FOLIAR RETENTION OF N-15 TRACERS - IMPLICATIONS FOR NET CANOPY EXCHANGE IN LOW-ELEVATION AND HIGH-ELEVATION FOREST ECOSYSTEMS

Nitrogen-15(N-15) labelled ammonium and nitrate were applied to unders tory saplings in a low-elevation deciduous forest and a high-elevation coniferous forest during the 1993 growing season to estimate the mini mum retention of inorganic N from wet deposition. Tissue samples were collected at both sites before and after: the tracer applications to d etermine the initial and the final N-15 content. The fraction of the t racer application retained by ceciduous leaves or by current-year leav es and stems on conifers was calculated from measurements of N-15 and total N content. At both study sites, tracer applications significantl y increased the N-15 content of sapling tissues, although there was no significant difference between the fractional retention of ammonium a nd nitrate. The results support earlier studies indicating that the fo liar retention of N-15 applied in simulated wet deposition is small. O ur estimates of annual N uptake from wet deposition by the forest cano py at the low- and high-elevation study sites were only 14 and 7%, res pectively. This study, along with prior studies of the different chemi cal forms of N deposition, indicates that canopy uptake of dry N depos ition is a major contributor to the net canopy exchange of N in both f orests. (C) 1998 Elsevier Science B.V.