What was done
The authors studied methane emissions from plots of rice grown in large vats filled with topsoil from surrounding fields and placed within greenhouse tunnels under ambient (350 ppm) and doubled (700 ppm) atmospheric CO2 concentrations.  Each of these greenhouse tunnels was further subdivided into four cells providing a range of temperatures from ambient - which prevailed in an "open cell" located outside of the tunnels - to as much as 5°C above ambient at the ends of the tunnels.

What was learned
As would be expected, atmospheric CO2 enrichment significantly enhanced rice biomass production both above (by up to 35%) and below (by up to 83%) ground.  However, in a truly unanticipated development, methane emissions from rice grown at 700 ppm CO2 were observed to be 10 to 45 times less than emissions from plants grown at 350 ppm CO2.  In the words of the authors, "the results of this study did not support our hypothesis that an effect of both increased carbon dioxide and temperature would be an increase in methane emissions."  Indeed, "both increased carbon dioxide and increased temperatures were observed to produce decreased methane emissions," except for the first 2°C increase above ambient, which did produce a slight increase in them.

What it means
Nearly everyone, the authors included, had long thought that atmospheric CO2 enrichment would lead to increased methane emissions from rice plots, the thought being that methane production would be stimulated by the CO2-induced increase in root biomass and soil microbial activity; and, in fact, a few prior studies had obtained results that seemed to confirm this hypothesis.

In checking for potential problems with their study, the authors could find none.  They thus stated that "the results presented here unequivocally support the conclusion that, during this study, methane emissions from Oryza sativa plants grown under conditions of elevated CO2 were dramatically reduced relative to plants gown in comparable conditions under ambient levels of CO2."  In fact, they even went on to replicate their experiment in a second year of sampling, obtaining essentially the same results.

So what's the story behind their amazing results?  Their best explanation for what happened in their experiment is that increased delivery of oxygen to the root zone or rhizosphere reduced methane production in the soil of the CO2-enriched tunnel.  As they describe it, "we hypothesize that CO2 enrichment led to the attenuation of methane production due to increased delivery of oxygen to the rhizosphere because of increased root biomass and porosity," whereby "the increased root biomass due to elevated CO2 may have more effectively aerated the soil, suppressing methane production," which requires strict anaerobic conditions.

The results of this study are so startling that they will need to be confirmed in other studies before they can be fully and confidently accepted.  Nevertheless, they may hold the explanation for another unanticipated phenomenon that has been observed in recent years: the dramatic slowdown in the rate of increase in atmospheric methane concentration.  Could it be that the ongoing rise in the air's CO2 concentration has been the stimulus for this phenomenon via the mechanism that is proposed in this paper?  Researchers should be rushing to fill this knowledge void as fast as they can run.