Effects of ectomycorrhizae and solution pH on [¹⁵N]ammonium uptake by coniferous seedlings

Abstract

Effects of mycorrhizae and solution pH on ammonium uptake and fluxes of hydrogen, calcium, and potassium in solution culture were determined using three Northwest coniferous species: Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), Sitka spruce (Piceasitchensis (Bong.) Carr.), and western hemlock (Tsugaheterophylla (Raf.) Sarg.). The ectomycorrhizal fungus was Hebelomacrustuliniforme (Bull, ex St. Amans) Quel. Ammonium uptake rates decreased with decreasing pH and were accompanied by decreasing H + release to the external solution. The mycorrhizal fungus H. crustuliniforme enhanced ammonium uptake and generally released fewer hydrogen ions per ammonium ion taken up than did nonmycorrhizal roots. This altered stoichiometry in mycorrhizal seedlings could mediate pH changes in the rhizosphere of soil-grown plants. During ammonium uptake, calcium ions were released at low pH and taken up at higher pH. Potassium efflux occurred at all hydrogen levels, with less efflux at higher pH. The mycorrhizal association generally increased calcium uptake or decreased calcium and potassium efflux rates compared with nonmycorrhizal seedlings. This research demonstrates that the ectomycorrhizal fungus Hebelomacrustuliniforme can significantly improve ammonium acquisition, a major growth-limiting nutrient in most Pacific Northwest forests, and that pH strongly affects ammonium uptake.