Mycorrhizal fungi supply nitrogen to host plants in Arctic tundra and boreal forests: 15N is the key signal

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Symbiotic fungi’s role in providing nitrogen to host plants is well-studied in tundra at Toolik Lake, Alaska, but little-studied in the adjoining boreal forest ecosystem. Along a 570 km north-south transect from the Yukon River to the North Slope of Alaska, the 15N content was strongly reduced in ectomycorrhizal and ericoid mycorrhizal plants including Betula, Salix, Picea mariana (P. Mill.) B.S.P., Picea glauca Moench (Voss), and ericaceous plants. Compared with the 15N content of soil, the foliage of nonmycorrhizal plants (Carex and Eriophorum) was unchanged, whereas content of the ectomycorrhizal fungi was very much higher (e.g., Boletaceae, Leccinum and Cortinarius). It is hypothesized that similar processes operate in tundra and boreal forest, both nitrogen-limited ecosystems: (i) mycorrhizal fungi break down soil polymers and take up amino acids or other nitrogen compounds; (ii) mycorrhizal fungi fractionate against 15N during production of transfer compounds; (iii) host plants are accordingly depleted in 15N; and (iv) mycorrhizal fungi are enriched in 15N. Increased N availability for plant roots or decreased light availability to understory plants may have decreased N allocation to mycorrhizal partners and increased 15N by 3‰-4‰ for southern populations of Vaccinium vitis-idaea L. and Salix. Fungal biomass, measured as ergosterol, correlated strongly with soil organic matter and attained amounts similar to those in temperate forest soils.

Le rôle des champignons symbiotiques dans l’apport en azote des hôtes est bien étudié dans la toundra, au lac Toolik, en Alaska, mais peu étudié dans l’écosystème adjacent que constitue la forêt boréale. Le long des 570 km du transect nord-sud allant de la rivière Yukon au versant nord de l’Alaska, le 15N était fortement réduit chez les plantes ectomycorhizes et mycorhizes éricoïdes, notamment Betula, Salix, Picea mariana (P. Mill.) B.S.P., Picea glauca Moench (Voss) et les éricacées. Le feuillage des plantes non mycorhizes (Carex et Eriophorum) demeure inchangé en fonction du contenu du sol en 15N, alors que le contenu des champignons ectomycorhizes est beaucoup plus élevé (e.g., Bolétacées, Leccinum et Cortinarius). On a supposé que des processus similaires opèrent dans la toundra et la forêt boréale, deux écosystèmes limités en azote : (i) les champignons mycorhizes scindent les polymères du sol et incorporent les acides aminés ou les autres composés azotés; (ii) les champignons mycorhizes favorisent le 15N lors du transfert des composés; (iii) les plantes hôtes sont conséquemment appauvries en 15N; et (iv) les champignons mycorhizes sont enrichis en 15N. La disponibilité accrue de l’azote pour les racines des plantes ou la plus faible disponibilité de lumière pour les plantes en sous-étage peut avoir diminué la portion d’azote allouée aux partenaires des mycorhizes et augmenté le 15N de 3‰-4‰ chez les populations de Vaccinium vitis-idaea L. et de Salix du sud. La biomasse fongique, mesuré sous forme d’ergostérol, est en forte corrélation avec la matière organique du sol et atteint des quantités similaires à celles qui sont retrouvées dans les sols forestiers des régions tempérées.

Document Type: Research Article

Publication date: January 1, 2009

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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