Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?

Authors

  • Torgny Näsholm,

    Corresponding author
    1. Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, SLU, Umeå, Sweden
    • Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
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  • Peter Högberg,

    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
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  • Oskar Franklin,

    1. International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria
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  • Daniel Metcalfe,

    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
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  • Sonja G. Keel,

    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
    2. Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
    3. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, USA
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  • Catherine Campbell,

    1. Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, SLU, Umeå, Sweden
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  • Vaughan Hurry,

    1. Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, Sweden
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  • Sune Linder,

    1. Southern Swedish Forest Research Centre, SLU, Alnarp, Sweden
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  • Mona N. Högberg

    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
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Author for correspondence:

Torgny Näsholm

Tel: +46 90 7868205

Email: torgny.nasholm@slu.se

Summary

  • Symbioses between plant roots and mycorrhizal fungi are thought to enhance plant uptake of nutrients through a favourable exchange for photosynthates. Ectomycorrhizal fungi are considered to play this vital role for trees in nitrogen (N)-limited boreal forests.
  • We followed symbiotic carbon (C)–N exchange in a large-scale boreal pine forest experiment by tracing 13CO2 absorbed through tree photosynthesis and 15N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community.
  • We detected little 15N in tree canopies, but high levels in soil microbes and in mycorrhizal root tips, illustrating effective soil N immobilization, especially in late summer, when tree belowground C allocation was high. Additions of N fertilizer to the soil before labelling shifted the incorporation of 15N from soil microbes and root tips to tree foliage.
  • These results were tested in a model for C–N exchange between trees and mycorrhizal fungi, suggesting that ectomycorrhizal fungi transfer small fractions of absorbed N to trees under N-limited conditions, but larger fractions if more N is available. We suggest that greater allocation of C from trees to ectomycorrhizal fungi increases N retention in soil mycelium, driving boreal forests towards more severe N limitation at low N supply.

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