Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?
Authors: Naesholm T, Hoegberg P, Franklin O, Metcalfe D, Keel SG, Campbell C, Hurry V, Linder S, Hoegberg MN
Publication Year: 2013
Reference: New Phytologist, Article in press (Published online 29 January 2013)
Abstract
-- 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.
KEYWORDS: Field experiment; 13C-15N pulse labelling; Modelling; Mycorrhiza; Nitrogen immobilization; Nitrogen limitation; Scots pine (Pinus sylvestris)
-- 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.
KEYWORDS: Field experiment; 13C-15N pulse labelling; Modelling; Mycorrhiza; Nitrogen immobilization; Nitrogen limitation; Scots pine (Pinus sylvestris)
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