Predicting the partitioning between aqueous and gaseous C across landscapes is difficult because many factors interact to control CO2 concentrations and removal as DIC. For example, carbonate minerals may buffer soil pH so that CO2 dissolves in porewaters, but nitrification of fertilizers may decrease pH so that carbonate weathering results in a gaseous CO2 efflux. Here, we investigate CO2 production and dissolution in an agricultural, first-order, mixed-lithology humid, temperate watershed. We quantified soil mineralogy and measured porewater chemistry, soil moisture, and pCO2 and pO2 as a function of depth at three hillslope positions for a year. The variation of soil moisture along the hillslope was the dominant control on the concentration of soil CO2, but mineralogy acted as a secondary control on the partitioning of CO2 between the gaseous and aqueous phases. The regression slopes of pCO2 vs. pO2 in the carbonate-bearing soils indicate a deficit of CO2 relative to O2 (p < 0.05). Additionally, we found no abiotic gaseous CO2 efflux from carbonate weathering. We concluded that in the calcareous soils, about a third of respired C dissolves and drains from the soil rather than diffusing out to the atmosphere. To represent the global scope of the reactions we evaluated at our local watershed, we used databases of carbonate minerals and land uses to map types of soil degassing behaviors. Based on our maps, the partitioning of respired soil CO2 to the aqueous phase may be globally common and should be accounted for in ecosystem C budgets and models.