Carbon and nitrogen isotope variations in tree-rings as records of perturbations in regional carbon and nitrogen cycles

Increasing anthropogenic pollution from urban centers and fossil fuel combustion can impact the carbon and nitrogen cycles in forests. To assess the impact of twentieth century anthropogenic pollution on forested system carbon and nitrogen cycles, variations in the carbon and nitrogen isotopic compositions of tree-rings were measured. Individual annual growth rings in trees from six sites across Ontario and one in New Brunswick, Canada were used to develop site chronologies of tree-ring delta N-15 and delta C-13 values. Tree-ring delta N-15 values were similar to 0.5% higher and correlated with contemporaneous foliar samples from the same tree, but not with delta N-15 values of soil samples. Temporal trends in carbon and nitrogen isotopic compositions of these tree-rings are consistent with increasing anthropogenic influence on both the carbon and nitrogen cycles since 1945. Tree-ring delta C-13 values and delta N-15 values are correlated at both remote and urban-proximal sites, with delta N-15 values decreasing since 1945 and converging on 1% at urban-proximal sites and decreasing but not converging on a single delta N-15 value in remote sites. These results indicate that temporal trends in tree-ring nitrogen and carbon isotopic compositions record the regional extent of pollution.