Biogeochemistry 45: 73–93, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. The fate of NH 4 NO 3 added to Sphagnum magellanicum carpets at f ve European mire sites B.L. WILLIAMS 1 , A. BUTTLER 2 , P. GROSVERNIER 2 , A.-J. FRANCEZ 3 , D. GILBERT 3 , M. ILOMETS 4 , J. JAUHIAINEN 5 , Y. MATTHEY 2 , D.J. SILCOCK 1 & H. VASANDER 5 1 Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK; 2 Laboratoire d’Ecologie Végétale,University of Neuchâtel, 2007 Neuchâtel, Switzerland; 3 Service d’Ecologie Végétale, U.M.R.-C.N.R.S. 6553 «Ecobio», University of Rennes 1, 35042 Rennes, France; 4 Institute of Ecology, Tallinn, Estonia; 5 Department of Forest Ecology, FIN-00014 University of Helsinki, Finland Accepted 3 July 1998 Key words: atmospheric deposition, moss, bog, nitrogen, phosphorus, water table Abstract. Nitrogen additions as NH 4 NO 3 corresponding to 0 (N0), 1 (N1), 3 (N3) and 10 (N10) g N m −2 yr −1 were made to Sphagnum magellanicum cores at two-week intervals in situ at four sites across Europe, i.e. Lakkasuo (Finland), Männikjärve (Estonia), Moidach More (UK) and Côte de Braveix (France). The same treatments were applied in a glasshouse experiment in Neuchâtel (Switzerland) in which the water table depth was artifi ially main- tained at 7, 17 and 37 cm below the moss surface. In the f eld, N assimilation in excess of values in wet deposition occurred in the absence of growth, but varied widely between sites, being absent in Lakkasuo (moss N:P ratio 68) and greatest in Moidach More (N:P 21). In the glasshouse, growth was reduced by lowering the water table without any apparent effect on N assimilation. Total N content of the moss in fi ld sites increased as the mean depth of water table increased indicating growth limitation leading to increased N concentrations which could reduce the capacity for N retention. Greater contents of NH 4 + in the underlying peat at 30 cm depth, both in response to NH 4 NO 3 addition and in the unamended cores conf rmed poor retention of inorganic N by the moss at Lakkasuo. Nitrate contents in the prof les at Lakkasuo, Moidach More, and Côte de Braveix were extremely low, even in the N10 treatment, but in Männikjärve, where the mean depth of water table was greatest and retention absent, appreciable amounts of NO 3 − were detected in all cores. It is concluded that peatland drainage would reduce the capture of inorganic N in atmospheric deposition by Sphagnum mosses. Introduction The principal peat forming plants, Sphagnum mosses, that grow on raised bogs are adapted to use the nutrients available to them at low concentrations in rainwater. Woodin et al. (1985) demonstrated the activity of the induca-