Elevated O3 Exerts Stronger Effects than Elevated CO2 on the Functional Guilds of Fungi, but Collectively Increase the Structural Complexity of Fungi in a Paddy Soil.

Global climate change is characterized by altered global atmospheric composition, including elevated CO2 and O3, with important consequences on soil fungal communities. However, the function and community composition of soil fungi in response to elevated CO2 together with elevated O3 in paddy soils remain largely unknown. Here we used twelve open-top chamber facilities (OTCs) to evaluate the interactive effect of CO2 (+ 200 ppm) and O3 (+ 40 ppb) on the diversity, gene abundance, community structure, and functional composition of soil fungi during the growing seasons of two rice cultivars (Japonica, Wuyujing 3 vs. Nangeng 5055) in a Chinese paddy soil. Elevated CO2 and O3 showed no individual or combined effect on the gene abundance or relative abundance of soil fungi, but increased structural complexity of soil fungal communities, indicating that elevated CO2 and/or O3 promoted the competition of species-species interactions. When averaged both cultivars, elevated CO2 showed no individual effect on the diversity or abundance of functional guilds of soil fungi. By contrast, elevated O3 significantly reduced the relative abundance and diversity of symbiotrophic fungi by an average of 47.2% and 39.1%, respectively. Notably, elevated O3 exerts stronger effects on the functional processes of fungal communities than elevated CO2. The structural equation model revealed that elevated CO2 and/or O3 indirectly affected the functional composition of soil fungi through community structure and diversity of soil fungi. Root C/N and soil environmental parameters were identified as the top direct predictors for the community structure of soil fungi. Furthermore, significant correlations were identified between saprotrophic fungi and root biomass, symbiotrophic fungi and root carbon, the pathotroph-symbiotroph and soil pH, as well as pathotroph-saprotroph-symbiotroph and soil microbial biomass carbon. These results suggest that climatic factors substantially affected the functional processes of soil fungal, and threatened soil function and food production, highlighting the detrimental impacts of high O3 on the function composition of soil biota.