RECONSTRUCTING LOCAL AND REGIONAL NITROGEN CYCLE PERTURBATION HISTORIES USING TREE-RING d15N VALUES

Anthropogenic perturbation of the nitrogen cycle has increased dramatically over the last fifty years. Absent direct instrumental records, it is important to develop reliable proxy-records to reconstruct historic change. Long life span, annual growth rings and the ubiquity of trees make them potentially good sentinels of environmental change. Nitrogen cycle perturbation, including addition of exogenous nitrogen can cause a change in the isotopic composition of bioavailable nitrogen. To determine whether tree-rings faithfully record nitrogen cycle perturbation, we first examined whether known historic local nitrogen cycle disturbances were recorded by tree-rings. We then examined the tree-ring based temporal record on a regional scale. We observed that trees from the perimeter of remnant stands recorded a marked 1.5-2.5‰ increase in the d¹⁵N values of their tree-rings relative to the values in trees from the center of the stand, with the shift synchronous with the tree-clearing and land-use change. This shift was most likely due to increased rates of nitrification and nitrate leaching in the soil as a result of tree-clearing combined with permanent changes in hydrology and probable fertilizer use accompanying the change in land-use. We found a strong correlation between carbon and nitrogen isotopic compositions of tree-rings along an urban to remote transect, suggesting that nitrogen isotopes in tree-rings can be used to infer the extent of regional nitrogen pollution. 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 d¹³C values and d¹⁵N values are correlated at both remote and urban-proximal sites, with d¹⁵N values decreasing since 1945 and converging on 1‰ at urban-proximal sites and decreasing but not converging on a single d¹⁵N value in remote sites. This work suggests that tree-rings faithfully record both nitrogen cycle perturbations and increasing nitrogen pollution on local and regional scales.