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TitleNitrogen isotopes in the soil-to-tree continuum - tree-rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions
AuthorSavard, M MORCID logo; Martineau, CORCID logo; Laganière, J; Bégin, C; Marion, J; Smirnoff, AORCID logo; Stefani, F; Bergeron, J; Rheault, K; Paré, DORCID logo; Séguin, A
SourceScience of the Total Environment vol. 780, 146581, 2021 p. 1-15, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200628
Mediapaper; on-line; digital
File formatpdf
NTS74D/11; 74D/12; 74D/13; 74D/14; 83G/01; 83G/02; 83G/07; 83G/08; 83G/09; 83G/10; 83G/15; 83G/16; 83H/03; 83H/04; 83H/05; 83H/06; 83H/11; 83H/12; 83H/13; 83H/14
Lat/Long WENS-115.0000 -113.0000 54.0000 53.0000
Lat/Long WENS-112.0000 -111.0000 57.0000 56.5000
Subjectsenvironmental geology; Nature and Environment; Science and Technology; nitrogen; pH patterns; soils; soil properties; pollutants
Illustrationsplots; location maps; tables
ProgramEnvironmental Geoscience Sources
Released2021 03 20
AbstractAnthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (d15N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring d15N changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and d15N values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive d15N signals. Our results reveal that airborne anthropogenic N with distinct d15N signals may directly modify the NO3 - d15N values in surface soils, but not the ones of NH4 +, the preferred N form of the studied trees. Hence, the tree-ring d15N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring d15N range, we report differences in: soil nutrient content and N transformation rates; d15N values of NH4 +, total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root d15N valueswith fungal identification to infer that hydrophobic EcMfungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots,which also assimilate N directly under less acidic conditions. Despite the complexities of soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring d15N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring d15Nvalues as a suitable indicator for environmental research on forest N cycling.
Summary(Plain Language Summary, not published)
Airborne nitrogenous contaminants or pollutants emitted by human activities can cause undesirable effects on forest ecosystems. Indicators allowing to understand changes in the natural nitrogen cycle must therefore be developed. This article aims to assess the potential of white spruce growth rings coupled with nitrogen isotopes (d15N) to reflect environmental conditions. A holistic approach was therefore adopted in order to deduce what are the current mechanisms controlling the d15N values of growth rings, by examining the steps of transfer from nitrogen in the air to nitrogenous components in the soil, then to fungi associated with the root system of spruce trees, the rootlets, and finally up to rings of spruce stems. The analytical characterizations include of course isotopic analyzes of the various compartments mentioned, but also soil chemistry, and the genomics of soil microbes (bacteria and fungi). Two Alberta study areas subject to annual anthropogenic nitrogen deposition differing in terms of quantity and isotopic values were selected, namely that of the oil sands region near Fort McMurray (3.9 to 4.8 kg/ha/y) and that of the coal-fired power plants near Edmonton (6.0 to 8.1 kg/ha/y). The results show the possible influence of the anthropogenic nitrogen inputs on soil microbial communities, which in turn modify the transformation processes of soil nitrogen compounds and their isotopic ratios. Thus, the d15N values of nitrogen assimilated by the roots and trunk of the spruce trees studied reflect the chemical conditions, mainly the pH and the rates of anthropogenic nitrogen supply. In the future, the study of long series of growth rings covering a temporal spectrum including pre- and syn-industrial operations periods could allow to better understand the role of environmental indicator of the growth-ring d15N values.

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