Large Canopy Exchange Fluxes of Inorganic and Organic Nitrogen and Preferential Retention of Nitrogen by Epiphytes in a Tropical Lowland Rainforest Nina Hinko-Najera Umana 1,2 and Wolfgang Wanek 2 * 1 Department of Forest and Ecosystem Science, The University of Melbourne, Water Street, Creswick, Victoria 3363, Australia; 2 Department of Chemical Ecology and Ecosystem Research, University of Vienna, Vienna Ecology Centre, Althanstrasse 14, A-1090 Vienna, Austria ABSTRACT Little is known about how tropical forest canopies interact with atmospheric nitrogen deposition and how this affects the internal nutrient dynamics and the processing of external nutrient inputs. The objectives of this study therefore were (1) to investigate gross and net canopy nitrogen (N) fluxes (retention and leaching) and (2) the effect of canopy components on net canopy N retention. Tracers were applied on detached branches in a tropical wet lowland rainforest, Costa Rica. A novel 15 N pool dilution method showed that gross canopy fluxes (retention and leaching) of NO 3 - , NH 4 + , and dissolved organic nitrogen (DON) were remarkably higher than net throughfall fluxes. Gross fluxes of NH 4 + and NO 3 - resulted in a negligible net flux whereas DON showed net uptake by the canopy. The highest quantity of 15 N was recovered in epi- phytic bryophytes (16.4%) although the largest biomass fraction was made up of leaves. The study demonstrates that tracer applications allow inves- tigation of the dynamic and complex canopy ex- change processes and that epiphytic communities play a major role in solute fluxes in tree canopies and therefore in the nutrient dynamics of tropical rain forests. Key words: artificial rainfall; leaching; 15 N pool dilution; canopy retention; throughfall. INTRODUCTION Atmospheric deposition of reactive nitrogen has increased globally and is projected to increase substantially within the coming decades in tropical forests (Galloway and others 2004). The fate of deposited reactive nitrogen is largely unknown for tropical ecosystems (Matson and others 1999). Most data come from short-term and long-term fertilization experiments where nitrogen has been added to the soils. These studies demonstrate Received 10 September 2009; accepted 3 February 2010; published online 9 March 2010 Electronic supplementary material: The online version of this article (doi:10.1007/s10021-010-9324-7) contains supplementary material, which is available to authorized users. Author contributions: N.H.-N. Umana performed the research, ana- lyzed the data and wrote the paper. W. Wanek designed the study, contributed the novel gross flux method and revised the paper. *Corresponding author; e-mail: wolfgang.wanek@univie.ac.at Ecosystems (2010) 13: 367–381 DOI: 10.1007/s10021-010-9324-7 Ó 2010 Springer Science+Business Media, LLC 367