Stabilization of Organic Carbon in Soils

The majority of organic carbon in soils turns over slowly, on time scales from centuries to millennia (Schimel et al. 1994). Soil minerals are known to stabilize soil organic carbon, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long residence-time organic carbon is not well known (Oades 1988). In recent research in Hawaii, it was found that soil mineralogy can exert an influence on soil carbon storage and turnover comparable to that commonly attributed to climate or vegetation (Torn et al. 1997). The accumulation and subsequent loss of organic content were largely driven by changes in millennial-cycling, mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity (Figure 1). This study was conducted on Andisol soils which, while providing a clear and dramatic example of soil carbon-mineral interactions, are not typical of soils worldwide. The next step in this research is to test for mineral control in more common parent materials and soil mineral transitions.

We are investigating the role of soil minerals in controlling the sequestration and loss of soil organic carbon at the ecosystem scale. Field sites are selected to form controlled natural gradients, where one factor varies in predictable patterns while minimizing variation in other factors that influence soil carbon dynamics. The main study sites currently being developed have natural gradients of soil development and soil mineralogy due to differences in substrate age.

The primary hypothesis guiding our work that as soil minerals develop over time they lose ability to stabilize carbon. This occurs following initial mineral formation from the parent material. Second, it is hypothesized that readily quantifiable properties of soil minerals can be identified that will allow a ranking of carbon-stabilization efficiency of major soil mineral groups. Examples of such properties are mineral surface area and charge density. The critical aspect of these hypotheses is that they link molecular-scale processes to controls of soil carbon storage at the ecosystem scale, and ultimately to the global scale.

Figure 1. (Left) Inventory of carbon in SOM to the depth of soil development (~1 m) in a chronsequence of Hawai'ian rainforest soils. (Right) The turnover time of SOM, as indicated by radiocarbon content, is signficantly correlated with the non-crystalline mineral content of all horizons of the Hawai'ian chronosequence soils (solid line; excluding the 4.1 million year site which had less than 10% non-crystalline mineral content in any horizon). The dashed line shows the regression for radiocarbon vs. non-crystalline mineral content for a separate moisture gradient on Hawai'i (Torn et al. 1997).

Fieldwork is starting this spring on a chronosequence of marine terraces north of Mendocino California (Chadwick 1994). The seven terraces range in age from 3.9 ka to 240 ka and are formed on quartz-plagioclase sandstone. Vegetation on all terraces is dominated by grassland typical of the cool, Mediterranean climate. Soil development and mineral composition of these sites has been studied by Chadwick et al. At each site, soil samples will be collected by horizon and analyzed for C, ¹⁴C, nutrients, and mineral composition. We are collaborating with Markus Kleber, of Halle, Germany, on the mineral analysis.

Since minerals appear to be one of the main mechanisms behind long-term storage of soil organic carbon, this work is aimed at contributing to three long-term goals: (1) incorporation of mineral controls into terrestrial carbon models; (2) an improved mapping of soil carbon stocks and rates of soil-atmosphere C exchange; and (3) an evaluation of the potential of different soils to sequester additional carbon over the long-term.

References

Schimel, D.S., Brasswell, B.H., Holland, E., McKeown, R., Ojima, D.S., Painter, T.H., Parton, W.J. & Townsend, A.R. Climatic, edaphic, and biotic controls over storage and turnover of carbon in soils. Global Biogeochemical Cycles 8, 279-294 (1994).
Oades, J.M. The retention of organic matter in soils. Biogeochemistry 5, 35-70 (1988).

Funding

This work was partially supported by a DOE DirectorÕs LDRD grant.

For more information on this project, please contact:

Margaret Torn
Geochemistry Department
Earth Sciences Division
ph: 510-495-2223
email:mstorn@lbl.gov