This study investigates how fungal-bacterial diversity and microbiome complexity influence ecosystem functioning in grassland ecosystems. The research manipulates soil microbiomes in controlled microcosms and finds that higher microbial diversity and network complexity enhance ecosystem functions related to nutrient cycling, such as multifunctionality. Microcosms with less developed microbial networks and reduced richness showed lower multifunctionality due to fewer taxa supporting the same function (redundancy) and lower diversity of taxa supporting different functions (reduced functional uniqueness). The study highlights the importance of microbial interactions within and among fungal and bacterial communities for enhancing ecosystem performance. It also shows that the extinction of complex ecological associations belowground can impair ecosystem functioning. Microbes are a large part of Earth's genetic diversity and play a central role in biogeochemical cycles, influencing ecosystem functioning and productivity. Experiments in microcosms and at global scales show that microbial diversity is linked to ecosystem functioning, implying that more diverse communities perform better. However, functional redundancy is not always present, as microbes are involved in multiple functions simultaneously. The study emphasizes the need to consider not only the total number of taxa but also how the reduction in species supporting a single function relates to the loss of multiple functions. The study hypothesizes that microbiome richness and network complexity promote ecosystem multifunctionality. It also suggests that greater richness provides a greater diversity of taxa that support multiple functions, leading to a positive relationship between functional uniqueness and ecosystem multifunctionality. The research uses next generation sequencing to characterize the fungal and bacterial soil microbiome and identifies taxa that contribute to ecosystem functions. It finds that more complex microbial networks contribute more to improved ecosystem function multifunctionality than simpler or low-diversity networks. The study shows that different microbes support different functions, highlighting the significance of functional diversity in microbial communities. It also demonstrates that the complexity of microbial networks and their relation to function is not necessarily determined by the number of taxa but by the number of associations among them. The findings indicate that maintaining a rich and diverse soil microbiome is crucial for supporting multiple ecosystem functions, as it increases the likelihood of having taxa that support any given function and enhances functional diversity. The study also shows that considering both fungal and bacterial communities together provides a better prediction of soil multifunctionality than considering them separately. The results suggest that microbial interactions and diversity are essential for maintaining ecosystem functioning and that the extinction of complex ecological associations can impair ecosystem services.This study investigates how fungal-bacterial diversity and microbiome complexity influence ecosystem functioning in grassland ecosystems. The research manipulates soil microbiomes in controlled microcosms and finds that higher microbial diversity and network complexity enhance ecosystem functions related to nutrient cycling, such as multifunctionality. Microcosms with less developed microbial networks and reduced richness showed lower multifunctionality due to fewer taxa supporting the same function (redundancy) and lower diversity of taxa supporting different functions (reduced functional uniqueness). The study highlights the importance of microbial interactions within and among fungal and bacterial communities for enhancing ecosystem performance. It also shows that the extinction of complex ecological associations belowground can impair ecosystem functioning. Microbes are a large part of Earth's genetic diversity and play a central role in biogeochemical cycles, influencing ecosystem functioning and productivity. Experiments in microcosms and at global scales show that microbial diversity is linked to ecosystem functioning, implying that more diverse communities perform better. However, functional redundancy is not always present, as microbes are involved in multiple functions simultaneously. The study emphasizes the need to consider not only the total number of taxa but also how the reduction in species supporting a single function relates to the loss of multiple functions. The study hypothesizes that microbiome richness and network complexity promote ecosystem multifunctionality. It also suggests that greater richness provides a greater diversity of taxa that support multiple functions, leading to a positive relationship between functional uniqueness and ecosystem multifunctionality. The research uses next generation sequencing to characterize the fungal and bacterial soil microbiome and identifies taxa that contribute to ecosystem functions. It finds that more complex microbial networks contribute more to improved ecosystem function multifunctionality than simpler or low-diversity networks. The study shows that different microbes support different functions, highlighting the significance of functional diversity in microbial communities. It also demonstrates that the complexity of microbial networks and their relation to function is not necessarily determined by the number of taxa but by the number of associations among them. The findings indicate that maintaining a rich and diverse soil microbiome is crucial for supporting multiple ecosystem functions, as it increases the likelihood of having taxa that support any given function and enhances functional diversity. The study also shows that considering both fungal and bacterial communities together provides a better prediction of soil multifunctionality than considering them separately. The results suggest that microbial interactions and diversity are essential for maintaining ecosystem functioning and that the extinction of complex ecological associations can impair ecosystem services.