Decadal Fate of a 15N Tracer in a Mixed Deciduous Forest.

The impacts of atmospheric nitrogen deposition on forest ecosystems depend on its fate. Past tracer studies show that litter and soils dominate the short-term fate of added ¹⁵N, yet few have examined longer-term dynamics or differences among forest types. This study is examining the decadal fate of a ¹⁵N-NO₃^- tracer in a stand evenly composed of trees with ectomycorrhizal and arbuscular mycorrhizal associations. The tracer was expected to slowly mineralize from its main initial fate in litter and surface soil, with some ¹⁵N moving to trees, some to deeper soil, and some net losses. Analyses for years 1 and 5-6 are complete, and year 10 sampling is scheduled for summer 2017. Recovery of added ¹⁵N in trees and litterfall totaled 11.3% at both year 1 and 5-6, as ¹⁵N redistributed from fine and especially coarse roots into litterfall, with small accumulations in woody tissues. Tree ¹⁵N acquisition occurred within the first year after the tracer addition, with no additional net transfer of ¹⁵N from detrital to plant pools by year 5-6. Ectomycorrhizal trees gained 50% more of the tracer than trees with arbuscular mycorrhizae at both timepoints. Tracer recovery rapidly decreased over time in surface litter material and accumulated in shallow and deep soil, perhaps through earthworm mixing. Overall, results showed redistribution of tracer ¹⁵N through trees and surface soils without any losses, as whole-ecosystem recovery remained constant between years 1 and 5-6 at 70% of the ¹⁵N addition. These results demonstrate persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale. New sampling of the tracer’s fate at year 10 will allow investigation of whether ¹⁵N eventually turns over from detrital pools to become available to plants over decadal timescales, and how tree species differ in their ability to acquire this N.