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Bacteria and fungi, representing two major soil microorganism groups, play an important role in the global carbon (C) cycle. Despite the critical role of fungi and bacteria in C cycling, our understanding of their roles in terrestrial C cycling was still unclear. In this dissertation, I investigated the biogeography of fungi and bacteria using a synthesized global dataset of fungal (FBC) and bacterial (BBC) biomass C of 0-30 cm. We observed clear distribution patterns of FBC, BBC, and FBC:BBC (F:B) ratio along latitude and environmental gradients including mean annual temperature, mean annual precipitation, net primary productivity, root C density, soil temperature, soil moisture, and edaphic factors. Fungal and bacterial biomass C and their ratio were dominated by different factors, with FBC and BBC predominated by edaphic properties and F:B ratio determined by climates. Combining the empirical model developed for F:B ratio with a global dataset of soil microbial biomass C, we estimated global stocks of living microbial biomass C as 12.6 (6.6~16.4) PgC for FBC and 4.3 (0.5~10.3) PgC for BBC in topsoil. To mechanistically understand microbial role in terrestrial C cycling, I first parametrized the CLM-Microbe model, a microbial-explicit model with fungal and bacterial regulatory role on soil processes represented, using the compiled time-series data of FBC and BBC from nine natural terrestrial biomes. The parameterization suggested the reasonable performance of the CLM-Microbe model in capturing the seasonal dynamics of FBC and BBC across biomes. On overage, the CLM-Microbe model explained 70% of the variation in FBC (P