Micro-aggregation of a pristine grassland soil selects for bacterial and fungal communities and changes in nitrogen cycling potentials

Microbial analysis at the micro scale of soil is essential to the overall understanding of microbial organization and interactions, and necessary for a better understanding of soil ecosystem functioning. While bacterial communities have been extensively described, little is known about the organization of fungal communities as well as functional potentials at scales relevant to microbial interactions. Fungal and bacterial communities and changes in nitrogen cycling potentials in the pristine Rothamsted Park Grass soil (bulk soil) as well as in its particle size sub-fractions (PSFs; > 250 m, 250-63 m, 63-20 m, 20-2 m, < 2 m and supernatant) were studied. The potential for nitrogen reduction was found elevated in bigger aggregates. The relative abundance of Basidiomycota deceased with decreasing particle size, Ascomycota showed an increase and Mucoromycota became more prominent in particles less than 20 m. Bacterial community structures changed below 20 m at the scale where microbes operate. Strikingly, only members of two bacterial and one fungal phyla (Proteobacteria, Bacteroidota and Ascomycota, respectively) were washed-off the soil during fractionation and accumulated in the supernatant fraction where most of the detected bacterial genera (e.g., Pseudomonas, Massilia, Mucilaginibacter, Edaphobaculum, Duganella, Janthinobacterium and Variovorax) were previously associated with exopolysaccharide production and biofilm formation. Overall, the applied method shows potential to study soil microbial communities at micro scales which might be useful in studies focusing on the role of specific fungal taxa in soil structure formation as well as research on how and by whom biofilm-like structures are distributed and organized in soil. ImportanceIntensive exploitation of soils has led to increasing environmental concerns such as pollution, erosion, emission of greenhouse gases and, in general, the weakening of its ecosystem services that are mainly regulated by microbial activity. Microbial activity and metabolism drive the formation of soil aggregates, ranging in size from a few micrometres to several millimetres. Understanding biological mechanisms related to aggregate size classes can provide insight into large-scale processes, but most research has focused on macroaggregates. Here, we investigated the microbial community and its functional changes at these smaller scales that are clearly more relevant for assessing microbial activity. We demonstrated that fungal communities are more sensitive to bigger size classes than bacteria, suggesting their dominant role in soil structure formation and turnover. We also identified preferential niches for reductive processes within the nitrogen cycle and a selection of specific taxa by analysing the water used for the wet-fractionation approach.