Soil depth-based community assembly processes in Ngari drylands, Tibet

Numerous studies have showed patterns in the surface soil fungal diversity and biogeography over the broad spatial scales around the world. However, less is known about variation in subsurface (15–30 cm of depth) fungal communities, especially under the extreme soil conditions of cold, infertile and water-deficient. Here we studied depth profiles of fungal assemblages in Ngari drylands, Tibet. The relative abundance of Basidiomycota and Zygomycota was higher in surface soil layers, while that of Ascomycota was higher in subsurface soil samples. The alpha diversity (including observed OTUs and predicted chao1) in surface soil layers was significantly higher than that in subsurface soil layers (P<0.001). Fungal communities significantly differed between the surface and subsurface soil layers, strongly correlating with the variation in soil C:N ratio and precipitation seasonality. Furthermore, fungal community dissimilarity between the surface and subsurface soil at each single site was significantly larger than that between different surface soil samples that are separated hundreds of kilometers apart. Both dispersal limitation and environmental selection had a weaker effect on communities in surface soil layers as reflected by negative deviations from the null-model expectations, which was also corroborated by (partial) Mantel test and multivariate analysis. 133 fungal cosmopolitans were observed in all the surface soil samples with accounting for 60% of total sequences, while only 4 OTUs shared among all the subsurface soil samples, which indicated high dispersal of fungal communities in surface soil layers on this extreme environments. Taken together, these results confirm the random distribution of surface soil fungal communities due to high dispersal and the absence of edaphic effects. Oppositely, the deterministic process with respect to soil, climate and space plays the overarching role in subsurface soil layers.