Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests J. A. MACDONALD* { ,N.B.DISE { ,E.MATZNER { ,M.ARMBRUSTER { §,P.GUNDERSEN } and M. FORSIUS** *Scottish Environment Protection Agency, 7 Whitefriars Crescent,Perth, PH2 0PA, UK, {Department of Earth Science, The Open University,MiltonKeynes,HK76AA,UK, {InstituteofTerrestrialEcosystemResearch,UniversityofBayreuth(BITO È K),Bayreuth, Germany, §Technical Institute of Dresden, Postfach 1117, D-01735 Tharandt, Germany, }Danish Forest and Landscape Research Institute, Hùrsholm Kongevej 11, DK-2970 Hùrsholm, Denmark, **Finnish Environment Institute, PO Box 140, FIN-00251 Helsinki, Finland Abstract The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundredandeightyoneforestsareexamined,fromcountriesacrossEuroperangingfrom boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1kgNha 21 y 21 in northern Norway and Finland to greater than 60kgNha 21 y 21 in the Netherlands and Czech Republic. The amount of NO 3 ± leached covers a smaller range, between 1 and 40kgNha 21 y 21 . Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P , 0.05) are observed between N input and NO 3 ± leached. The slope of the relationship for those sites where C/N ratio is < 25('nitrogen enriched' sites) is twice that for those sites where C/N ratio is . 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO 3 ± leaching also shows a relationship with soil pH, with high rates of NO 3 ± leachingonlyobservedatsiteswithapH , 4.5andNinputs . 30kgNha 21 y 21 .Treeage andspecieshavenosignificantimpactontheecosystemresponsetoNinputataregional scale. Keywords: C/N ratio, forest, nitrate leaching, nitrogen deposition Received 4 May 2001; revised version received 22 March 2002 and accepted 26 March 2002 Introduction Forests across Europe receive a wide range of nitrogen inputs from the atmosphere as wet and dry deposition, typically at rates between 1 and 60 kg N ha 1 y 1 . Most forest ecosystems are naturally N limited and a wide range of ecosystem responses to this elevated N input has been recorded, including increased forest growth (Tamm, 1991), increases in foliar N concentration, and increased leaching of nitrate (Aber et al., 1995; Boxman et al., 1998). Deposition of N has also been implicated in forest decline (Nihlga Êrd, 1985). This led to the develop- ment of the hypothesis of `nitrogen saturation', variously defined as `an ecosystem where N losses approximate or exceed the inputs of N' (A Ê gren & Bosatta, 1988) or one where `the availability of inorganic N is in excess of total combined plant and microbial nutritional demand' (Aber et al., 1989). Indicators of N saturation can provide valu- able tools in the prediction of ecosystem response to elevated N inputs. A large number of studies have monitored input and output fluxes of N and other major elements in forest ecosystems. In addition, experimental manipulation Correspondence: Jannette A MacDonald, fax 44 (0)1738 630997, e-mail: jannette.macdonald@sepa.org.uk Global Change Biology (2002) 8, 1028±1033 1028 ß 2002 Blackwell Science Ltd