Effects of abrupt and gradual increase of atmospheric CO₂ concentration on methanotrophs in paddy fields

The simulation of abrupt atmospheric CO₂ increase is a common way to examine the response of soil methanotrophs to future climate change. However, atmosphere is undergoing a gradual CO₂ increase, and it is unknown whether the previously reported response of methanotrophs to abrupt CO₂ increase can well represent their response to the gradual increase. To improve the understanding of the effect of elevated CO₂ (eCO₂) on methanotrophs in paddy ecosystems, the methane oxidation potential and communities of methanotrophs were examined via open top chambers under the three following CO₂ treatments: an ambient CO₂ concentration (AC); an abrupt CO₂ increase by 200 ppm above AC (AI); a gradual CO₂ increase by 40 ppm each year until 200 ppm above AC (GI). Relative to AC treatment, AI and GI treatments significantly (p < 0.05) increased the methane oxidation rate by 43.8% and 36.7%, respectively, during rice growth period. Furthermore, the abundance of pmoA genes was significantly (p < 0.05) increased by 62.4% and 32.5%, respectively, under AI and GI treatments. However, there were no significant variations in oxidation rate or gene abundance between the two eCO₂ treatments. In addition, no obvious change of overall community composition of methanotrophs was observed among treatments, while the proportions of Methylosarcina and Methylocystis significantly (p < 0.05) changed. Taken together, our results indicate similar response of methanotrophs to abrupt and gradual CO₂ increase, although the magnitude of response under gradual increase was smaller and the abrupt increase may somewhat overestimate the response.