Abstract: Agricultural production is influenced by atmospheric CO₂ concentration. Fungi are the key promoters of organic matter degradation and nutrient cycling in soil, but their responses to the increase of CO₂ concentration are not clear. Miseq high-throughput sequencing technology was used to investigate the fungal community structure under four CO₂ concentrations (ambient, 2%, 4%, 6%) in microcosm experiments on typical fluvo-aquic soil in North China Plain. The results showed that fungal α diversity decreased significantly at 4% and 6% CO₂ concentration. Ascomycota, Basidiomycota, Mortierellomycota dominated the fungal community. The relative abundance of Basidiomycota and Glomeromycota decreased with the increases of CO₂ concentration, while that of Ascomycota increased. The dominant genera included Mortierella, Fusarium, Citripora, Stachybotrys, and Humicola. The relative abundance of Citripora decreased with the increases of CO₂ concentration, but that of Mortierella increased twofold in the condition of 2% CO₂ concentration. CO₂ concentration positively correlated with the relative abundance of Humicola phialophoroides and Sarocladium hominis, but negatively correlated with the relative abundance of Citripora afrocitrina. The fungal network topology in 2% CO₂ treatment was similar to the control, but with more edges. The network topology changed obviously in 4% and 6% CO₂ treatments, differentiating into two network modules. The keystone taxa were Aspergillus terreus, Trichoderma yunnanense, and Spizellomyces sp. This study clarified the response of fungi to different CO₂ concentrations in fluvo-aquic soil, and provided theoretical reference for studying the impacts of elevated CO₂ on ecosystem functions of farmland.

Key words: CO₂ concentration, soil fungi, community composition, network structure