Insights into the soil microbial communities in New Zealand indigenous tussock grasslands

New Zealand indigenous tussock grassland ecosystems are highly valued for their biodiversity, as well as their natural and cultural heritage values. Extensive conversion of grasslands to agricultural land through burning, grazing and oversowing has led to calls for conservation of tussock grasslands. Areas of tussock grassland are gradually being converted into reserves for conservation and scientific research. Grassland soils harbour a diverse group of microorganisms that regulate and contribute to many biogeochemical processes. To better understand and predict how soil microbial communities will respond to environmental change, it is important to first examine the baseline microbial communities in undisturbed grasslands. Specifically this work assessed whether active and total microbial community composition and abundance differed between edaphically different indigenous grassland soils dominated by two Chionochloa tussock species. In this thesis, the abundances of nitrogen (N) cycling genes were correlated with differences in edaphic properties of the two grassland soils. High-throughput sequencing of the 16S rRNA gene identified significant differences in the microbial communities between two grassland soils dominated by closely related Chionochloa tussock grasses. These differences in composition and relative abundances were evident in both the active (rRNA-based) and total (rDNA-based) microbial communities. Amplicon sequencing of the nifH gene, encoding for nitrogenase gene, also identified differences in the total nitrogen (N2)-fixing communities between the two grassland soils. The active community was less diverse than the total community in both soils, and many rare phyla were enriched in the active microbial community. Results from this study showed that more than half of all operational taxonomic units (OTUs) in the Chionochloa grassland soils could be dormant. Furthermore, transcript ratios of nifH and amoA (encoding for ammonia-monooxygenase) to rpoB showed significant inter-site variation, as quantified by RT-qPCR. This thesis showed differential abundances of N cycling genes in the active microbial community in natural, undisturbed grassland soils with different edaphic properties. Characterising the structure, abundance and functional potential of microbial communities is important as they may serve as bioindicators reflecting not only the N status of soils, but general health of soils in the event of anthropogenic and natural disturbances