Multiyear fate of a¹⁵N 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 N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a 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 N moving to trees, some to deeper soil, and some net losses. Recovery of added N in trees and litterfall totaled 11.3% both 1 and 5-6 years after the tracer addition, as N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree N recovery amounted to 12-14 kg C per kg of N deposition. Tree N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of 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 N recovered in wood occurred in tree rings formed prior to the 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 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 N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.

Tree mycorrhizal type predicts within‐site variability in the storage and distribution of soil organic matter

“…We did note higher pH in AM‐dominated soils, which could favor nonsymbiotic N‐fixation (Limmer & Drake, ), but AM soils have high inorganic N availability compared to ECM soils (Phillips et al., ), which should constrain N‐fixation (Vitousek, Menge, Reed, & Cleveland, ). Instead, our results support previous evidence that ECM‐associated trees are able to take up and retain greater amounts of N (Goodale, ) by mining N directly from soil organic matter (Courty et al., ; Phillips, Finzi, & Bernhardt, ; but see Pellitier & Zak, ), resulting in a redistribution of N from mineral soils to plant biomass and organic soil horizons.…”

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi‐site dual nitrate isotopes

“…Besides the smallest N deposition rates (Table ), N1 has least NO 3 − leaching from hillslope soils (Figure ) and lacks significant changes in NO 3 − concentration along the flow path. This indicates that N1 is N‐limited, with N largely being assimilated by plants and immobilized in soils, similar to what is commonly seen in N‐limited temperate forest ecosystems in Northeastern America (Goodale, ; Nadelhoffer, Colman, Currie, Magill, & Aber, ).…”

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

“…Both fungi of different functional guilds and plants possess multiple NH 4 + and NO 3 − and amino acid transporters to acquire simple N compounds (Casieri et al ., ; Giovannetti et al ., ; Nehls & Plassard, ). Goodale () estimated that N uptake of EcM trees may exceed that of AM trees by 50% in temperate forests.…”