Elevated atmospheric CO2 reduces yield-scaled N2O fluxes from subtropical rice systems: Six site-years field experiments

Increasing levels of atmospheric CO2 are expected to enhance crop yields and alter soil greenhouse gas fluxes from rice paddies. While elevated CO2 ( ECO2) effects on CH4 emissions from rice paddies have been studied in some detail, little is known how ECO2 might affect N2O fluxes or yield-scaled emissions. Here, we report on a multi-site, multi-year in-situ FACE (free-air CO2 enrichment) study, aiming to determine N2O fluxes and crop yields from Chinese subtropical rice systems as affected by ECO2. In this study, we tested various N fertilization and residue addition treatments, with rice being grown under either ECO2 (+200 mu mol/mol) or ambient control. Across the six site-years, rice straw and grain yields under ECO2 were increased by 9%-40% for treatments fertilized with >= 150 kg N/ha, while seasonal N2O emissions were decreased by 23%-73%. Consequently, yield-scaled N2O emissions were significantly lower under ECO2. For treatments receiving insufficient fertilization (<= 125 kg N/ha), however, no significant ECO2 effects on N2O emissions were observed. The mitigating effect of ECO2 upon N2O emissions is closely associated with plant N uptake and a reduction of soil N availability. Nevertheless, increases in yield-scaled N2O emissions with increasing N surplus suggest that N surplus is a useful indicator for assessing N2O emissions from rice paddies. Our findings indicate that with rising atmospheric CO2 soil N2O emissions from rice paddies will decrease, given that the farmers' N fertilization is usually sufficient for crop growth. The expected decrease in N2O emissions was calculated to compensate 24% of the simultaneously observed increase in CH4 emissions under ECO2. This shows that for an agronomic and environmental assessment of ECO2 effects on rice systems, not only CH4 emissions, but also N2O fluxes and yield-scaled emissions need to be considered for identifying most climate-friendly and economically viable options for future rice production