Above- and below-ground linkages of semi-arid perennial tussock grasslands

Executive Summary Despite the important role that plant-microbe relationships play in grassland structure and dynamics, our understanding of these relationships in the grassy ecosystems of southern Australia is limited. This thesis aimed to contribute towards a greater understanding of the linkages between the above-ground producer and below-ground decomposer communities in the critically endangered Northern Plains Grassland Communities of the Riverine Plains of south-eastern Australia. Firstly, a major functional group of soil microbes was focused on, the arbuscular mycorrhizal fungi (AMF), which play a role in the growth and nutrition of plants, particularly in the acquisition of soil phosphorus. Secondly, the dynamics of whole microbial communities was studied by phospholipid fatty acid analysis. A bait plant study (Chapter 2) showed that whilst the two predominant soil types of a Northern Plains grassland community differed in both vegetation and soil resource availability, they did not differ in the availability of propagules of AMF or the distribution of species forming arbuscular mycorrhizas (AM). The spatially variable distribution of AMF propagules across the grassland may have implications for the plant community. In a growth experiment (Chapter 3) the four dominant grasses of this grassland (Rytidosperma erianthum, R. caespitosum, R. setaceum and R. auriculatum) showed a negative growth response to the formation of AM, with no increase in plant tissue P in the one species where it could be tested in (R. caespitosum). This may have implications for plant community structure, such as promoting the growth of subordinate plant species. It also highlights the need to incorporate closely related plant species in studies of plant community/AMF community dynamics. Changes in vegetation and microbes were investigated along an agricultural de-intensification gradient (Chapter 4). With agricultural de-intensification, grasslands progressed toward plant community level traits for slow, conservative growth and resource use, alongside conversion from a bacterial- a to fungal-dominated soil flora. This is likely to be associated with changes in important ecosystem functions such as decreases in decomposition and productivity, and more efficient and retentive cycling of nutrients. A lag in fungal biomass recovery indicates potentially different drivers of the above- and below-ground sub-system recovery toward a more native grassland state. It was demonstrated with a nutrient addition experiment (Chapter 5) that nutrient availability may not be one of the strong drivers of the shift toward fungal dominance with agricultural de-intensification. The ratio of fungi to bacteria did, however, vary with seasonal vegetation changes. This indicates that microbial community dominance likely plays a role in the partitioning of soil resources for vegetation growth across seasons in this temporally dynamic semi-arid system. In conclusion, this study demonstrated variability in the spatial distribution and function of AMF communities in relation to vegetation and soil properties in the Northern Plains grassland community of south-eastern Australia. It highlighted a predictable shift from bacterial to fungal dominance alongside a shift toward resource-conservative plant community level traits with agricultural de-intensification, and suggested a role for soil microbes in grassland seasonal dynamics. In the face of continued anthropogenic pressures upon the grassy ecosystems of southern Australia, a greater understanding of the role of plant-microbe interactions in the provision of essential ecosystem functions is required.