Received: 14 March, 2007. Accepted: 1 May, 2007. Invited Review Functional Plant Science and Biotechnology ©2007 Global Science Books How Important is Aerobic Methane Release by Plants? Miko U. F. Kirschbaum 1,2*  Ülo Niinemets 3,4  Dan Bruhn 5,6  Anthony J. Winters 7,8 1 Landcare Research, Private Bag 11052, Palmerston North, New Zealand 2 Environmental Biology Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra ACT 2601, Australia 3 Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 64, Tartu 51014, Estonia 4 Department of Plant Physiology, University of Tartu, 23 Riia St., Tartu 51010, Estonia 5 Plant and Soil Science Laboratory, Dept of Agric. Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark 6 Biosystems Department, Risø National Laboratory, Frederiksborgvej 399, DK-4000 Roskilde, Denmark 7 School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia 8 Ecosystem Dynamics Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra ACT 2601, Australia Corresponding author: * KirschbaumM@LandcareResearch.co.nz ABSTRACT The first research paper describing aerobic methane release from living plants and dead organic matter was published in early 2006. These original findings have yet to be independently repeated and confirmed. Instead, the only other detailed study that has been published did not find any significant aerobic emissions of methane. Concerns remain about possible artefacts, especially with respect to methane adsorption and desorption. Several questions are yet to be answered, such as identification of a plausible biochemical mechanism for the process, how CH 4 emissions might change with light, temperature or the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Various studies have assessed the likely magnitude of aerobic methane release within a global context. Different estimates based on more or less sophisticated approaches have all indicated that the magnitude of aerobic methane release must be relatively moderate and contribute between 0-10% of modern and 0-30% of pre-industrial/pre-agricultural methane emissions. In the context of land-use change, consideration of aerobic CH 4 emissions from different plant types is only a small factor for overall greenhouse gas balances. Any carbon- offset benefit from planting trees is likely to be about 100 times as effective as any possible detrimental effect due to increased aerobic methane release. Land-use change, including the draining of wetlands, the establishment of paddy rice farming, or the introduction of ruminant animals, would produce emission changes that significantly outweigh any potential changes arising from differences in aerobic methane release by different plant types. _____________________________________________________________________________________________________________ Keywords: adsorption, aerobic, climate-change mitigation, desorption, dissolution, global budget, Kyoto Protocol, land-use change, methane oxidation, trace gas CONTENTS INTRODUCTION...................................................................................................................................................................................... 138 EXPERIMENTAL EVIDENCE FOR AEROBIC METHANE RELEASE BY PLANTS ......................................................................... 139 Potential problems with methane flux measurements ............................................................................................................................ 140 Field measurements ............................................................................................................................................................................... 141 THE GLOBAL SIGNIFICANCE OF METHANE RELEASE BY PLANTS............................................................................................ 142 CONTRASTING LAND-USE OPTIONS ................................................................................................................................................. 143 CONCLUSIONS ........................................................................................................................................................................................ 144 ACKNOWLEDGEMENTS ....................................................................................................................................................................... 144 REFERENCES........................................................................................................................................................................................... 144 _____________________________________________________________________________________________________________ INTRODUCTION Methane is the second-most important greenhouse gas, con- tributing about 20% to the current radiative forcing of the enhanced greenhouse effect (Ramaswamy et al. 2001). It has been intensively studied and it had been thought that all of its sources and sinks had been identified. Hence, it came as a significant surprise when Keppler et al. (2006) reported a new finding of aerobic methane release by living plants and even dead tissue. However, the question must be asked whether the ap- parent findings of significant methane emissions were actu- ally just artefacts. The observed rates were exceedingly small, and measuring the minute emission fluxes at current high atmospheric methane concentration backgrounds cons- titutes a challenge to any experimental setup. Dueck et al. (2007) used a different experimental approach to overcome some of these measurement challenges and did not report any significant methane emissions in their system. If aerobic methane release does indeed occur, the ques- tion arises as to its global significance. There are inherent difficulties in extrapolating from a few measurements in the laboratory to global emissions from a variety of different plant species and under greatly varying conditions. Keppler et al. (2006) provided estimates of global emissions based on their measurements and derived large values, with as much as 1/3 of global emissions attributed to aerobic me- thane release. However, the Keppler et al. (2006) method for scaling methane fluxes has been questioned by a number of workers (Kirschbaum et al. 2006; Parsons et al. 2006), and their glo- bal estimates were significantly smaller. Additional studies