Multiyear fate of a 15N tracer in a mixed deciduous forest: retention,redistribution, and differences by mycorrhizal association |
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| Authors: | Christine L Goodale |
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| Institution: | Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA |
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| 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 15N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a 15N‐ 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 15N moving to trees, some to deeper soil, and some net losses. Recovery of added 15N in trees and litterfall totaled 11.3% both 1 and 5–6 years after the tracer addition, as 15N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree 15N recovery amounted to 12–14 kg C per kg of N deposition. Tree 15N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of 15N 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 15N recovered in wood occurred in tree rings formed prior to the 15N 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 15N through trees and surface soils without any losses, as whole‐ecosystem recovery remained constant between 1 and 5–6 years at 70% of the 15N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale. |
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| Keywords: | 15N tracer arbuscular mycorrhizae ectomycorrhizae nitrogen deposition nitrogen fate nitrogen retention nitrogen uptake |
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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 15N moving to trees, some to deeper soil, and some net losses. Recovery of added 15N in trees and litterfall totaled 11.3% both 1 and 5–6 years after the tracer addition, as 15N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree 15N recovery amounted to 12–14 kg C per kg of N deposition. Tree 15N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of 15N 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 15N recovered in wood occurred in tree rings formed prior to the 15N 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 15N through trees and surface soils without any losses, as whole‐ecosystem recovery remained constant between 1 and 5–6 years at 70% of the 15N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.