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Comparison of ammonium, nitrate and proton fluxes in mycorrhizal and non-mycorrhizal roots of lodgepole pine in contrasting nitrogen treatments

Relatively few comparative studies exist of nitrogen (N) uptake by ectomycorrhizal (EM) and non-mycorrhizal roots under controlled conditions. Instantaneous and long-term N uptake by EM and non-mycorrhizal roots of lodgepole pine seedlings (Pinus contorta (Douglas) var. latifolia (Engelm.)) were compared after 9 - 12 months of preconditioning under high or low N supply, with NH4+ and NO3- in proportions of 90:10 and 10:90. EM roots were inoculated with Amphinema byssoides or Laccaria bicolor. Instantaneous net NH4+, NO3- and H+ fluxes were measured simultaneously on seedling roots with a microelectrode ion flux measurement system, and seedling organ biomass and N content were measured to infer long-term N uptake. Net NH4+ uptake was observed in EM roots of lodgepole pine but NH4+ efflux was observed in non-mycorrhizal roots. Greater instantaneous N uptake in EM roots was supported by higher root and shoot N contents in seedlings associated with L. bicolor than in non-inoculated seedlings. Absence of NH4+ efflux in EM roots suggests that root colonization by EM fungi has the potential to reduce futile cycling of NH4+. EM lodgepole pine appeared to prefer NH4+ over NO3- as an N source and instantaneous NH4+ uptake rates were higher in NH4-starved roots. H+ efflux was observed in most roots. Future work should focus on a greater variety of EM species to explore differences among species in N uptake.