Biotic controls on CO 2 and CH 4 exchange in wetlands – a closed environment study TORBEN R. CHRISTENSEN 1, *, NICOLAI PANIKOV 1,2,3 , MIHAIL MASTEPANOV 1,2 , ANNA JOABSSON 1 , ANGELA STEWART 1 , MATS ÖQUIST 4 , MARTIN SOMMERKORN 1 , SEBASTIAN REYNAUD 1 and BO SVENSSON 4 1 Climate Impacts Group, Department of Ecology (now: Department of Physical Geography and Eco- system Analysis), Lund University, S-223 62 Lund, Sweden; 2 Russian Academy of Sciences, Institute of Microbiology, Moscow, 117811, Russia; 3 Dept. of Chemistry & Chemical Biology, Stevens Institute of Technology, NJ, United States; 4 Department of Water and Environmental Studies, Linköping University, S-581 83 Linköping, Sweden; *Author for correspondence Received 9 March 2002; accepted in revised form 8 July 2002 Key words: 14 C labelling, CO 2 exchange, Controlled environment studies, Methane emissions, Peat- lands Abstract. Wetlands are significant sources of the important greenhouse gas CH 4 . Here we explore the use of an experimental system developed for the determination of continuous fluxes of CO 2 and CH 4 in closed ecosystem monoliths including the capture of 14 CO 2 and 14 CH 4 following pulse labelling with 14 CO 2 . We show that, in the ecosystem studied, ebullition (bubble emission) may account for 18 to 50% of the total CH 4 emission, representing fluxes that have been difficult to estimate accurately in the past. Furthermore, using plant removal and 14 C labelling techniques, we use the system to detail the direct influence of vascular plants on CH 4 emission. This influence is observed to be dependent on the amount of vascular plants present. The results that may be produced using the presented experimental set-up have implications for an improved understanding of wetland ecosystem/atmosphere interactions, includ- ing possible feedback effects on climate change. In recent years much attention has been devoted to ascertaining and subsequently using the relationship between net ecosystem productivity and CH 4 emis- sion as a basis for extrapolation of fluxes across large areas. The experimental system presented may be used to study the complex relationship between vascular plants and CH 4 emission and here we show examples of how this may vary considerably in nature between and even within ecosystems. Introduction Growing evidence from recent climate and biogeochemical studies indicates that carbon cycling in boreal and arctic wetlands strongly influences the global climate (IPCC (Intergovernmental Panel on Climate Change) 1995). In particular, exchange of the greenhouse gases CO 2 and CH 4 is important, both for the dynamics of the “natural” climate and for possible man-induced climate change. Northern peatlands and wet tundra regions contain large amounts of stored organic carbon, and their potential for exchange of greenhouse gases (CO 2 , CH 4 ) with the atmosphere is therefore great (Gorham 1991; Oechel et al. 1993, 2000; Christensen et al. 1999). Biogeochemistry 64: 337–354, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.