Rapid nitrogen loss from ectomycorrhizal pine germinants signaled by their fungal symbiont

Joshua M Smith,Melanie D Jones,Matthew D Whiteside

doi:10.1007/s00572-020-00959-7

Joshua M Smith, Melanie D Jones + Show 1 more

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https://doi.org/10.1007/s00572-020-00959-7

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Abstract

Ectomycorrhizal fungi supply their plant partners with nitrogen but can also retain substantial amounts. The concentration of nitrogen in the soil and the amount of carbon supplied from the host seem to influence the proportion of N retained by the fungus. In an experiment designed to determine whether differential supply of nitrogen to two plants influenced nitrogen transfer from fungus to plant within a mycorrhizal network, we observed rapid, substantial loss of nitrogen from pine seedlings. The loss occurred when the mycorrhizal fungus experienced a sudden increase in nitrogen supply. We grew Pinus contorta seedlings in association with Suillus tomentosus in low-nitrogen microcosms where some nitrogen was accessible only by hyphae. After 70 days, foliage of some seedlings was treated with nitrogen. Three days later, hyphal nutrient media were replaced with water or a solution containing nitrogen. Foliar treatment did not affect nitrogen transfer by the fungus to shoots, but by day 75, seedling nitrogen contents had dropped by 60% in microcosms where nitrogen had been added to the hyphal compartments. Those seedlings retained only 55% of the nitrogen originally present in the seed. Loss of nitrogen did not occur if water was added or the hyphae were severed. Because of the severing effect, we concluded that S. tomentosus triggered the loss of seedling nitrogen. Nitrogen may have been lost through increased root exudation or transfer to the fungus. Access to nitrogen from nutrient-rich germinants would benefit rhizosphere microorganisms, including ectomycorrhizal fungi colonizing pine from spores after wildfire.

Highlights

IntroductionEM fungi can transfer substantial amounts of N to their hosts (Hobbie and Högberg 2012; Akroume et al 2019), they retain varying amounts of N to meet their own needs (Hobbie et al.2008; Näsholm et al 2013)
In boreal and montane forests of the Northern Hemisphere, where nitrogen (N) is typically deficient (Hunt et al 1988; Brockley 2001b; Högberg et al 2017), ectomycorrhizal (EM) fungi mineralize organic forms of N and translocate a portion of the solubilized N to their plant partnersElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.(Bending and Read 1995; Perez-Moreno and Read 2000; Högberg et al 2007; Nicolás et al 2019)
The results provide no evidence that N status of the seedlings influenced N translocation from the shared fungus, 15N enrichment of most shoots demonstrates that functional ectomycorrhizas had been established in which N absorbed from the wells was passed by hyphae to their plant partners

Summary

Introduction

EM fungi can transfer substantial amounts of N to their hosts (Hobbie and Högberg 2012; Akroume et al 2019), they retain varying amounts of N to meet their own needs (Hobbie et al.2008; Näsholm et al 2013). Sometimes such a high proportion of N is reserved by the fungus that plant growth is negatively affected (Abuzinadah and Read 1989; Colpaert et al 1996; Corrêa et al 2008; Franklin et al 2014). The form of N supplied and the EM fungal species present will influence N transfer to the plant (Abuzinadah and Mycorrhiza (2020) 30:407–417

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