Changes in root symbionts during long-term soil and ecosystem development and their ecological role for the maintenance of plant diversity

Soil microbiota are increasingly acknowledged as key drivers of plant diversity and community assembly. Indeed, plant communities can be strongly influenced by their associated root microorganisms, such as mutualistic symbionts and soil-borne pathogens. The two main root mutualistic fungal associations are with: i) arbuscular mycorrhizal (AM) fungi, where the fungus enhances the acquisition of inorganic phosphorus (P) and other relatively immobile nutrients; and ii) ectomycorrhizal (ECM) fungi, which enhance plant acquisition of mainly organic sources of nitrogen (N) and P, and provide defence against pathogens. On the other hand, soil-borne pathogens such as oomycetes are parasitic root symbionts that usually cause damping-off in roots; they can have strong detrimental effects on plant health. In this thesis, I studied shifts in root colonisation of AM and ECM fungi associated with two plant species (Acacia rostellifera and Melaleuca systena), which are both capable of forming multiple symbioses, along a south-western Australian dune chronosequence representing two million years of soil and ecosystem development, and along which there are major changes in soil N and P availability Furthermore, I evaluated the importance of soil fertility and soil inoculum potential effects on the ECM fungal community composition and richness within individual plant species along the chronosequence. I also studied the role that native species of pathogenic Phytophthora and mutualistic ECM fungi may play in plant species coexistence in severely P-impoverished soils in the Southwest Australian Biodiversity Hotspot. To do so, I collected roots and soils from the Jurien Bay chronosequence and grew seedlings of both species in a glasshouse in soils collected from the same chronosequence and measured seedling biomass, mycorrhizal root colonisation, and ECM fungal community composition in roots. I found that AM fungal root colonisation declined with soil age, while ECM root colonisation increased.
This switch in importance between different mutualistic root symbionts is likely related to a decline in total and mineral P, but an increase in the organic fraction of total P. In addition, ECM fungal communities associated with roots from the same two plant species changed in composition during ecosystem development, and this was also related to soil chemical properties, such as pH and different forms of soil P. Finally, I found that native Phytophthora species reduced seedling biomass of non-mycorrhizal Proteaceae, while not affecting ECM plant species when both types of plants were competing with each other. Hence, native Phytophthora reduced differences in competitive ability between Proteaceae and ECM plant species, likely contributing to the coexistence of these plant species of contrasting strategies. I surmise that this could be related to a trade-off between P-acquisition efficiency and pathogen defence, where Proteaceae are more efficient at acquiring P, but more susceptible to soil-borne pathogens. I conclude that the importance of different symbionts and their communities is strongly associated with edaphic properties, especially P. Furthermore, I show that soil microbiota could play a role in maintaining plant diversity. Soil microbiota are an important part of ecosystems and more studies focusing on them will further our knowledge on plant communities, especially in highly diverse ecosystems.