Taxon-specific fungal preference for distinct soil particle size fractions

Soil particlesize fractions (PSFs), i.e. clay, fine silt, coarse silt, and sand with particulate organic matter (POM), differ in mineralogical and organic composition and thereby in their sorption and desorption of biotic and abiotic compounds. Utilising a fractionation protocol, including gentle ultra-sonication, wet sieving and centrifugation, previous studies with soils from a long-term fertilisation experiment demonstrated that many soil bacterial taxa are preferentially associated with specific soil PSFs. The present study applied a similar experimental set-up to characterise fungal preferences for PSFs. Fungal abundance and diversity was analysed by qPCR and high-throughput sequencing of the ribosomal internal transcribed spacer region of fungal genomes from soil DNA. Fungal community composition of non-fractionated soil and of pooled PSFs differed by less than 6.4%, indicating that the fractionation protocol allowed for recovery of most fungi. The diversity of fungal taxa differed significantly between the PSFs. The sand/POM fraction contained several species of Dothideomycetes and Cadophora. Members of Sordariomycetes had their highest abundance in fine silt while Penicillium and Mucoraceae were most abundant in the clay fraction. In contrast, clay was depleted in sequences of Sordariales, and Mortierella was less abundant in the sand/POM fraction. Despite the potential cross-contamination of PSFs and simultaneous interactions of fungal hyphae with different PSFs, our data indicate that most of the dominant soil fungal taxa show a distinct preference for specific PSFs. Considering that PSF-specific preferences were also detected for soil bacteria, interactions between microorganisms and soil particle surfaces should not be neglected to explain how microbial diversity is spatially structured in soils.