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

The interacting effects of mycorrhizae and pH on nitrate uptake by three native Northwest coniferous species were examined. Seedlings of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), Sitka spruce (Piceasitchensis (Bong.) Carr.), and western hemlock (Tsugaheterophylla (Raf.) Sarg.) were grown for several months, and then half of the seedlings were inoculated with the ectomycorrhizal fungus Hebelomacrustuliniforme (Bull, ex St. Amans) Quél. In a series of experiments, pH was continuously maintained at set levels and fluxes of nitrate, potassium, calcium, and hydroxide ions were measured. While both coniferous species and mycorrhizae did affect nitrate uptake rates, the major effect on nitrate uptake rates was due to solution pH. Nitrate uptake rates generally increased with increasing pH from pH 2.5 to 7.5, as did calcium flux rates, which changed from efflux at low pH to uptake at higher pH levels. Potassium was released from the roots at all pH levels. Hydroxide ion release rates decreased with increasing pH, resulting in hydrogen ion release above pH 5.5. Mycorrhizal roots often released fewer hydroxide ions per nitrate ion taken up than did nonmycorrhizal roots, leading to the suggestion that mycorrhizae may act as rhizosphere buffers. Among the three coniferous species, Douglas-fir roots released more hydroxide ions per nitrate taken up than did western hemlock; Sitka spruce values were intermediate. These apparent species effects may be related to the less acidic mineral soil environment where Douglas-fir roots are often found, and to the more acidic forest floor environment where western hemlock roots primarily grow.