Foliar loading and metabolic assimilation of dry deposited nitric acid air pollutants by trees

Abstract

Dry deposition of nitric acid vapor (HNO₃) is a major contributor to eutrophication of natural ecosystems. Although soil fertilization by nitrogen deposition is considered to be the primary pathway for changes in plant nutrient status and shifts in ecological structure, the aerial portion of plants offer many times the surface area in which to collect atmospheric HNO₃. As much as 60% of deposited nitrogen may be retained in the canopy and not land on the soil surface below. Although uptake and assimilation appears to contribute to retention, only a small percentage of dry deposition is recovered in assimilated N pools. To test the importance of biological activity on the process and measurements of dry deposition, we used controlled environmental chambers to compare deposition to living and freeze-dried foliage of four tree species using ¹⁵N-labeled HNO₃. In living trees, assimilation was determined by ¹⁵N incorporation into free amino acids and proteins in leaves and roots. From 10% to 60% of the retained HNO₃ was incorporated into the biologically active nitrogen pool. The remainder was bound to foliar surfaces in an insoluble form in either living or freeze-dried foliage. The importance of the boundary layer conditions emerged as a primary factor controlling dry deposition characteristics and measurements.

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