A growing body of experimental and observational studies have indicated that plant and animal diversity drives ecosystem function and stability in terrestrial ecosystems. Therefore, it is of paramount importance to identify the consequences of biodiversity loss to assess long-term sustainability of ecosystem functions and services (e.g. climate regulation and nutrient cycling). Soil microbial communities represent one of the most diverse and complex natural communities and are responsible for many ecologically and economically important processes. For example, soil microbes play critical roles in regulating nutrient cycling, decomposition of organic matter, and gas emission. These ecological processes are fundamental for human wellbeing. Despite their importance and complexity, soil microbes receive little attention in the ongoing debate regarding global biodiversity loss, global change and conservation issues, primarily due to the perceived functional redundancy and the large diversity of the soil microbial community. This is no accident; there is a lack of theoretical and experimental protocols which demonstrate microbial regulation of soil ecosystem processes. Consequently, much less is known of the role of microbial diversity in controlling ecosystem functioning. This is critical knowledge gap which hinders inclusion of microbial community response in simulation models or management and policy decisions. Therefore, this research investigated the relationship between microbial diversity and ecosystem functioning (BEF) and resistance to better understand the consequences of microbial diversity loss on ecosystem function and sustainability. To achieve this, my project was divided into experimental (Chapters 2- 4) and observational (Chapters 3 and 5) studies. Each chapter in this thesis will highlight the importance of microbial diversity in ecosystem function. In summary, my study provided direct evidence of relationship and the shape of relationships between microbial diversity and ecosystem functions and suggested that any loss of microbial diversity will have at least proportional decline in the process rates. Particularly microbial community richness and composition were found to be important, yet independent drivers of multiple ecosystem functions. These results highlight that, belowground microbial diversity is as important as above ground diversity in maintaining ecosystem services. This study is also the first to investigating the drivers of microbial nitrifiers at the global scale. Overall, results from this thesis demonstrate that microbial diversity should be explicitly considered in all biodiversity conservation debates and management decisions and indicated that inclusion of microbial data in predictive models are required to improve predictions to ensure that informed environmental policy decisions are made to sustain ecosystem function under predicted global climate change scenarios
