Elevated atmospheric CO₂ reduces yield-scaled N₂ O fluxes from subtropical rice systems: Six site-years field...

Increasing levels of atmospheric CO₂ are expected to enhance crop yields and alter soil greenhouse gas fluxes from rice paddies. While elevated CO₂ ( $$ ) effects on CH₄ emissions from rice paddies have been studied in some detail, little is known how $$ 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 $$ . In this study, we tested various N fertilization and residue addition treatments, with rice being grown under either $$ (+200 μmol/mol) or ambient control. Across the six site-years, rice straw and grain yields under $$ were increased by 9%-40% for treatments fertilized with ≥150 kg N/ha, while seasonal N₂ O emissions were decreased by 23%-73%. Consequently, yield-scaled N₂ O emissions were significantly lower under $$ . For treatments receiving insufficient fertilization (≤125 kg N/ha), however, no significant $$ effects on N₂ O emissions were observed. The mitigating effect of $$ 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 $$ . This shows that for an agronomic and environmental assessment of $$ 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.