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

Abstract Increasing levels of atmospheric CO 2 are expected to enhance crop yields and alter soil greenhouse gas fluxes from rice paddies. While elevated CO 2 ( ) effects on CH 4 emissions from rice paddies have been studied in some detail, little is known how might affect N 2 O fluxes or yield‐scaled emissions. Here, we report on a multi‐site, multi‐year in‐situ FACE (free‐air CO 2 enrichment) study, aiming to determine N 2 O fluxes and crop yields from Chinese subtropical rice systems as affected by . In this study, we tested various N fertilization and residue addition treatments, with rice being grown under either (+200 μmol/mol) or ambient control. Across the six site‐years, rice straw and grain yields under were increased by 9%–40% for treatments fertilized with ≥150 kg N/ha, while seasonal N 2 O emissions were decreased by 23%–73%. Consequently, yield‐scaled N 2 O emissions were significantly lower under . For treatments receiving insufficient fertilization (≤125 kg N/ha), however, no significant effects on N 2 O emissions were observed. The mitigating effect of upon N 2 O emissions is closely associated with plant N uptake and a reduction of soil N availability. Nevertheless, increases in yield‐scaled N 2 O emissions with increasing N surplus suggests that N surplus is a useful indicator for assessing N 2 O emissions from rice paddies. Our findings indicate that with rising atmospheric CO 2 soil N 2 O emissions from rice paddies will decrease, given that the farmers’ N fertilization is usually sufficient for crop growth. The expected decrease in N 2 O emissions was calculated to compensate 24% of the simultaneously observed increase in CH 4 emissions under . This shows that for an agronomic and environmental assessment of effects on rice systems, not only CH 4 emissions, but also N 2 O fluxes and yield‐scaled emissions need to be considered for identifying most climate‐friendly and economically viable options for future rice production.