B41G-0510
The importance of hydrologic and lithologic controls on pCO2 and pO2 dynamics in the soil atmosphere in a temperate forest at meters depth 

Thursday, 17 December 2015
Poster Hall (Moscone South)
Hyojin Kim, Pennsylvania State University Main Campus, University Park, PA, United States, Gary E Stinchcomb, Baylor University-Geology, State College, PA, United States and Susan L Brantley, Earth and Environmental Systems Institute, Penn State, Univ. Pk, PA, United States
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.