Soluble soil aluminum alters the relative uptake of mineral nitrogen forms by six mature temperate broadleaf tree species: possible implications for watershed nitrate retention.

Increased availability of monomeric aluminum (Al) in forest soils is an important adverse effect of acidic deposition that reduces root growth and inhibits nutrient uptake. There is evidence that Al exposure interferes with NO uptake. If true for overstory trees, the reduction in stand demand for NO could increase NO discharge in stream water. These effects may also differ between species that tolerate different levels of soil acidity. To examine these ideas, we measured changes in relative uptake of NO and NH by six tree species in situ under increased soil Al using a N-labeling technique, and measured soluble soil Al levels in a separate whole-watershed acidification experiment in the Fernow Experimental Forest (WV). When exposed to added Al, the proportion of inorganic N acquired as NO dropped 14% across species, but we did not detect a reduction in overall N uptake, nor did tree species differ in this response. In the long-term acidification experiment, we found that soluble soil Al was mostly in the free Al form, and the concentration of Al was 65 μM higher (250%) in the mineral soil of the acidified watershed vs. an untreated watershed. Thus, increased levels of soil Al under acidic deposition cause a reduction in uptake of NO by mature trees. When our N uptake results were applied to the watershed acidification experiment, they suggest that increased Al exposure could reduce tree uptake of NO by 7.73 kg N ha year, and thus increase watershed NO discharge.