Soil Carbon Dioxide Flux Partitioning in a Calcareous Watershed With Agricultural Impacts
Caitlin Hodges, Susan L. Brantley, Melika Sharifironizi, Brandon Forsythe, Qicheng Tang, Nathan Carpenter, Jason P. Kaye
Abstract
Abstract Predicting the partitioning between aqueous and gaseous C across landscapes is difficult because many factors interact to control carbon dioxide (CO 2 ) concentrations and removal as dissolved inorganic carbon (DIC). For example, carbonate minerals buffer soil pH and allow CO 2 dissolution in porewaters, but nitrification of fertilizers may decrease pH so that carbonate weathering results in a gaseous CO 2 efflux. Here, we investigate CO 2 partitioning in an agricultural, first‐order, mixed‐lithology humid, temperate watershed. We quantified soil mineralogy and measured porewater chemistry, soil moisture, and soil pCO 2 and pO 2 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 CO 2 , but mineralogy acted as a secondary control on the partitioning of CO 2 between gaseous and aqueous phases. Regression slopes of pCO 2 versus pO 2 in the carbonate‐bearing soils indicate a deficit of aerobically respired CO 2 relative to O 2 ( p < 0.05). Additionally, nitrification of upslope fertilizers did not lower soil pH and therefore did not cause a gaseous CO 2 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 CO 2 to the aqueous phase could be important in estimating ecosystem C budgets and models.