Multiyear fate of a 15 N tracer in a mixed deciduous forest: retention, redistribution, and differences by mycorrhizal association

Abstract

The impact of atmospheric nitrogen deposition on forest ecosystems depends in large part on its fate. Past tracer studies show that litter and soils dominate the short-term fate of added ¹⁵ N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a ¹⁵ N-NO3- tracer over 5-6 years in a mixed deciduous stand that was 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 ¹⁵ N moving to trees, some to deeper soil, and some net losses. Recovery of added ¹⁵ N in trees and litterfall totaled 11.3% both 1 and 5-6 years after the tracer addition, as ¹⁵ N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree ¹⁵ N recovery amounted to 12-14 kg C per kg of N deposition. Tree ¹⁵ N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of ¹⁵ N from detrital to plant pools. In both years, ectomycorrhizal trees gained 50% more of the tracer than did trees with arbuscular mycorrhizae. Much of the ¹⁵ N recovered in wood occurred in tree rings formed prior to the ¹⁵ N addition, demonstrating the mobility of N in wood. Tracer recovery rapidly decreased over time in surface litter material and accumulated in both shallow and deep soil, perhaps through mixing by earthworms. Overall, results showed redistribution of tracer ¹⁵ N through trees and surface soils without any losses, as whole-ecosystem recovery remained constant between 1 and 5-6 years at 70% of the ¹⁵ N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.

Keywords: 15N tracer; arbuscular mycorrhizae; ectomycorrhizae; nitrogen deposition; nitrogen fate; nitrogen retention; nitrogen uptake.