Soil carbon dioxide and oxygen concentrations indicate mineralogy plays a key role in controlling soil pCO2

Soil CO₂ and O₂ are coupled in some processes (e.g. respiration) but uncoupled in others (e.g. mineral weathering), such that simultaneous measurement of these two gases can yield insight into an array of soil chemical reactions and biogeochemical processes. Because soil CO₂ production and O₂ consumption are tightly coupled when aerobic respiration and diffusion persist in the soil system, the deviations from that coupling can be interpreted to signify various biotic and abiotic reactions. Here, we used such measurements as a function of depth to understand mineral, hillslope, and seasonal controls on soil pCO₂ relative to pO₂ in three watersheds of different bedrock lithology. We made our measurements over a growing season in three neighboring humid, temperate watersheds underlain by three different sedimentary bedrocks – acidic shale, calcareous shale, and acidic sandstone. Across these three watersheds, we expected to observe different soil pCO₂ vs. pO₂ patterns. For example, in calcareous soils we anticipated to observe a greater signature of soil CO₂ consumption through weathering reactions than in silicate-dominated systems. Additionally, based on prior work, we anticipated a strong metal oxidation signature in the acidic soils.

Our results point to the strong control of parent material on the deviation of soil pCO₂ from the theoretical values for aerobic respiration and diffusion. In the two acidic parent materials we observed a signature of seasonal metal redox cycling, with metal oxidation in the early growing season as soils drain and reoxygenate, and metal reduction in the late growing season when warm moist soils drive soil respiration rates to higher than the diffusion rate of O₂. On the other hand, in the calcareous watershed, soil pCO₂ and pO₂ measurements did not suggest a seasonal redox cycle and instead indicate a consistent deficit of CO₂ relative to the O₂ consumed through aerobic respiration. Corresponding measurements of porewater chemistry indicate that this deficit is not solely attributable to carbonate mineral weathering, but also from consistent dissolution and transport downslope of respired CO₂. We calculate that the effects of these processes can impact soil CO₂ efflux to the atmosphere by up to 35%. Such results challenge our understanding of the soil carbon cycle. Employing coupled pCO₂ and pO₂ measurements in other systems will deepen understanding of soil C fluxes by identifying where and when factors other than aerobic respiration and diffusion control C flux out of the soil.