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

Elevated atmospheric carbon dioxide (eCO(2)) greatly impacts greenhouse gas (GHG) emissions of CH4 and N2O from rice fields. Although eCO(2) generally stimulates GHG emissions in the short term (<5 years) experiments, the responses to long-term ( >= 10 years) eCO(2) remain poorly known. Here we show, through a series of experiments and metaanalysis, that the eCO(2) 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 [O-2] under eCO(2). Moreover, the eCO(2) 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 eCO(2) while reduced under long-term eCO(2). The eCO(2)-induced increase in yield and biomass and the ratio of mcrA genes/pmoA genes declined with eCO(2) duration, indicating an eCO(2) stimulation of methanogenesis lower than that of methanotrophy over time by fewer increased substrates. Upscaling the results of meta-analysis, the eCO(2)-induced global paddy CH4 and N2O emissions shifted from an increase ( +0.17 Pg CO2-eq year 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 eCO(2) on GHG emissions changes over time, and this should be considered in future climate change research.

Automated summary

Experimental results and meta-analysis suggest that long-term elevated atmospheric carbon dioxide (eCO2) may decrease methane (CH4) and nitrous oxide (N2O) emissions from rice paddies, contrasting with the short-term stimulatory effect of eCO2. The impact of eCO2 on greenhouse gas emissions changes over time and should be considered in future climate change research.