Acidic deposition due to fossil-fuel combustion may increase the solubility of monomeric aluminum (Al3+) in soils, which reduces root growth and interferes with nutrient uptake. The effects of elevated levels of Al3+ on plant growth have been studied extensively in the lab, but field studies do not consistently find reduced tree growth. However, even in the absence of an overall effect on growth, soluble Al3+ can still impact plant mineral nutrition, and there is some evidence that Al3+ exposure can inhibit nitrate (NO3) uptake by plants. If the uptake of NO3 by overstory trees is reduced by elevated levels of available Al3+, this could contribute to the low N retention and high NO3 loss that has been observed in some Appalachian forests. To examine the feasibility of this idea, we measured soluble soil Al levels (chelated and free monomeric) in a long-term, whole-watershed acidification experiment in the Fernow Experimental Forest near Parsons, WV. We then used a 15N-labeling technique to measure the relative uptake of NH4 vs. NO3, with and without added soluble Al3+, by six major overstory species in a nearby stand of deciduous broadleaf trees.
Soluble soil Al was mostly in the free Al3+ form, which is the form that is most harmful to plant nutrition. Al3+ was ~65 μM higher (~250%) in the mineral soil of the acidified watershed vs. a reference watershed, but there was no difference in soil Al in the organic soil horizon. Without added Al3+, NO3 accounted for 59% of the total uptake of N from the labeled pool, with no difference between species. However, the share of inorganic N acquired as NO3 by trees dropped to 45% when Al3+ was added, and this reduction did not differ between species. Our results suggest that soluble soil Al3+ can impact tree nutrition even in the absence of a reduction in overall uptake of N. In an initial assessment of the potential impact of Al on NO3 discharge, we estimate that the reduction in uptake of NO3 could cause up to 7.73 kg NO3-N ha-1 yr-1 to be lost from the acidified watershed, which is 76% of the treatment-induced increase in stream NO3-N discharge. Thus, increased Al3+ exposure due to soil acidification could reduce tree uptake of NO3, and increase NO3 discharge from forested catchments.