Soil Carbon Dioxide Flux Partitioning in a Calcareous Watershed With Agricultural Impacts

Predicting the partitioning between aqueous and gaseous C across landscapes is difficult because many factors interact to control carbon dioxide (CO₂) concentrations and removal as dissolved inorganic carbon (DIC). For example, carbonate minerals buffer soil pH and allow CO₂ dissolution in porewaters, but nitrification of fertilizers may decrease pH so that carbonate weathering results in a gaseous CO₂ efflux. Here, we investigate CO₂ partitioning in an agricultural, first-order, mixed-lithology humid, temperate watershed. We quantified soil mineralogy and measured porewater chemistry, soil moisture, and soil pCO₂ and pO₂ as a function of depth at three hillslope positions. 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 gaseous and aqueous phases. Regression slopes of pCO₂ versus pO₂ in the carbonate-bearing soils indicate a deficit of aerobically respired CO₂ relative to O₂ (p < 0.05). Additionally, nitrification of upslope fertilizers did not lower soil pH and therefore did not cause a gaseous CO₂ flux from carbonate weathering. We concluded that in the calcareous soils, up to 43% of respired C potentially dissolves and drains from the soil rather than diffusing out to the atmosphere. To explore the possible implications of the reactions we evaluated, 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 could be important in estimating ecosystem C budgets and models.