Elevated atmospheric CO2 reduces yield-scaled N2 O 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 ( E CO 2 ) effects on CH4 emissions from rice paddies have been studied in some detail, little is known how E CO 2 might affect N2 O 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 N2 O fluxes and crop yields from Chinese subtropical rice systems as affected by E CO 2 . In this study, we tested various N fertilization and residue addition treatments, with rice being grown under either E CO 2 (+200 μmol/mol) or ambient control. Across the six site-years, rice straw and grain yields under E CO 2 were increased by 9%-40% for treatments fertilized with ≥150 kg N/ha, while seasonal N2 O emissions were decreased by 23%-73%. Consequently, yield-scaled N2 O emissions were significantly lower under E CO 2 . For treatments receiving insufficient fertilization (≤125 kg N/ha), however, no significant E CO 2 effects on N2 O emissions were observed. The mitigating effect of E CO 2 upon N2 O emissions is closely associated with plant N uptake and a reduction of soil N availability. Nevertheless, increases in yield-scaled N2 O emissions with increasing N surplus suggests that N surplus is a useful indicator for assessing N2 O emissions from rice paddies. Our findings indicate that with rising atmospheric CO2 soil N2 O emissions from rice paddies will decrease, given that the farmers' N fertilization is usually sufficient for crop growth. The expected decrease in N2 O emissions was calculated to compensate 24% of the simultaneously observed increase in CH4 emissions under E CO 2 . This shows that for an agronomic and environmental assessment of E CO 2 effects on rice systems, not only CH4 emissions, but also N2 O fluxes and yield-scaled emissions need to be considered for identifying most climate-friendly and economically viable options for future rice production.