Acclimation of CH₄ emissions from paddy soil to atmospheric CO₂ enrichment in a growth chamber experiment

Elevated levels of atmospheric CO₂ (eCO₂) promote rice growth and increase methane (CH₄) emissions from rice paddies, because increased input of plant photosynthate to soil stimulates methanogenic archae. However, temporal trends in the effects of eCO₂ on rice growth and CH₄ emissions are still unclear. To investigate changes in the effects of eCO₂ over time, we conducted a two-season pot experiment in a walk-in growth chamber. Positive effects of eCO₂ on rice leaf photosynthetic rate, biomass, and grain yield were similar between growing seasons. However, the effects of eCO₂ on CH₄ emissions decreased over time. Elevated CO₂ increased CH₄ emissions by 48%–101% in the first growing season, but only by 28%–30% in the second growing season. We also identified the microbial process underlying the acclimation of CH₄ emissions to atmospheric CO₂ enrichment: eCO₂ stimulated the abundance of methanotrophs more strongly in soils that had been previously exposed to eCO₂ than in soils that had not been. These results emphasize the need for long-term eCO₂ experiments for accurate predictions of terrestrial feedbacks.