Technical Abstract: We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28 mg/g in a frigid-dry region (Alberta/British Columbia), 25 mg/g in a frigid-wet region (Maine), 11 mg/g in a thermic-dry region (Texas), and 12 mg/g in a thermic-wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg C/g SOC/d), greater net N mineralization (2.8 vs 1.3 mg inorganic N/g SOC/24 d), and greater SMBC (53 vs 21 mg SMBC/g SOC). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg C/g SOC/d) and lower SMBC (31 vs 43 mg SMBC/g SOC), but had inconsistent effects on net N mineralization. Although thermic regions are not able to retain as high a level of SOC as frigid regions, biologically active soil fractions appear to be as high per mass of soil and even 2- to 3-time greater per unit of SOC in thermic compared with frigid regions. Soil microbial activity and biomass are much more intimately linked across major differences in climate than microbial biomass/activity with total organic C.