Effects of Fractionation Methods on Soil Aggregate Microbial Community Composition: Settling vs. Wet Sieving

Soil microbial communities play a vital role in biogeochemical cycling and ecosystem functions at the aggregate scale. Soil fractionation methods can affect the interpretation of microbial spatial distribution and ecological processes in aggregates. However, there is a lack of comparative studies illustrating the differences between wet sieving and settling fractionation on soil aggregate microbial communities. In this study, we compared bacterial and fungal community composition and diversity within three soil aggregates sizes separated by wet sieving and settling tube apparatus. Settling fractionation increased the relative abundance of Verrucomicrobia and Planctomycetes by 80.1%-93.6% and 54.5%-100%, respectively, but reduced that of Actinobacteria and Firmicutes by 16.7%-36.9% and 41.4%-95.7% respectively, in the three aggregate size fractions in comparison with wet sieving. However, the relative abundance of major genus (relative abundance > 0.2%) rather than phylum of fungal communities were significantly different for the two fractionation methods (P < 0.05). Compared with wet sieving, settling fractionation resulted in the detection of significantly less bacterial diversity (decreased by 14.2-18.7% for OTU richness and 6.8-6.9% for Shannon index) but more fungal diversity (increased by 12.3-51.3% for OTU richness and 13.8-35.1% for Shannon index) in finest soil aggregates. Distinct aggregate-disrupting energy from settling fractionation and wet sieving resulted in different size distributions and soil physicochemical and biological properties, leading to the differentiation of microhabitats and microbial communities. Settling fractionation and wet sieving both revealed significant differences in microbial communities among aggregate size fractions (P < 0.05). Here, our findings highlight that wet sieving should be favored in numerous studies, while settling fractionation may prove particularly suitable for research within the context of erosion.