Climatic modifiers of the response to nitrogen depositionin peat-forming Sphagnum mosses: a meta-analysisJ. Limpens1*, G. Granath2*, U. Gunnarsson2*, R. Aerts3, S. Bayley4, L. Bragazza5,6,7, J. Bubier8, A. Buttler6,7,9,L. J. L. van den Berg10, A-J. Francez11, R. Gerdol5, P. Grosvernier12, M. M. P. D. Heijmans1,M. R. Hoosbeek13, S. Hotes14, M. Ilomets15, I. Leith16, E. A. D. Mitchell17, T. Moore18, M. B. Nilsson19,J-F. Nordbakken20, L. Rochefort21, H. Rydin2, L. J. Sheppard16, M. Thormann22, M. M. Wiedermann19,B. L. Williams23and B. Xu241Nature Conservation & Plant Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708 PB Wageningen, the Netherlands;2Department ofPlant Ecology, Evolutionary Biology Centre, Uppsala University, Norbyva¨gen 18D, SE-752 36 Uppsala, Sweden;3Faculty of Earth and Life Sciences,Systems Ecology, Free University of Amsterdam, De Boelelaan 1085 1081 HV Amsterdam, the Netherlands;4Department of Biological Sciences,University of Alberta, Edmonton, AB, Canada T6G 2E9;5Department of Biology and Evolution, University of Ferrara, Corso Ercole I d’Este 32, I-44100Ferrara, Italy;6WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Wetlands Research Group, Site Lausanne, station 2, CH-1015Lausanne, Switzerland;7Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Laboratory of Ecological Systems – ECOS, station 2, CH-1015 Lausanne,Switzerland;8Mount Holyoke College, Environmental Studies Program, Clapp Laboratory, 50 College Street, South Hadley, MA 01075, USA;9Laboratoryof Chrono-Environnement, UMR 6249 CNRS – INRA, Universite´de Franche-Comte´, Besanc¸on, France;10Aquatic Ecology and Environmental Biology,Radboud University Nijmegen, 6525 AJ Nijmegen, the Netherlands;11UMR 6553 ECOBIO & FR90 CAREN, Rennes University, CNRS, Campus deBeaulieu, 263 avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France;12LIN’eco, Case postale 80, 2732 Reconvilier, Switzerland;13Earth SystemScience – Climate Change, Department of Environmental Sciences, Wageningen University, PO Box 47, 6700AA Wageningen, the Netherlands;14Department of Animal Ecology, Justus-Liebig-University, Heinrich-Buff-Ring 26-32 (IFZ), D-35392 Giessen, Germany;15Department of LandscapeEcology, Institute of Ecology, Tallinn University, Uus-Sadama 5, EE-10120 Tallinn, Estonia;16CEH Edinburgh Bush Estate Penicuik, EH26 0QB, UK;17Laboratory of Soil Biology, University of Neuchaˆtel, Rue Emile-Argand 11, CH-2000 Neuchaˆtel, Switzerland;18Department of Geography, McGillUniversity, 805 Sherbrooke St. W. Montreal, QC, Canada H3A 2K6;19Soil Science, Biogeochemistry Group, Department of Forest Ecology &Management, Swedish University of Agricultural Sciences, S-901 83 Umea˚, Sweden;20The Norwegian Forest and Landscape Institute, Postbox 115,1431 A˚s, Norway;21Department of Plant Sciences, Universite´Laval, 2425 rue de l’Agriculture, Quebec, QC, Canada G1V 0A6;22Aquilon EnvironmentalConsulting Ltd. 3111 Spence Wynd SW, Edmonton, AB, Canada T6X 0H7;23Macaulay Land Use Research Institute, Aberdeen, UK;24Department ofPlant Biology, Southern Illinois University Carbondale 1125 Lincoln Drive, Carbondale, IL 62901, USAAuthor for correspondence:Juul LimpensTel: +31 317483173Email: [email protected]: 25 January 2011Accepted: 31 January 2011New Phytologist (2011)doi: 10.1111/j.1469-8137.2011.03680.xKey words: carbon, climate, global change,meta-regression, nitrogen, peatlands,productivity, Sphagnum.Summary• Peatlands in the northern hemisphere have accumulated more atmospheric car-bon (C) during the Holocene than any other terrestrial ecosystem, makingpeatlands long-term C sinks of global importance. Projected increases in nitrogen(N) deposition and temperature make future accumulation rates uncertain.• Here, we assessed the impact of N deposition on peatland C sequestrationpotential by investigating the effects of experimental N addition on Sphagnummoss. We employed meta-regressions to the results of 107 field experiments,accounting for sampling dependence in the data.• We found that high N loading (comprising N application rate, experiment dura-tion, background N deposition) depressed Sphagnum production relative tountreated controls. The interactive effects of presence of competitive vascularplants and high tissue N concentrations indicated intensified biotic interactionsand altered nutrient stochiometry as mechanisms underlying the detrimental Neffects. Importantly, a higher summer temperature (mean for July) and increased*These authors contributed equally to this work.NewPhytologistResearch 2011 The AuthorsNew Phytologist  2011 New Phytologist TrustNew Phytologist (2011) 1www.newphytologist.comannual precipitation intensified the negative effects of N. The temperature effect wascomparable to an experimental application of almost 4 g N m)2yr)1for each 1 Cincrease.• Our results indicate that current rates of N deposition in a warmer environment willstrongly inhibit C sequestration by Sphagnum-dominated vegetation.IntroductionAt the scale of millennia, peatlands in the northern hemi-sphere have significantly affected the earth’s atmosphere(Frolking & Roulet, 2007) by steadily sequestering CO2inthe form of partly decomposed organic material (peat),mostly formed by peat forming Sphagnum mosses (Rydin& Jeglum, 2006). Although fairly resilient to small distur-bances in climate (Belyea & Baird, 2006), Sphagnum-dominated peatlands are now experiencing a hithertounprecedented combination of stresses such as increases innitrogen (N) deposition (Galloway et al., 2008), tempera-ture, and drought frequency (Dise, 2009). To what extentthese stresses will affect future carbon (C) sequestrationrequires urgent attention (Dise, 2009).Sphagnum-dominated peatlands are extremely nutrientpoor ecosystems and commonly rely on atmospheric inputsas their sole sources of external nutrients, resulting in a plantcommunity sensitive to increases in N deposition (Bobbinket al., 2003). The vegetation consists mainly of slow-growingericaceous dwarf shrubs and cyperaceous graminoids root-ing in a soil matrix of living and dead peat mosses. Thecompetitive balance between Sphagnum and vascular plantsis maintained by their asymmetrical competition for nutri-ents. Sphagnum uses N derived from atmosphericdeposition, and efficiently relocates nutrients from older tis-sue (Rydin & Clymo, 1989), whereas vascular plantsdepend more on N released during decomposition oforganic material