing trend is statistically linked to the increasing number of registered cars in the Quebec Province, which produce N emissions characterized by low isotopic values. We suggest that this long-term trend could have been caused by a change to a dominant source of N emissions with low δ15N values, and/or by the microbiological processes, such as the release of N from mycorrhizas under N-limited environment. Importantly, we demonstrate that this clear isotopic pattern is matching that of other species (pine and beech trees) growing under different ecological conditions, at 260km from our study site. Therefore, we suggest that nitrogen oxides emitted by car exhausts, themselves linked to the amount of transport-related N deposition at the provincial scale, could have caused the observed trends and have participated to the acidification of soil. This investigation allows suggesting that the δ15N values of trees have the potential to reflect perturbations of the soil and air nitrogen dynamics caused by anthropogenic N emissions at a large scale.