Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?
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 (13) CO(2)  absorbed through tree photosynthesis and (15) N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community. We detected little (15) N 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 (15) N 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.
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.|Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, SLU, SE-901 85, Umeå, Sweden.
Torgny
T
Näsholm
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.
Peter
P
Högberg
International Institute for Applied Systems Analysis (IIASA), A-2361, Laxenburg, Austria.
Oskar
O
Franklin
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.
Daniel
D
Metcalfe
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.|Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland.|Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, 08544, USA.
Sonja G
SG
Keel
Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, SLU, SE-901 85, Umeå, Sweden.
Catherine
C
Campbell
Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, SE-901 87, Umeå, Sweden.
Vaughan
V
Hurry
Southern Swedish Forest Research Centre, SLU, PO Box 49, Alnarp, SE-230 53, Sweden.
Sune
S
Linder
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.
Mona N
MN
Högberg
D001272:Atmosphere / Q000737:chemistry; D002244:Carbon / Q000378:metabolism; D002247:Carbon Isotopes; D008954:Models, Biological; D038821:Mycorrhizae / Q000187:drug effects / Q000502:physiology*; D009584:Nitrogen / Q000494:pharmacology*; D009587:Nitrogen Isotopes; D018517:Plant Roots / Q000382:microbiology; D012988:Soil Microbiology; D014197:Trees / Q000187:drug effects / Q000254:growth & development* / Q000382:microbiology*
D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D002244:Carbon; D009584:Nitrogen