Elevated CO2 and/or O3 shift the functional processes and structural complexity of soil protists in a paddy soil

The functional composition of soil protists represents the integrated response of soil functioning and yet is poorly understood compared to other soil microbes. Given the importance of soil protists and the rise of global climate changes, we conducted open-top chambers facilities (OTC) with elevated CO2 (eCO2, + 200 ppm), O3 (eO3, +40 ppb) and combined treatment (eCO2 + eO3) to investigate the responses of protist functional groups during the growing seasons of two rice varieties. When averaged both varieties, eCO2 showed no individual effect on the functional composition or diversity of soil protists. Whereas, eO3 greatly increased the relative abundance and diversity of parasites by 50.7 % and 38.7 %, respectively. Instead, eO3 reduced the relative abundance of consumers by 49.7 %, indicating eO3 rather than eCO2 potentially weakened the top-down effects of consumer protists. Meanwhile, the decrease of 16.9 % in the total links of the protist network under the combined effect of eCO2 and eO3, is lower than the decreases of 23.2 % and 20.5 % under individual effects. However, the tradeoff could not fully offset the negative impact of climate change on the functional stability of soil protists, though combined treatment increased the observed ASVs and Faith's phylogenetic distance, indicating that the combined effect of eCO2 and eO3 on soil protist community were more complexed than under individual effect. This is the first study that evaluated the individual versus combined effects of eCO2 and eO3 on soil protist communities and thereby strongly contributes to our understanding of soil function and food production in agroecosystems in the future. •Elevated CO2 showed no individual effect on the functional composition or diversity of protists.•Elevated O3 increased the relative abundance and diversity of parasites, but reduced consumers.•Both elevated CO2 and O3 reduced the structural complexity of soil protists.•Elevated O3 plays a more important role in soil protists than elevated CO2.