October 12, 2017

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Synthetic Iron (Hydr)Oxide-Glucose Associations in Subsurface Soil: Effects on Decomposability of Mineral-Associated Carbon

Soil minerals stabilize highly decomposable compounds like glucose.

The Science

Recent field studies suggest that interactions with soil mineral phases can stabilize otherwise biodegradable organic matter (OM) in soils against microbial decomposition. To directly assess the effect of organo-mineral associations on an easily decomposable substrate (glucose), the research team conducted a series of laboratory incubations with well-characterized minerals (goethite and ferrihydrite) and native soils from three soil depths. Indeed, while free glucose added to soil was completely respired by microbes within 80 days, almost no glucose that had been sorbed to minerals before incorporation into soil was respired (~100% versus 0.4%, respectively).

The Impact

(1) This study provides direct evidence that even the most chemically labile organic substrates can be protected from microbial decomposition via association with mineral phases [in this case iron (hydro)oxide]. (2) These results support the emerging view that molecular structure is not the sole determinant of soil organic carbon (SOC) stability. (3) The efficacy of the laboratory approach demonstrates that microbial respired CO2 can be used as a tracer for OM desorption in soil, creating additional research opportunities.

Summary

Empirical field-based studies have provided indirect evidence of the capacity of soil minerals to stabilize organic carbon in soil. However, uncertainties remain as to the effect of mineral association on the bioavailability of organic compounds. To assess the impact of mineral association on the decomposition of glucose, an easily respirable organic substrate, a series of laboratory incubations was conducted with soils from 15, 50, and 85 cm. 13C-labeled glucose was added either directly to native soil or sorbed to one of two synthetic iron (Fe) (hydr)oxides (goethite and ferrihydrite) that differ in crystallinity and affinity for glucose. This study demonstrates that association with Fe (hydr)oxide minerals effectively reduced decomposition of glucose by ~99.5% relative to the rate of decomposition for free glucose in soil. These results emphasize the key role of mineral-organic associations in regulating the fluxes of carbon from soils to the atmosphere by enhancing the persistence of SOC stocks.

Principal Investigator

Margaret Torn
Lawrence Berkeley National Laboratory
mstorn@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was supported as part of the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, under Contract No. DE-AC02-05CH11231.

References

Porras, R. C., C. E. Hicks Pries, M. S. Torn, and P. S. Nico. "Synthetic iron (hydr)oxide-glucose associations in subsurface soil: Effects on decomposability of mineral associated carbon". Science of The Total Environment 613–614 342–351  (2017). https://doi.org/10.1016/j.scitotenv.2017.08.290.