520 www.frontiersinecology.org © The Ecological Society of America G lobal warming is caused by increased atmospheric concentrations of the greenhouse gases (GHGs) car- bon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ). Terrestrial ecosystems are important sources and sinks of these GHGs, all of which are produced and con- sumed through biological processes including photosyn- thesis, decomposition, nitrification, denitrification, methanogenesis, and CH 4 oxidation (eg Schlesinger 1997). Increased atmospheric CO 2 concentration and ele- vated air/soil temperatures (hereafter elevated CO 2 and temperature; please note also that, unless stated otherwise, the following text refers to terrestrial ecosystems) can directly and indirectly alter these processes. Depending on the direction and magnitude of the alteration, elevated CO 2 and temperature could either accelerate or decelerate the rate of global warming. The effects of elevated CO 2 and temperature on N 2 O and CH 4 fluxes in terrestrial ecosystems have been studied less frequently than the effects on CO 2 exchange. This is not surprising given that CO 2 exchange rates are usually orders of magnitude greater than the exchange rates of N 2 O and CH 4 (Schlesinger 1997). However, N 2 O and CH 4 have higher global warming potentials (GWPs) than that of CO 2 . Thus, although CO 2 is – per molecule – the most important GHG, N 2 O and CH 4 are more efficient in warming the atmosphere (the GWPs of N 2 O and CH 4 are 298 and 25 times that of CO 2 , respectively, over a 100-year period; Forster et al. 2007). Global warming is therefore more sensitive to changes in the exchange of N 2 O and CH 4 relative to that of CO 2 . Process-based ecosystem models applied at regional and continental scales have recently estimated that net N 2 O and CH 4 emissions increased during the past 40 years and could further increase in the future because of elevated CO 2 and temperatures (Xu et al. 2010, 2012; Tian et al. 2012). Although important for long-term and large-scale predic- tions of climate-change feedbacks, modeling efforts still leave a lot of uncertainty, mostly due to our limited under- standing of the underlying mechanisms governing N 2 O and CH 4 fluxes in different ecosystems (Tian et al. 2012). SPECIAL ISSUE Effects of elevated carbon dioxide and increased temperature on methane and nitrous oxide fluxes: evidence from field experiments Feike A Dijkstra 1* , Stephen A Prior 2 , G Brett Runion 2 , H Allen Torbert 2 , Hanqin Tian 3 , Chaoqun Lu 3 , and Rodney T Venterea 4 Climate change could alter terrestrial ecosystems, which are important sources and sinks of the potent green- house gases (GHGs) nitrous oxide (N 2 O) and methane (CH 4 ), in ways that either stimulate or decrease the mag- nitude and duration of global warming. Using manipulative field experiments, we assessed how N 2 O and CH 4 soil fluxes responded to a rise in atmospheric carbon dioxide (CO 2 ) concentration and to increased air tempera- ture. Nitrous oxide and CH 4 responses varied greatly among studied ecosystems. Elevated CO 2 often stimulated N 2 O emissions in fertilized systems and CH 4 emissions in wetlands, peatlands, and rice paddy fields; both effects were stronger in clayey soils than in sandy upland soils. Elevated temperature, however, impacted N 2 O and CH 4 emissions inconsistently. Thus, the effects of elevated CO 2 concentrations on N 2 O and CH 4 emissions may fur- ther enhance global warming, but it remains unclear whether increased temperature generates positive or neg- ative feedbacks on these GHGs in terrestrial ecosystems. Front Ecol Environ 2012; 10(10): 520–527, doi:10.1890/120059 In a nutshell: • Net emissions of nitrous oxide (N 2 O) and methane (CH 4 ) from terrestrial ecosystems could increase or decrease in response to climate change, thereby either accelerating or decelerating global warming • Field experiments examining the effects of climate change on N 2 O and CH 4 emissions provide important information that may help improve long-term predictions with process-based models • A rise in atmospheric carbon dioxide concentration often increased N 2 O emissions in fertilized systems and CH 4 emis- sions in wetlands, peatlands, and rice paddy fields; such increases may enhance global warming • Conversely, responses of N 2 O and CH 4 emissions to elevated temperatures have been inconsistent in many ecosystems 1 Department of Environmental Sciences, Faculty of Agriculture and Environment, University of Sydney, Sydney NSW, Australia * (feike.dijkstra@sydney.edu.au); 2 USDA–ARS National Soil Dynamics Laboratory, Auburn, AL; 3 School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL; 4 USDA–ARS Soil and Water Management Research Unit, St Paul, MN