Interactions among soil biology, nutrition, and performance of actinorhizal plant species in the H.J. Andrews Experimental Forest of Oregon

Abstract

The study examined the effect of Frankia, macronutrients, micronutrients, mycorrhizal fungi, and plant-growth-promoting fluorescent Pseudomonas sp. on total biomass, nodule weight, and nitrogen fixation of red alder ( Alnus rubra) and snowbrush ( Ceanothus velutinus) under greenhouse conditions. The soil samples were collected from a 10-year-old clearcut on the H.J. Andrews Experimental Forest, Oregon. Within the clearcut, four sampling points were selected along a slope gradient. Red alder and snowbrush plants were greenhouse-grown in a mix of soil–vermiculite–perlite (2:1:1) for 6 and 12 months, respectively. Plants were inoculated with Frankia and a fluorescent Pseudomonas sp. Some of the red alder were also inoculated with ectomycorrhizal Alpova diplophloeus, and some snowbrush with endomycorrhizal Glomus intraradix. There was no interaction between treatment and slope location for either species. There were significant treatment effects for red alder, but not for snowbrush. Red alder seedlings given Frankia and macronutrients produced more biomass and had greater nitrogen fixation than seedlings grown without additions; adding A. diplophloeus increased nitrogen fixation by 33% over that obtained with Frankia plus macronutrients. Frankia, macronutrients, and the mycorrhizal fungus together increased nitrogen fixation by 136% over the control. Adding only micronutrients to Frankia and macronutrients, however, reduced nitrogen fixation by nearly one half; the presence of the mycorrhizal fungus appeared to buffer these negative effects. Pseudomonas inoculation did not affect any of the measured variables. Slope location of soil affected the two plant species differently. Red alder seedlings grown in upper slope soil had greater biomass and nitrogen fixation than those grown in soil from the lower slope. In contrast, snowbrush plants had greater biomass, nodule weight, and nitrogen fixation when grown in bottom slope soil rather than on soil from any of the other slope positions.