Short-term elevated O3 exerts stronger effects on soil nitrification than does CO2, but jointly promotes soil denitrification

Aims Atmospheric CO 2 and O 3 concentrations have been increasing, with important consequences on the biogeochemical cycle in agroecosystems. However, little is known about the interactive effect of elevated CO 2 and O 3 on soil nitrogen cycling processes mediated by soil microbes. Methods We conducted open-top chamber facilities to assess the impacts of short-term elevated CO 2 and O 3 on soil nitrification and denitrification rates, and the abundances of soil N cycling-related genes for two rice cultivars (Nanjing 5055 vs. Wuyujing 3) in paddy soil. Results Elevated CO 2 potentially increased the abundance of nitrification-related genes (AOA amoA + 40.9%, AOB amoA + 23.4%, nxrB + 8.6%). Elevated O 3 potentially reduced the abundances of AOA and AOB amoA , nxrA , and nxrB by 3.1–23.8%. Combined treatment showed detrimental effects on the abundances of AOA and AOB amoA , and nxrA by 17.6–36.0%, indicating that short-term elevated O 3 exerted stronger effects on soil nitrification than CO 2 . Similarly, both individual and combined treatments decreased the abundance of comammox amoA . Additionally, the individual and combined treatments stimulated the abundance of denitrification-related genes by 4.2–11.9%, except narG . Accordingly, the denitrification rates were significantly increased by 77.2–89.1% under all treatments, particularly for Nanjing 5055. Furthermore, the abundance of nifH mediating N fixation was reduced by elevated CO 2 and combined treatments. Conclusions Elevated CO 2 and O 3 may promote soil N losses by increasing the abundances of denitrification-related genes, restraining N fixation-related genes, and potentially threatening food production, highlighting the detrimental impacts of ongoing elevated CO 2 and O 3 on soil N retention capacity in the future.