Abstract

Accurate measurements of soil CO₂ concentrations (pCO₂) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO₂, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO₂ distribution can predictably vary with topographic position. We measured soil pCO₂ at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO₂ (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO₂ generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO₂ because it controls soil depth, texture, porosity, and moisture, which govern soil CO₂ diffusive fluxes. The highest pCO₂ values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO₂ diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO₂ because they are shallower (≤0.6 m) and drier, resulting in high CO₂ diffusion through soil profiles. Aspect was a minor predictor of soil pCO₂: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO₂ than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO₂ was similar across the watershed, the amplitude of the pCO₂ peak was higher in the deep soils due to more variable moisture content. The high pCO₂ observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO₂ alone. CO₂ is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO₂ dynamics would be improved by including landscape controls on soil pCO₂.

Citation

Hasenmueller, E.A., Jin, L., Stinchcomb, G.E., Lin, H., Brantley, S.L., Kaye, J.P. (2015): Topographic controls on the depth distribution of soil CO2 in a small temperate watershed. Applied Geochemistry, vol 63, 58-69. DOI: 10.1016/j.apgeochem.2015.07.005

This Paper/Book acknowledges NSF CZO grant support.