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

ABSTRACT

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.