Contrasted effects of simulated drought on the production and oxidation of methane in a mid-Wales wetland C. Freeman a, * , G.B. Nevison a ,H.Kang a , S. Hughes b , B. Reynolds b , J.A. Hudson c a School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK b Centre for Ecology & Hydrology, Bangor Research Unit, University of Wales, Bangor, Gwynedd LL57 2UP, UK c Centre for Ecology & Hydrology, Wallingford, Oxfordshire OX10 8BB, UK Received 20 September 2000; received in revised form 7 June 2001; accepted 9 July 2001 Abstract WetlandsareamajorcontributortotheglobalCH 4 budget.Currently,thereisaconsensusviewthatdroughtrestrainsCH 4 emissionsfrom wetlands,andthatthisarisesduetoasuppressionofCH 4 productionandstimulationofCH 4 oxidationunderthemoreaerobicconditionsthat accompany lower water table levels. Our data con®rm that under drought conditions, CH 4 production is lower 273%, P , 0.05) largely becauseitiscon®nedtothedeeper,moreanaerobic,depths.Lowerproductionrateswerefoundtopersistatleast1monthbeyondtheendof thedrought289%, P , 0.01).However,wefoundnoevidenceofincreasedratesofCH 4 oxidationduringthedrought,andincontrast,atthe end of the simulation observed signi®cantly less CH 4 oxidationinthedroughttreatedsystemP , 0.05). These ®ndings suggest that under droughtconditions,theregulationofwetlandmethane¯uxoccursprimarilythroughchangesinmethaneproduction,withmethaneoxidation playing only a secondary role. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methane; Production; Oxidation; Peatland; Drought; Water table 1. Introduction Various physico-chemical or biological factors such as temperature, type and amount of substrate, vegetation or pH have been proposed as the main controlling variables for CH 4 emission from wetlands. Much attention has been paid to the effects of ¯uctuating water table levels. This is largely because climate change and the drier summers predicted for high latitudes by climate models Mitchell andWarrilow,1987)maylowerwetlandwatertablelevels, which through increased soil aeration may alter CH 4 ¯uxes from northern wetlands. Such wetlands represent approxi- mately 50% of the global wetland area and almost 30% of global wetland CH 4 emissions Matthews and Fung, 1987). Thus, water table drawdown may result in a feedback to climate change Freeman et al., 1993a; Bridgham et al., 1995). The net ¯ux of CH 4 from a wetland is governed by the balance between rates of CH 4 production methano- genesis) and rates of CH 4 consumption CH 4 oxidation). For a complete understanding of the factors governing CH 4 release to the atmosphere, it is, thus, essential to appreciate the rates at which CH 4 consumption are occur- ring. Although it has been noted that the methods for ®eld assessment of CH 4 consumption are under debate, it is generally accepted that a large and variable 1±90%) part oftheCH 4 producedinpeatisconsumedbyCH 4 oxidation inoxicregionssuchastherhizosphereandthepeatsurface Segers, 1998). The distribution and availability of oxygen inthewetlandsoilpro®lehasbeenidenti®edasakeyregu- lator of aerobic CH 4 oxidation. For example, temporal changes in factors that affect oxygen availability such as the presence of active photosynthetic organisms King, 1990), and diurnal variations in water table levels, have been shown to in¯uence CH 4 oxidation Bosse et al., 1993). A larger fraction of the CH 4 has been found to be oxidized at sites with low average water tables, and those resultssuggestthatlownetemissionratesintheseenviron- ments are caused not only by lower CH 4 production rates, butalsobyconditionsmorefavourableforthedevelopment of CH 4 -oxidizing bacteria in these environments Sundh et al., 1994). However, those researchers have also found that overall, net CH 4 emission rates were negatively, but only weakly, correlated with the potential oxidation, suggesting thatwhileaerobicoxidationcancontrolemissionratesfrom comparatively dry peatlands, other factors also in¯uence the emission. Sundh et al., 1995). In the light of such Soil Biology & Biochemistry 34 2002) 61±67 0038-0717/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S0038-071701)00154-7 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: 144-1248-382353; fax: 144-1248- 370731. E-mail address: c.freeman@bangor.ac.uk C. Freeman).