This chapter explores the advances in understanding the role of soil microbiomes in carbon cycling in soil. The author begins by reviewing the contributions of bacterial and fungal necromass to soil carbon, and the roles of soil microbial biomass in carbon cycling. Here, he describes how soil microbiomes regulate carbon cycling through both catabolic and anabolic processes, modulated by metabolic properties and processes such as life history strategy, carbon use efficiency, and priming. The author then turns his attention to how the role of soil microbiomes in soil carbon cycling can be modelled, how these models represent our best conceptual understanding of the structure of the system, and the ways that quantitative predictions are improved by explicitly including the functional traits of the soil microbiome and its activities. Lastly, the author provides a case study of soil microbiomes as an indicator of biological soil health.