Seasonal variations in soil fungal communities and co-occurrence networks along an altitudinal gradient in the cold temperate zone of China: A case study on Oakley Mountain

Cold temperate forests carry out functions that are essential to global biogeochemical cycling and climate regulation. The biogeography of soil fungi has attracted much attention in recent years; however, the seasonal patterns of soil fungal community structure and diversity along altitudinal gradients under the unique climatic conditions at high latitudes remain unclear, which limits our insight into soil microbial interactions and the mechanisms of community assembly. In this study, Illumina MiSeq sequencing was used to investigate the spatiotemporal changes in soil fungal communities along an altitudinal gradient (from 750 m to 1420 m) on Oakley Mountain in the northern Greater Khingan Mountains. Altitude had significant impacts on the relative abundances of the dominant phyla and classes of soil fungi, and the interaction of altitude and season significantly affected the relative abundances of Ascomycota and Basidiomycota. The number of soil fungal taxa and Faith's phylogenetic diversity index tended to monotonically decline with increasing elevation. Soil moisture, soil temperature and pH were the main factors affecting fungal community structure in May, July and September, respectively. The soil dissolved organic carbon content significantly shaped the soil fungal community composition along the altitudinal gradient throughout the growing season. Compared to that in May and July, the soil fungal network in September had more nodes and links, a higher average degree and a higher average clustering coefficient. The nine module nodes in the co-occurrence network were all Ascomycota taxa, and the identities of the keystone taxa of soil fungi in the network showed obvious seasonality. Our results demonstrated that altitude has stronger effects than season on soil fungal community structure and diversity at high latitudes. In addition, the co-occurrence network of soil fungi exhibited obvious seasonal succession, which indicated that the keystone taxa of soil fungi exhibit niche differentiation among seasons.