Effects of elevated nitrate and aluminum on the growth and nutrition of red spruce (Picea rubens) seedlings

Acidic deposition in high-elevation forests in the Appalachian Mountains of the eastern United States has been implicated in the decline of red spruce (Picea rubens Sarg.). Elevated soil acidity may increase soil Al availability and toxicity to roots. Enhanced soil solution NO3(-1) concentrations, resulting from precipitation inputs and enhanced soil organic matter mineralization, may exacerbate Al toxicity by increasing root Al uptake. We exposed red spruce seedlings to 350, 500, 800 or 1400 micromolar NO3(-1) and 0 or 200 micromolar Al in a factorial design in sand-nutrient solution culture to test if increased NO3(-1) concentrations enhance Al uptake and toxicity. In addition to significant reductions in seedling growth parameters resulting from Al exposure, we found significant interactions between NO3(-1) and Al seedling height growth rate, needle weight, shoot weight and root weight. Differences in these parameters between Al treatments became more pronounced as solution NO3(-1) concentration increased and reflected an Al-mediated inhibition of seedling response to increasing NO3(-1) concentration. Solution NO3(-1) concentrations above 500 micromolar induced root nitrate reductase (NR) activity, whereas shoot NR activity increased in response to NO3(-1) up to 500 micromolar and declined above that concentration. In contrast, exposure to Al depressed NR activity of roots but tended to stimulate needle NR activity. Foliar N concentrations increased in seedlings grown in cultures containing between 350 and 500 micromolar NO3(-1) with no change above 500 micromolar. Increasing concentrations of NO3(-1) depressed foliar P concentrations, with reductions being greatest in seedlings exposed to 1400 micromolar NO3(-1). Exposure to Al increased foliar Ca, K and Al concentrations, decreased foliar P concentrations, and inhibited increases in foliar Mg concentration in response to increasing NO3(-1). The consistent interactions between NO3(-1) and Al for growth root NR activity and foliar Mg concentration were the result of an inhibition of seedling response to NO3(-1) mediated by Al in solution, rather than enhanced Al toxicity resulting from growth in the presence of elevated NO3(-1) concentrations.