Soil Biodiversity of <i>Eucalyptus saligna</i> : Insights Into Bacterial and Nematode Communities

Soil microorganisms and nematodes are key regulators of soil function, yet their co-occurrence in Eucalyptus rhizospheres remains poorly understood. In this exploratory study, we characterised bacterial and nematode communities associated with Eucalyptus saligna soils in Limpopo, South Africa, using high-throughput 16S rRNA gene sequencing and nematode surveys. Bacterial assemblages were dominated by Proteobacteria (42%), Acidobacteria (28%), Actinobacteria (12%) and Planctomycetes (9%). Eleven bacterial genera occurred across all sites, with Rhizobiales (prominence value, PV = 315,350) and Xanthobacteraceae (PV = 292,930) emerging as the most prominent taxa. Nematode surveys identified 19 genera, including plant-parasitic such as Meloidogyne (PV = 5759.1) and abundant free-living such as Tylolaimorphus (PV = 4150.0) and Acrobeloides (PV = 2900.0). Principal component analysis showed that bacterial communities were associated with soil pH, salinity and nitrogen forms, whereas nematode assemblages were associated with phosphate and sand content, together explaining 83%-90% of total variance. Network analysis indicated that all sampling sites functioned as central hubs (degree = 19-23; eigenvector centrality = 0.90-1.0), integrating nematode-bacteria associations. Key connector taxa included Acrobeloides, Wilsonema and Aphelenchoides, as well as bacteria such as Rhizobiales and Acidothermus. These findings provide a baseline framework for understanding belowground biodiversity and co-occurrence patterns in Eucalyptus plantation soils.