Stable nitrogen isotopes in tree rings are an integrator of historical changes in nitrogen cycling dynamics in red spruce (Picea rubens Sarg.) forests in West Virginia

Background/Question/Methods

The availability of soil nitrogen (N) constrains the ability of temperate forests to sequester carbon (C) and mitigate future climate warming. While there are many ways to characterize the N status of an ecosystem’s present state (i.e., mineralization, soil and foliar δ¹⁵N), there remain roadblocks to extending N status records back in time. As such, we have a limited understanding of how N deposition altered C sequestration prior to regional forest inventory efforts. To fill this knowledge gap, we investigated the extent to which tree ring δ¹⁵N integrates N cycling dynamics and its relationship with carbon isotope discrimination (∆¹³C). We analyzed tree ring δ¹⁵N and δ¹³C from fifteen red spruce trees (Picea rubens Sarg.) across three sites spanning a latitudinal transect in West Virginia, USA. We first examined whether tree ring δ¹⁵N from 2011-2013 tracked present indices of site N availability. From 1940-2013, we analyzed the relationships between tree ring δ¹⁵N with time and historical N loading. Further, we investigated the relationship between tree ring δ¹⁵N and growth, as well as changes in physiology derived from ∆¹³C.

Results/Conclusions

Across all red spruce sites, tree ring δ¹⁵N for 2011-2013 was positively related to foliar δ¹⁵N (R² = 0.59, p < 0.05), and was marginally related to potential rates of N mineralization (R² = 0.51, p = 0.07). Averaged across all sites from 1940-1990, tree ring δ¹⁵N did not change significantly from a mean of 0.404 ‰. After 1990, tree ring δ¹⁵N progressively declined, on average, by 0.082 ‰ each year. The reduction in tree ring δ¹⁵N after 1990 was positively related to the decline in national emissions of NO_x (R² = 0.85, p < 0.01) and local wet deposition of NO₃^- (R² = 0.74, p < 0.01). Further, reductions in tree ring δ¹⁵N after 1990 were synchronous with increases in tree ring ∆¹³C (R² > 0.92, p < 0.05), and occurred alongside increased growth of the red spruce trees. Together, these data suggest a tighter, more closed N cycle after 1990 which likely resulted from decreases in exogenous inputs of N coupled with increased red spruce growth and may have broad implications for forest ecosystem recovery to pollution, including increased water quality and fewer occurrences of eutrophication as pollution declines.