Foliar retention of 15N tracers:: implications for net canopy exchange in low- and high-elevation forest ecosystems

Nitrogen-15(N-15) labelled ammonium and nitrate were applied to understory saplings in a low-elevation deciduous forest and a high-elevation coniferous forest during the 1993 growing season to estimate the minimum retention of inorganic N from wet deposition. Tissue samples were collected at both sites before and after: the tracer applications to determine the initial and the final N-15 content. The fraction of the tracer application retained by ceciduous leaves or by current-year leaves and stems on conifers was calculated from measurements of N-15 and total N content. At both study sites, tracer applications significantly increased the N-15 content of sapling tissues, although there was no significant difference between the fractional retention of ammonium and nitrate. The results support earlier studies indicating that the foliar retention of N-15 applied in simulated wet deposition is small. Our estimates of annual N uptake from wet deposition by the forest canopy at the low- and high-elevation study sites were only 14 and 7%, respectively. This study, along with prior studies of the different chemical forms of N deposition, indicates that canopy uptake of dry N deposition is a major contributor to the net canopy exchange of N in both forests. (C) 1998 Elsevier Science B.V.