A comment on the quantitative significance of aerobic methane release by plants

Abstract

Abstract. A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH 4 emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH 4 emissions as 10-60 Mt CH 4 year −1 . This estimate is much smaller than the 62-236 Mt CH 4 year −1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH 4 budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH 4 emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH 4 emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH 4 emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH 4 emission, how CH 4 emissions change with light, temperature and 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. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.

A novel pathway of direct methane production and emission by eukaryotes including plants, animals and fungi: An overview

“…Keppler et al (2006) made the first attempt to extrapolate the magnitude of this newly discovered CH 4 source of plants on measured CH 4 emission rates and NPP (net primary production of roots and shoots) and suggested a range of 62e236 Tg CH 4 yr À1 from plants. However, it seems that the magnitude of this source has been overestimated because of the following reasons: (1) the involvement of plant roots in the process and the extent that roots contribute to CH 4 production have not been defined, whereas the NPP allocated to roots was included in the calculation; (2) the loss of vegetation biomass during the growing season and plant selfshading may cause uncertainties (Kirschbaum et al, 2006;Megonigal and Guenther, 2008); (3) the length of the growing season was multiplied twice when multiplying the measured leaf emission rate by the annual growth rate and growing season length (Parsons et al, 2006).…”

“…Several subsequent scaling-up estimates were achieved using different assumptions, but the data for leaf emission rates were collected during the original study ) using bottom-up approaches (Butenhoff and Khalil, 2007;Houweling et al, 2006;Kirschbaum et al, 2006;Parsons et al, 2006) (Table 2). By assuming UV radiation-induced CH 4 emissions from pectin as representing foliar CH 4 emissions, Bloom et al (2010) suggested a global contribution of 0.2e1.0 Tg plantderived CH 4 yr À1 .…”

“…Furthermore, this assumption is consistent with CH 4 released with diverse d 13 C observed in different experiments due to various sources of CH 4 from soil water (Nisbet et al, 2009). An adsorption and desorption model for ambient air has also been proposed (Kirschbaum et al, 2006). However, based on Henry's law, Watanabe et al (2012) argued that the amount of actual CH 4 release is much greater (approximately 1000-fold) than theoretically calculated due to the magnitude of water vapour emissions.…”

Sensitivity of wetland methane emissions to model assumptions: application and model testing against site observations

“…Together, previous modeling studies suggest rice paddies and wetlands can release 100-380 Tg CH 4 yr −1 to the atmosphere. Further, recent studies identified a new source of tropical methane from nonwetland plants that could add as much as 10-60 Tg CH 4 yr −1 to the global budget (Keppler et al, 2006;Kirschbaum et al, 2006), although this source has been disputed and is still poorly quantified (Dueck et al, 2007).…”

Continuous measurement of methane flux over a larch forest using a relaxed eddy accumulation method

“…The global methane production rates from living plants were estimated to be 62-236 Tg CH 4 year −1 , which may have a considerable effect on the global methane budget. This study generated a vigorous scientific debate and many subsequent studies were performed (e.g., Kirschbaum et al 2006;Kitaoka et al 2007;Beerling et al 2008;Kirschbaum and Walcroft 2008;Bowling et al 2009). In subsequent experiments, emission rates were found to be quite distinct in each experiment (e.g., Keppler et al 2006;Kirschbaum et al 2006;Kitaoka et al 2007), and some studies failed to measure any emissions (Beerling et al 2008;Kirschbaum and Walcroft 2008).…”