Below-ground plant traits influence tundra plant acquisition of newly thawed permafrost nitrogen

Rebecca E. Hewitt, D. Lee Taylor, Hélène Genet, A. David McGuire, Michelle C. Mack

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

The release of permafrost-derived nitrogen (N) has the potential to fertilize tundra vegetation, which in turn may stimulate productivity and thus offset carbon (C) losses from thawing permafrost. Below-ground plant traits may mediate ecosystem response to permafrost thaw and associated feedbacks to the atmosphere by differentially conferring access to deep, newly thawed permafrost N. Yet, identifying roots and quantifying root N uptake from deep, cold soils in complex plant communities has proved challenging to date. We investigated plant acquisition of experimentally added 15 N isotope tracer applied at the permafrost boundary in graminoid- and shrub-dominated tundra at Eight Mile Lake, Alaska, when the thaw front was close to its maximum depth, simulating the release of newly thawed permafrost N. We used molecular tools to verify species and estimate biomass, nitrogen, and isotope pools. Root biomass depth distributions follow an asymptotic relationship with depth, typical of other ecosystems. Few species had roots occurring close to the thaw front. Rubus chamaemorus, a short-statured non-mycorrhizal forb, and Carex bigelowii, a sedge, consistently had the deepest roots. Twenty-four hours after isotope addition, we observed that deep-rooted, non-mycorrhizal species had the highest 15 N enrichment values in their fine root tissue indicating that they access deep N late in the growing season when the thaw front is deepest. Deep-rooted plants are therefore able to immediately take up newly thawed permafrost-derived N. During the following growing season, herbaceous, non-mycorrhizal plants allocated tracer above-ground before woody, mycorrhizal plants. Ectomycorrhizal deciduous and ericoid mycorrhizal evergreen shrubs, by contrast, did not have immediate access to the deep N tracer and assimilated it into new foliar tissue gradually over the following growing season. Synthesis. Graminoids and forbs that have immediate access to deep N represent a modest C sink compared to C emissions from thawing permafrost. However, the effects of deep N fertilization on shrubs over longer time-scales may stimulate productivity and account for a more considerable N and C sink, thus constraining the permafrost C-climate feedback.

Original languageEnglish (US)
Pages (from-to)950-962
Number of pages13
JournalJournal of Ecology
Volume107
Issue number2
DOIs
StatePublished - Mar 2019

Keywords

  • Alaska
  • isotope N
  • moist acidic tundra
  • mycorrhizae
  • roots
  • shrub expansion

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Ecology
  • Plant Science

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