Abstract

Quantifying soil gas dynamics is needed for global C cycle models, projecting feedbacks between climate change and terrestrial ecosystem C balance, and assessing bedrock weathering through enhanced acidity from CO₂ dissolution into porewaters. We seek to link patterns in soil CO₂ fluxes with dissolved inorganic C (DIC) in porewaters, which will allow modelling of CO₂ loss from soils to the atmosphere and through the advective outflux of DIC-containing porewaters from watersheds. Soil CO₂ fluxes were measured at the land surface and calculated at depth using Fick’s Law, pCO₂, and soil properties for a first-order catchment at the Susquehanna Shale Hills Critical Zone Observatory [1]. We measured soil CO₂ along a planar slope and swale depression, with discrete
sampling depths at the ridge, mid-slope, and valley. We compared CO₂ flux data with DIC for soil porewaters and groundwater [2]. Surface CO₂ flux positively correlates with soil temperature and moisture up to 0.25 m³ m^-3; above this value it is negatively correlated with soil moisture. On average, surface CO₂ flux is higher for the wetter swale depression (4.67±2.96 μmol m ^-2 s^-1) than the drier planar slope (3.67±2.48 μmol m^-2 s^-1). However, there is high spatial variability along the two transects that does not correlate with hillslope position or soil depth, but is likely the result of heterogeneous leaf litter distribution (R² = 0.69). At depth, CO₂ fluxes to the atmosphere decrease rapidly (to ~1 μmol m² s^-1 at 10 cm and to nearly zero below 30 cm) and are positively correlated with porosity and negatively correlated with soil moisture. Under drier conditions and at >50 cm depth, we observe small CO₂ fluxes towards the groundwater. This downward flux is confirmed by observations of groundwater δ ¹³CDIC that indicate mixing between DIC from soil CO₂ and ankerite dissolution in the parent shale [2].

[1] Hasenmueller et al. (2015) Appl. Geochem. 63, 58-69.

[2] Jin et al. (2014) Geochim. Cosmochim. Acta 142, 260-280.

Citation

Elizabeth A. Hasenmueller*, Pamela L. Sullivan, Julie N. Weitzman, Susan L. Brantley, and Jason P. Kaye (2018): Soil CO2 Fluxes Through a Temperate Watershed. Abstract 11g:338 presented at 2018 Goldschmidt Meeting, Boston, MA 12-17 August.

This Paper/Book acknowledges NSF CZO grant support.