Effects of elevated CO2 concentration on CH4 and N2O emissions from paddy fields: A meta-analysis

The inevitable rise of atmospheric CO2 concentration plays an important role in regulating the carbon (C) and nitrogen (N) cycling in the rice-cropping system. Elucidating the effects of elevated CO2 concentration (ECO2) on CH4 and N2O emissions from paddy fields is essential for evaluating agricultural production in response to global climate change. In this study, we conducted a global meta-analysis to assess the overall effect of ECO2 on CH4 and N2O emissions from paddy fields, aiming at providing a guideline for sustainable C and N management in paddy fields under future climate conditions. The results showed that, overall, ECO2 significantly increased CH4 emissions from rice fields by 23% (P<0.05), but reduced N2O emissions by 22% (P<0.05). With a long duration (>10 yr) of ECO2, ECO2 significantly reduced CH4 and N2O emissions from paddy fields by 27% and 53%, respectively (P<0.05). Along with the increasing levels of ECO2, the stimulating effect of ECO2 on CH4 emissions showed a trend of "weakening firstly and then strengthening", while its effect on N2O emissions changed from stimulation to inhibition. Agronomy managements (e.g., N application rates, straw incorporations, water regimes, and rice cultivars) affected the effects of ECO2 on CH4 and N2O emissions from paddy fields. With no or half amount of straw incorporation, ECO2 increased CH4 emissions by 27% or 49% (P<0.05) from paddy fields, respectively, while non-significant effects on CH4 emissions from paddy fields were observed under full straw incorporation. With the increasing amount of straw incorporation, the reductions in N2O emissions from paddy fields were enhanced by ECO2. Compared with a continuous flooding regime, intermittent irrigation weakened the promoted effect on CH4 emissions but stimulated the inhibited effect on N2O emissions from paddy fields under ECO2. Therefore, under the future condition of ECO2, it is recommended to adopt the appropriate agricultural management measures, such as combining straw incorporation and intermittent irrigation, and optimizing N application and using rice cultivars of high-yield with lower emissions. In addition, it is necessary to conduct comprehensive studies at multi-scale, with multi-factor, and by multi-method to effectively reduce the uncertainty of quantifying the response of CH4 and N2O emissions from paddy fields to future ECO2.