Short-term elevated O.sub.3 exerts stronger effects on soil nitrification than does CO.sub.2, but jointly promotes soil denitrification

Aims Atmospheric CO.sub.2 and O.sub.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.sub.2 and O.sub.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.sub.2 and O.sub.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.sub.2 potentially increased the abundance of nitrification-related genes (AOA amoA + 40.9%, AOB amoA + 23.4%, nxrB + 8.6%). Elevated O.sub.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.sub.3 exerted stronger effects on soil nitrification than CO.sub.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.sub.2 and combined treatments. Conclusions Elevated CO.sub.2 and O.sub.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.sub.2 and O.sub.3 on soil N retention capacity in the future.