Preface 18Soils are integral to the function of all terrestrial ecosystems and for sustaining food and fibre 19production. An overlooked aspect of soils is their potential to mitigate greenhouse gas (GHG)

| In the Anthropocene, in which we now live, climate change is impacting most life on Earth. Microorganisms support the existence of all higher trophic life forms. To understand how humans and other life forms on Earth (including those we are yet to discover) can withstand anthropogenic climate change, it is vital to incorporate knowledge of the microbial 'unseen majority'. We must learn not just how microorganisms affect climate change (including production and consumption of greenhouse gases) but also how they will be affected by climate change and other human activities. This Consensus Statement documents the central role and global importance of microorganisms in climate change biology. It also puts humanity on notice that the impact of climate change will depend heavily on responses of microorganisms, which are essential for achieving an environmentally sustainable future.

Microbial processes have a central role in the global fluxes of the key biogenic greenhouse gases (carbon dioxide, methane and nitrous oxide) and are likely to respond rapidly to climate change. Whether changes in microbial processes lead to a net positive or negative feedback for greenhouse gas emissions is unclear. To improve the prediction of climate models, it is important to understand the mechanisms by which microorganisms regulate terrestrial greenhouse gas flux. This involves consideration of the complex interactions that occur between microorganisms and other biotic and abiotic factors. The potential to mitigate climate change by reducing greenhouse gas emissions through managing terrestrial microbial processes is a tantalizing prospect for the future.

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Measuring Methanogenesis After carbon dioxide, methane is the second most important greenhouse gas, and an important species in terms of its role in atmospheric chemistry. The sources and sinks of methane, particularly the natural ones, are too poorly quantified, however, even to explain why the decades-long, steady increase of its concentration in the atmosphere was interrupted between 1999 and 2006. Bloom et al. (p. 322 ) use a combination of satellite data, which indicate water table depth and surface temperature, and atmospheric methane concentrations to determine the location and strength of methane emissions from wetlands, the largest natural global source. The constraints placed on these sources should help to improve predictions of how climate change will affect wet-land emissions of methane.

Summary• Recent studies demonstrating an in situ formation of methane (CH 4 ) within foliage and separate observations that soil-derived CH 4 can be released from the stems of trees have continued the debate about the role of vegetation in CH 4 emissions to the atmosphere. Here, a study of the role of ultraviolet (UV) radiation in the formation of CH 4 and other trace gases from plant pectins in vitro and from leaves of tobacco (Nicotiana tabacum) in planta is reported.• Plant pectins were investigated for CH 4 production under UV irradiation before and after de-methylesterification and with and without the singlet oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO). Leaves of tobacco were also investigated under UV irradiation and following leaf infiltration with the singlet oxygen generator rose bengal or the bacterial pathogen Pseudomonas syringae.• Results demonstrated production of CH 4 , ethane and ethylene from pectins and from tobacco leaves following all treatments, that methyl-ester groups of pectin are a source of CH 4 , and that reactive oxygen species (ROS) arising from environmental stresses have a potential role in mechanisms of CH 4 formation.• Rates of CH 4 production were lower than those previously reported for intact plants in sunlight but the results clearly show that foliage can emit CH 4 under aerobic conditions.

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