Australian Acacia species impact soil bacterial communities abroad but not at home

Background and Aims: Invasive Australian Acacia species are known to modify soil physicochemical properties and microbial communities, yet their effects on soil microbial co-occurrence networks and functioning across native and non-native ranges remain poorly understood. Here, we investigated these impacts by comparing soil physicochemical properties, soil bacteriome diversity, composition and nitrogen-cycling guilds in sites with and without Acacia species across native (Australia) and non-native regions where they have become naturalized or invasive (New Zealand, South Africa). 

Methods: We reconstructed bacterial co-occurrence networks to examine network topological features and associated hub taxa (i.e., highly interactive/connected bacteria) in relation to soil physicochemical variables and the presence of acacia trees. 

Results: Acacia presence was associated with soil acidification and increases in soil organic matter and NH₄⁺-N in New Zealand and South Africa, but not in Australia. Networks associated with acacia soils in New Zealand and South Africa showed lower node-level closeness than those in Australia, indicating that soil bacterial taxa are less centrally connected in the non-native ranges than in the native range of acacias. Relative abundance of hub taxa was strongly correlated with soil pH, moisture, and nutrient availability. 

Conclusion: Our findings highlight the importance of combining co-occurrence network analysis with differential abundance testing of core taxa and exploring the associations between highly connected (central) soil microbes, changes in soil physicochemical properties, and nitrogen-cycling guilds. Understanding how microbial networks respond to invasive plant species is essential for predicting the recovery of soils following management interventions. For instance, this knowledge can guide restoration strategies, identifying instances where passive recovery is sufficient and when active interventions, such as soil amendments or microbial inoculation, are necessary to restore native communities and ecosystem processes.