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TitleOn the significance of long-term trends in tree-ring N isotopes - The interplay of soil conditions and regional NOx emissions
AuthorSavard, M MORCID logo; Marion, J; Bégin, C; Laganière, J
SourceScience of the Total Environment vol. 857, 3, 159580, 2022 p. 1-11, Open Access
logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20220219
Mediapaper; digital; on-line
File formatpdf
Subjectsenvironmental geology; Nature and Environment; geochemistry; nitrogen; isotopic studies; isotopes; climate; water utilization; statistical analyses; soils; Anthropology; Emissions
Illustrationsplots; graphs; tables
ProgramEnvironmental Geoscience Sources
Released2022 11 02
AbstractAs anthropogenic nitrogen (N) emissions have been rising for decades, it is critical to develop natural archives that help understand how natural processes were modified in the past. Tree-ring d15N values may represent such an indicator but its validity as faithful record of N cycling changes is still debated. Here we produce long-tree-ring d15N series for five white spruce stands from two boreal regions submitted to moderate industrial N inputs. The obtained d15N series show sharp differences among stands, even from the same region, despite the fact that they show similar increases in intrinsicwater use efficiency (iWUE), a proxy for foliar strategies derived from d13C values. The statisticalmodeling of these series and the basal area increment (BAI) of the trees allow to suggest that the mechanisms controlling the isotopic fractionation of N assimilated by tree rings are decoupled from the foliar strategies under the anthropogenic N emissions. The iWUE trends mainly reflect rise of pCO2 and changes in air quality. The long-term d15N trends echo different biogeochemical processes responding to N deposition due to distinct original soil pH at the various sites. We contend that tree-ring d15N series can record changes in the forest N cycle, but their rigorous interpretation requires laborious work, particularly an understanding of the biogeochemistry in the soil immediately around the investigated trees. "Seek simplicity and distrust it", Alfred North Whitehead.
Summary(Plain Language Summary, not published)
Anthropogenic nitrogen (N) oxides emissions have now increased the atmospheric load to levels largely surpassing the natural emissions at global scale, with possible undesirable effects on forest ecosystems. This article aims at assessing if white spruce growth rings coupled with nitrogen isotopes (d15N) can reflect past environmental conditions, and understanding their response mechanisms under N emissions. We produced tree-ring d15N series of 92 years or more, from two exposed Albertan regions: the oil sands area (3.9-4.8 kg/ha/y), near Fort McMurray, and the coal fired power plants area, near Edmonton (6.0-8.1 kg/ha/y). Our results show that, under the N inputs, soil biogeochemistry (pH and microbial communities) is the main driver for various d15N responses in trees. Thus, the d15N values in the studied trees reflect changes in the forest N cycle, but in complex responses linked to the same contamination, showing that the forest ecosystem has numerous interdependent biogeochemical N compartments.

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