The importance of hydrologic and lithologic controls on pCO2 and pO2 dynamics in the soil atmosphere in a temperate forest at meters depth

Abstract:

To improve our understanding of how and when carbon is delivered to deeper depths and what processes are responsible for mineralization of carbon at depth, this study monitored the CO₂ and O₂ variability in the soil atmosphere from land surface down to the regolith-bedrock boundary on two different rock types (i.e., to 7.2m on granite and 1.4m on diabase) in Virginia at a bi-weekly interval for two years. In both sites, at the shallow depth (<0.7-1m) CO₂ and O₂ concentrations sharply increased (to values as high as 2-5% v/v) and decreased (to ~15%) with increasing depth, displaying a CO₂:O₂ = 1:-1 mole ratio. At deeper depths on granite, the CO₂ varied between 6% (summer) and 10% (winter), maintaining CO₂:O₂ =1:-1 year round. However, strong seasonality was observed in the diabase’s deeper depths: during the spring and summer, the CO₂ concentrations increased up to ~4% as its O₂ decreased in a 1:-2.7 ratio. During the fall and winter on diabase, the CO₂ and O₂ concentrations varied by 2-3 % and 17-18%, respectively, but always displayed the 1:-1 ratio. We attribute these observations to the important role of fractures in regolith in delivering oxygen and labile organic matter in regolith down to 7 m. These substrates sustain a highly productive aerobic microbial community. In the diabase, the exchange between the surface and the subsurface may be seasonally limited due to the swelling of the smectite-rich layer at 0.5 m depth. Consequently, oxygen and organic matter are poorly replenished at depth and the CO₂ production decreases below that of granite. The 1:-2.7 stoichiometric ratio in this mafic rock is attributed to the coupling of Fe-reductive carbon dissimilation and Fe-oxidation. Our study highlights i) a newly designed sensor system for soil gas at depth; ii) the use of soil gas ratios to infer metabolic pathways; and, surprisingly, iii) evidence for measurable diurnal gas cycles as deep as 7 meters beneath the temperate forest land surface.