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

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Abstract
Effects of mycorrhizae and solution pH on ammonium uptake and fluxes of H, Ca and K in solution culture were determined using 3 Northwest coniferous species: Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Sitka spruce (Picea sitchensis (Bong.) Carr.), and western hemlock (Tsuga heterophylla (Raf.) Sarg.). The ectomycorrhizal fungus was Hebeloma crustuliniforme (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 H+ 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, Ca2+ were released at low pH and taken up at higher pH. K+ efflux occurred at all H+ levels, with less efflux at higher pH. The mycorrhizal association generally increased Ca uptake or decreased Ca and K efflux rates compared with nonmycorrhizal seedlings. This research demonstrates that the ectomycorrhizal fungus H. crustuliniforme can significantly improve ammonium acquisition, a major growth-limiting nutrient in most Pacific Northwest forests [USA], and that pH strongly affects ammonium uptake.