Applied Geochemistry 26 (2011) S132–S134
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Seasonal dynamics of CO2 profiles across a soil chronosequence, Santa Cruz, California Marjorie Schulz a,⇑, David Stonestrom a, Guntram Von Kiparski b, Corey Lawrence a, Carrie Masiello c, Art White a, John Fitzpatrick a a b c
US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA Dept. of Environmental Sciences, University of California, Room 2327 Geology, Riverside, CA 92521, USA Department of Earth Science MS 126, Rice University, 6100 Main St., Houston, TX 77005, USA
a r t i c l e
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Article history: Available online 23 March 2011
a b s t r a c t Concentrations of CO2 in soil atmosphere and CO2 efflux were measured across a marine terrace soil chronosequence near Santa Cruz, California. Soil development, specifically the formation of an argillic horizon, has created a two-tier soil gas profile in the older terrace soils. The soil above the argillic horizon has seasonal variations in soil CO2 associated with plant respiration. The older soils with dense argillic horizons maintain a year round 1%CO2 below the argillic horizon. The CO2efflux during the growing season is higher on the older terraces. Published by Elsevier Ltd.
1. Introduction Exchanges of CO2 between soils, plants and the atmosphere rank among the lesser known controls on the terrestrial C budget (Sugden et al., 2004). Few data sets address the range of time and space scales sufficient to develop the processes-level understanding needed to constrain the interactions of these pools. Multi-year instrumented profile studies, as presented here, aid in filling these gaps by demonstrating how relationships among soil–gas C pools change in response to seasonal dynamics. Applying this approach to a soil chronosequence shows how these relationships evolve through geologic time to become controlled by pedogenic state. In this study, the concentrations of CO2 in soil gas and underlying regolith were measured in a marine terrace soil chronosequence NW of Santa Cruz, California (White et al., 2008) over a period of 4 a (10/2001–04/2004). The soils were instrumented with gas samplers to depths up to 9.25 m. The climate is Mediterranean with cool wet winters and warm dry summers. Soils are developed on uplifted marine terraces overlain by reworked granitic sediments with a lesser smectite component. The current vegetation is coastal prairie, dominated by European annual grasses, with scattered shrubs and oaks. Soils in the chronosequence are classified as Mollisols (Aniku and Singer, 1990) and have associated high organic C in the A horizon. These soils contain secondary kaolinite and Fe-oxides which become progressively more concentrated in ⇑ Corresponding author. Tel.: +1 650 329 4518. E-mail address:
[email protected] (M. Schulz). 0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2011.03.048
argillic horizons of the older terraces (between the depths of 80 and 180 cm). The argillic horizon is an important diffusion boundary that influences CO2 concentration. Details of the soil mineralogy and bulk chemistry can be found in White et al. (2008). The ages of the 5 terraces, dated using cosmogenic Be10 and Al26 (Perg et al., 2001) are: Terrace 1 = 65 ka, Terrace 2 = 90 ka, Terrace 3 = 137 ka, Terrace 4 = 194 ka and Terrace 5 = 226 ka. 2. Results Shallow soil gas samplers (less than 1 m depth) showed seasonal variations in CO2 concentration; high (up to 3% CO2 by volume) during the wet growing season and low values (approaching atmospheric) during the dry summer (Fig. 1). At depths >1 m, CO2 concentrations on all but the youngest terrace are relatively constant, averaging 1% CO2 on Terraces 2, 3 and 5 (Fig. 2). The CO2 concentrations in Terrace 1 vary throughout the year at all depths. Terrace 4 is anomalous, with deep CO2 concentrations of up to 7% CO2 (Fig. 3). Fluxes of CO2 at the soil surface were measured on Terraces 2, 3 and 5 (Fig. 4) during the growing season (January–March) and the dormant season (June–October). Soil respiration averaged over the growing season varied as follows: Terrace 3 (3.7 ± 0.7 lmol m 2 s 1) > Terrace 5 (2.9 ± 0.6 lmol m 2 s 1) > Terrace 2 (2.3 ± 0.4 lmol m 2 s 1). The dormant season is characterized by a substantial drop in soil respiration with fluxes across the three soil terraces as follows: Terrace 5 = 0.7 ± 0.1 lmol m 2 s 1, Terrace 3 = 0.6 ± 0.1 lmol m 2 s 1, and Terrace 2 = 0.5 ± 0.05 lmol m 2 s 1.
M. Schulz et al. / Applied Geochemistry 26 (2011) S132–S134
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3. Discussion High CO2 concentrations at depths >1 m have been observed by others (Haas et al., 1983; Amundson and Davidson, 1990; Wood et al., 1993; Richter and Markewitz, 1995; Bacon and Keller, 1998; Trumbore et al., 2006; Fierer et al., 2005). For several of these studies, the majority of the CO2 diffused up through the unsaturated zone from a deep source (Haas et al., 1983; Wood et al., 1993; Bacon and Keller, 1998). The other studies (Amundson and David-
son, 1990; Richter and Markewitz, 1995; Trumbore et al., 2006; Fierer et al., 2005) have shown deep CO2 to result from multiple sources. Possible deep CO2 sources include: root and fungal respiration, decomposition of illuviated SOM or DOM by microbial activity, microbial degradation of DOC in percolating pore water, degassing of percolation charged with CO2 in the shallow soil zone, calcite precipitation, and perhaps even deep seismic (fault) sources. Future work to determine which of these processes is dominant in the terrace soils is planned, and will include constraining and modeling the source of the deep CO2. The argillic horizon found in the older terrace soils regulates the deep soil from the seasonal swing in soil moisture (White et al., 2009) and, it seems, also buffers the deep CO2 from seasonal CO2 variation, maintaining relatively constant concentrations (Fig. 2). Terrace 1 does not have a well developed argillic horizon and therefore has a CO2 concentration which changes seasonally throughout its depth. Terrace 2 has an argillic horizon that may not be as impervious as the older terraces, as the deep CO2 concen-
Fig. 2. CO2 concentrations on Terraces 1, 2, 3 and 5.
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Soil CO2 Flux ( µ mol m-2 s-1)
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horizon the CO2 content shows little to no seasonal fluctuation. The CO2 flux data show a higher flux on the older terraces. More detailed measurements of soil C pools with depth will allow constraint of the sources of deep CO2. Future work with coupled modeling of soil geochemical weathering and C cycling will shed light on what is controlling the patterns observed at the Santa Cruz Terraces.
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trations in Terrace 2 do fluctuate slightly between 0.5 and 1.5% CO2 at the deeper soil depths. The sustained high (1%) CO2 at depth has implications for mineral weathering as the high CO2 will enhance weathering reactions by lowering solute pH. It is interesting that the soil CO2 efflux during the growing season of the older terrace soils is slightly higher than the younger terraces, which is likely due to elevated plant/microbial activity. The older terraces have slightly higher soil moisture (White et al., 2009), in part due to the clay content, this extra moisture may be enough to prime plants and microbes. Terrace 4 is anomalous due to its high CO2 concentration. This is the only terrace with an indication of a CO2 source at depth. The argillic horizon seems to act as a diffusion restrictor for the high CO2 here. Further investigation of Terrace 4 showed that it was anthropogenically disturbed from the making of charcoal on the site. There is abundant charcoal in and around the Terrace 4 site and there are other chemical and pedogenic indicators of this disturbance. The current hypothesis is that the source of the high CO2 concentrations in the deep regolith of Terrace 4 is the breakdown of pyroligneous acid generated during the charcoaling process. In summary, soil CO2 concentrations in the chronosequence only vary seasonally through the entire regolith in the youngest Terrace 1, which does not have a developed argillic horizon. In the older soils the seasonal variation in CO2 content is restricted to the top meter (above the argillic horizon). Below the argillic
We would like to thank Jennifer Harden, Dan Bain and Chris Swanton for insights and guidance over the length of this project. Kristin Manies and Chris Fuller provided helpful reviews. References Amundson, R.G., Davidson, E.A., 1990. Carbon dioxide and nitrogenous gases in the soil atmosphere. J. Geochem. Explor. 38, 1341. Aniku, J.R.F., Singer, M.J., 1990. Pedogenic iron oxide trends in a marine terrace chronosequence. Soil Sci. Soc. Am. J. 54, 147–152. Bacon, D.H., Keller, C.K., 1998. Carbon dioxide respiration in the deep vadose zone: implications for groundwater dating. Water Resour. Res. 34, 3069–3077. Fierer, N., Chadwick, O.A., Trumbore, S.E., 2005. Production of CO2 in soil profiles of a California annual grassland. Ecosystems 8, 412–429. Haas, H., Fisher, D.W., Thorstenson, D.C., Weeks, E.P., 1983. Distribution of gaseous 12 CO2, 13CO2, and 14CO2 in the sub-soil unsaturated zone of the Western US Great Plains. Radiocarbon 25, 315–346. Perg, L.A., Anderson, R.S., Finkle, R.C., 2001. Use of a new 10Be and 26Al inventory method to date marine terraces, Santa Cruz, California, USA. Geology 29, 879– 882. Richter, D.D., Markewitz, D., 1995. How deep is soil? BioScience 45, 600–609. Sugden, A., Stone, R., Ash, C., 2004. Ecology in the underworld. Science 304, 1613. Trumbore, S.E., Da Costa, E.S., Nepstad, D.C., De Camargo, P.B., Martinelli, L.A., Ray, D., Restom, T., Silver, W., 2006. Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration. Global Change Biol. 12, 217–229. White, A.F., Schulz, M.S., Vivit, D.V., Blum, A.E., Stonestrom, D.A., Anderson, S.P., 2008. Chemical weathering of a marine terrace chronosequence, Santa Cruz, California I: Interpreting rates and controls based on soil concentration-depth profiles. Geochim. Cosmochim. Acta 72, 36–68. White, A.F., Schulz, M.S., Stonestrom, D.A., Vivit, D.V., Fitzpatrick, J., Bullen, T.D., Maher, K., Blum, A.E., 2009. Chemical weathering of a marine terrace chronosequence, Santa Cruz, California II: solute profiles, gradients and the comparisons of contemporary and long-term weathering rates. Geochim. Cosmochim. Acta 73, 2769–2803. Wood, B.D., Keller, C.K., Johnstone, D.J., 1993. In situ measurement of microbial activity and controls on microbial CO2 production in the unsaturated zone. Water Resour. Res. 29, 647–659.
