Soluble soil aluminum alters the relative uptake of different mineral nitrogen forms by six mature temperate broadleaf tree species

Background/Question/Methods

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.

Results/Conclusions

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.