COS 99-8 - 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

Wednesday, August 9, 2017: 4:00 PM
B117, Oregon Convention Center
Justin M. Mathias and Richard B. Thomas, Department of Biology, West Virginia University, Morgantown, WV

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.


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 NOx (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.