Fungal alpha diversity influences stochasticity of bacterial and fungal community assemblies in soil aggregates in an apple orchard

Abstract

Understanding the drivers of variations in the microbial community in response to different soil aggregates and agricultural practices is important in ecology. Bacteria and fungi always co-exist in different soil aggregates and the variations in their communities are driven by agricultural practices, but the factors that control the assembly of soil bacterial and fungal communities are not fully understood. In this study, we examined bacterial and fungal community assemblies in soil macroaggregates (>0.25 mm), microaggregates (0.053–0.25 mm), and smaller microaggregates (silt+clay, <0.053 mm) in an apple orchard treated with a cover crop and fertilizer. The microbial community assembly processes were analyzed based on the normalized stochasticity ratio (NST) index. The NST index was developed with 50% set as the boundary point between more deterministic (<50%) and more stochastic (>50%) assemblies. The bacterial community diversity, composition, and stochasticity of assemblies were affected more by agricultural practices and aggregates than the fungal community. Bacterial community assemblies were more stochastic in silt+clay than macroaggregates, and more stochastic (NST > 50%) than fungal communities in soil aggregates. The bacterial NST was also negatively correlated with the fungal diversity, and the fungal NST was positively correlated with the fungal diversity. Co-occurrence networks suggested that bacteria and fungi interacted less strongly in the network obtained for silt+clay compared with that for macroaggregates. The results indicated that fungi affected the bacterial community assemblies in soil aggregates, and the stochasticity of the bacterial community assemblies increased as the interactions between bacteria and fungi decreased in the soil aggregates. This study enhances our mechanistic understanding of bacterial and fungal community assembly processes and the co-existence patterns of bacteria and fungi in soil aggregates.