Abstract

Accurate predictions of soil respiration dynamics are necessary for constructing models of the global carbon (C) cycle and projecting feedbacks between climate change and terrestrial ecosystem C balance. Moreover, quantifying soil CO₂ concentrations (pCO₂) is important for our understanding of bedrock weathering because it provides an active source of acidity through CO₂ dissolution into soil pore water. We seek to determine whether there are predictable patterns in soil pCO₂ and CO₂ fluxes across landscape positions within a first-order catchment that can lead to better models of soil respiration by using remotely mappable topographic information. Our experimental design captures the spatial variability of soil CO₂ by explicitly monitoring both pCO₂ and CO₂ flux at hillslope transects for a planar slope and swale depression (sampling sites include discrete depths for ridge tops, mid-slopes, and valley floors) over two years in the Susquehanna/Shale Hills Critical Zone Observatory (SSHCZO) catchment. The sampling sites have the same bedrock composition, but different soil series, soil moisture conditions, and vegetation depending on hillslope position.

Our results show that soil pCO₂ is controlled by topographic position. The highest pCO₂ values for all sites were measured during the growing season, and the average soil pCO₂ was highest in the landscape positions with the deepest soils (i.e., swale depression mid-slope: 1.6 m thick soil, pCO₂ = 8940 ± 5720 ppmv; planar slope valley floor: 0.7 m thick soil, pCO₂ = 5450 ± 4910 ppmv; swale depression valley floor: 0.9 m thick soil, pCO₂ = 4850 ± 3130 ppmv). The lowest pCO₂ values were found at the ridge tops and the planar slope mid-slope, which had soil depths of < 0.4 m and average pCO₂ values of < 1800 ppmv. The pCO₂ increased with depth for all sites throughout the year, with the exception of three sites that had high pCO₂ anomalies at shallower depths in the profile that only occurred during the growing season and were associated with preferential water flowpaths. Soil pCO₂ increased with both soil temperature and moisture, the latter largely being a function of soil depth and landscape position. The observed pattern in soil pCO₂ suggests that topographic positions with deeper soils have higher weathering potential.

Like soil pCO₂, surface CO₂ flux positively correlates with soil temperature and moisture, and on average, is higher for sites in the wetter swale depression (CO₂ flux = 4.67 ± 2.96 μmol m-2s-1) than those in the drier planar slope (CO₂ flux = 3.67 ± 2.48 μmol m-2s-1). However, flux becomes negatively correlated with soil moisture above 0.25 m³ m^-3. Further, there is high spatial variability along the individual transects that does not correlate with hillslope position or soil depth. Rather, the surface CO₂ flux pattern along each transect is the result of heterogeneous leaf litter distribution. This relationship is most pronounced during the growing season but persists into the winter. Our results suggest that while topography controls pCO₂, the surface CO₂ flux is instead influenced by the variability of leaf litter distribution, which is challenging to quantify.

Citation

Hasenmueller, E.A., Jin, L., Smith, L.A., Kaye, M.W., Lin, H., Brantley, S.L., Kaye, J.P. (2013): Depth and Topographic Controls on Soil Gas Concentrations and Fluxes in a Small Temperate Watershed. Abstract EP13C-0876 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.