Elevated CO₂ does not necessarily enhance greenhouse gas emissions from rice paddies

Elevated atmospheric carbon dioxide (eCO₂) greatly impacts greenhouse gas (GHG) emissions of CH₄ and N₂O from rice fields. Although eCO₂ generally stimulates GHG emissions in the short term (<5 years) experiments, the responses to long-term (≥10 years) eCO₂ remain poorly known. Here we show, through a series of experiments and meta-analysis, that the eCO₂ does not necessarily increase CH₄ and N₂O emissions from rice paddies. In an experiment of free-air CO₂ enrichment for 13-15 years, CH₄ and N₂O emissions were decreased by 11-54% and 33-54%, respectively. The decline of CH₄ emissions was related to the reduction of CH₄ production and enhancement of CH₄ oxidation via raising soil Eh and soil-water interface [O₂] under eCO₂. Moreover, the eCO₂ significantly decreased NH₄⁺-N content, suggesting a reduction of soil nitrification and thereby N₂O emissions. A meta-analysis showed that CH₄ and N₂O emissions were stimulated under short-term eCO₂ while reduced under long-term eCO₂. The eCO₂-induced increase in yield and biomass and the ratio of mcrA genes/pmoA genes declined with eCO₂ duration, indicating an eCO₂-stimulation of methanogenesis lower than that of methanotrophy over time by fewer increased substrates. Upscaling the results of meta-analysis, the eCO₂-induced global paddy CH₄ and N₂O emissions shifted from an increase (+0.17 Pg CO₂-eq year^-1) in the short term into a decrease (-0.11 Pg CO₂-eq year^-1) in the long term. Our findings suggest that the effect of eCO₂ on GHG emissions changes over time, and this should be considered in future climate change research.