3 years ago

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

Jonathan R. Cumming, Mary Beth Adams, Mark B. Burnham, William T. Peterjohn

Abstract

Increased availability of monomeric aluminum (Al3+) in forest soils is an important adverse effect of acidic deposition that reduces root growth and inhibits nutrient uptake. There is evidence that Al3+ exposure interferes with NO3 uptake. If true for overstory trees, the reduction in stand demand for NO3 could increase NO3 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 NO3 and NH4 + by six tree species in situ under increased soil Al3+ using a 15N-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 Al3+, the proportion of inorganic N acquired as NO3 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 Al3+ form, and the concentration of Al3+ was ~65 μM higher (~250%) in the mineral soil of the acidified watershed vs. an untreated watershed. Thus, increased levels of soil Al3+ under acidic deposition cause a reduction in uptake of NO3 by mature trees. When our 15N uptake results were applied to the watershed acidification experiment, they suggest that increased Al3+ exposure could reduce tree uptake of NO3 by 7.73 kg N ha−1 year−1, and thus increase watershed NO3 discharge.

Publisher URL: https://link.springer.com/article/10.1007/s00442-017-3955-8

DOI: 10.1007/s00442-017-3955-8

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