December 19, 2022

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Do Soil Minerals Protect or Degrade Organic Matter?

Study reveals that manganese oxides can both stabilize and destabilize small organic compounds.

Small organic molecules exhibit varied interactions with manganese (Mn) oxides that either stabilize carbon (C) through sorption to the mineral surface (e.g., phthalate) or destabilize C through oxidative degradation to form carbon dioxide (CO2).

[Reprinted with permission from Li, H., et al. "Effects of C/Mn Ratios on the Sorption and Oxidative Degradation of Small Organic Molecules on Mn-Oxides." Environmental Science & Technology 57 (1), 741–50 (2022). DOI:10.1021/acs.est.2c03633. Copyright 2023 American Chemical Society.]

The Science

Much of the organic carbon (C) stored in soils is associated with soil minerals. Therefore, understanding how soil minerals interact with a variety of organic compounds is essential to anticipating soil C storage and fluxes and their contributions to global climate change. Most studies examining C stabilization by soil minerals have focused on iron and aluminum oxides without considering the importance of less abundant manganese (Mn) oxides that have high sorption capacity and reactivity. This study demonstrates that organic compounds experience varied interactions with Mn oxides that primarily result in degradation but can also lead to organic C stabilization on the mineral surface.

The Impact

The study indicates that Mn oxides effectively oxidize organic compounds to release CO2 but also demonstrate a high capacity to adsorb and immobilize organic compounds. These stabilizing and destabilizing interactions may influence soil C storage and transformation.

Summary

Mn oxides are reactive soil minerals that can bind or oxidize organic compounds, but their role in regulating soil C storage is relatively unexplored. To better understand Mn-C interactions, researchers reacted five small organic compounds with Mn oxides to evaluate the potential for organic C to either bind to and be stabilized on the mineral surface or to be destabilized through oxidation reactions that produce carbon dioxide (CO2) gas. Mn-C interactions primarily resulted in organic C oxidation coupled to Mn oxide dissolution, although select compounds attached to the mineral surface without transformation. Also, a high proportion of organic C was degraded at low C/Mn ratios while increasing proportions were immobilized in solids at high C/Mn ratios.

Principal Investigator

Elizabeth Herndon
Oak Ridge National Laboratory
[email protected]

Program Manager

Jennifer Arrigo
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was sponsored by the Laboratory-Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, project number 9958. Support was also received from the Critical Interfaces Science Focus Area sponsored by the Biological and Environmental Research (BER) Program within DOE’s Office of Science and from the U.S. Department of Agriculture-National Institute of Food and Agriculture (USDA-NIFA), Hatch Project NC02898. This research used resources at Beamline 12-BM of the Advanced Photon Source, a user facility operated for DOE’s Office of Science by Argonne National Laboratory under contract DE-AC02-06CH11357. This work was performed in part by the Molecular Education, Technology and Research Innovation Center at North Carolina State University.

References

Li, H., et al. "Effects of C/Mn Ratios on the Sorption and Oxidative Degradation of Small Organic Molecules on Mn-Oxides." Environmental Science & Technology 57 (1), 741–50  (2022). https://doi.org/10.1021/acs.est.2c03633.