Biogeochemistry 50: 73–93, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Throughfall chemistry in a loblolly pine plantation under elevated atmospheric CO 2 concentrations J. LICHTER 1 , M. LAVINE 2,3 , K.A. MACE 1 , D.D. RICHTER 3 & W.H. SCHLESINGER 1,3 1 Department of Botany, Duke University, Durham, NC 27708, U.S.A.; 2 Institute of Statistics and Decision Science, Duke University, Durham, NC 27708, U.S.A.; 3 Nicholas School of the Environment, Duke University, Durham, NC 27708, U.S.A. Received 19 July 1999; accepted 30 November 1999 Key words: atmosphere-canopy interaction, atmospheric CO 2 , atmospheric deposition, dis- solved organic carbon, FACE experiment, loblolly pine (Pinus taeda L.), throughfall chemistry Abstract. Accelerated tree growth under elevated atmospheric CO 2 concentrations may influ- ence nutrient cycling in forests by (i) increasing the total leaf area, (ii) increasing the supply of soluble carbohydrate in leaf tissue, and (iii) increasing nutrient-use efficiency. Here we report the results of intensive sampling and laboratory analyses of NH + 4 , NO - 3 , PO 3- 4 ,H + , K + , Na + , Ca 2+ , Mg 2+ , Cl - , SO 2- 4 , and dissolved organic carbon (DOC) in throughfall precipitation during the first 2.5+ years of the Duke University Free-Air CO 2 Enrichment (FACE) experiment. After two growing seasons, a large increase (i.e., 48%) in throughfall deposition of DOC and significant trends in throughfall volume and in the deposition of NH + 4 , NO - 3 ,H + , and K + can be attributed to the elevated CO 2 treatment. The substantial increase in deposition of DOC is most likely associated with increased availability of soluble C in plant foliage, whereas accelerated canopy growth may account for significant trends toward decreasing throughfall volume, decreasing deposition of NH + 4 , NO - 3 , and H + , and increasing deposition of K + under elevated CO 2 . Despite considerable year-to-year variability, there were seasonal trends in net deposition of NO - 3 ,H + , cations, and DOC associated with plant growth and leaf senescence. The altered chemical fluxes in throughfall suggest that soil solu- tion chemistry may also be substantially altered with continued increases in atmospheric CO 2 concentrations in the future. Introduction Forest canopies capture dusts, aerosols, and gases in dry deposition and dis- solved ions in precipitation and cloud droplets. Intercepted chemicals may be absorbed by plant foliage or microbes living on leaf surfaces, or they may be washed off leaves and enter the soil system in throughfall solution. Typically, NH + 4 , HNO 3 , and H + are absorbed by forest canopies, whereas base cations