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

Abstract

Relatively few comparative studies exist of nitrogen (N) uptake by ectomycorrhizal (EM) and nonmycorrhizal roots under controlled conditions. Instantaneous and long-term N uptake by EM and nonmycorrhizal 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 nonmycorrhizal 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 noninoculated 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.