Revealing the Molecular Basis for the Stability of Organic Matter in Alkaline Soils

The Science

A team of scientists examined organic matter stabilization processes in alkaline soils and found that most of the organic matter was bound within calcium aggregates, rather than adsorbing to iron or aluminum minerals, as would be expected in acidic environments.

The Impact

Understanding how soil organic matter stores carbon and nutrients and acts as a sink for atmospheric carbon dioxide is important to both agriculture and the global climate. Until now, most research has focused on organic matter’s adsorption to mineral surfaces in acidic conditions. This study adds new insight into organic matter accumulation in alkaline soils and suggests a different, unexpected stabilization mechanism.

Summary

The scientists studied alkaline soil samples taken from eastern Washington State, conducting soil sorption experiments with two types of iron-binding compounds known as siderophores: pyoverdine and enterobactin. They examined the minerology and distribution of carbon- and nitrogen-containing siderophores on individual fine particles within the soil. The results indicated the siderophores aggregated with calcium-rich organic matter coatings rather than with bare mineral surfaces. This discovery suggests an adsorption mechanism by which organics aggregate within alkaline soils via cation bridging. The mechanism results in greater sorption of the more water soluble siderophore onto soil particles and may help scientists understand the composition of organic carbon and nutrients that accumulate in alkaline soils. The experiments used methods including X-ray diffraction, ⁵⁷Fe-Mössbauer spectroscopy, X-ray photoemission spectroscopy, nanoscale secondary ion mass spectroscopy, and transmission electron microscopy. All of these capabilities are available at the Environmental Molecular Sciences Laboratory (EMSL) as part of its Biogeochemical Transformations and Isotope & Chemical Analysis Integrated Research platforms. The team also included scientists from Oregon State University, Sandia National Laboratories, and Iowa State University.

Principal Investigator(s)

Rene M. Boiteau
Oregon State University
[email protected]

Ravi Kukkadapu
Environmental Molecular Sciences Laboratory
[email protected]

Funding

This work was supported by the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, and was conducted at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility. The Linus Pauling Postdoctoral Fellowship at Pacific Northwest National Laboratory also supported the work through funds from its Laboratory-Directed Research and Development Program.

References

Boiteau, R.M., Kukkadapu, R., Cliff, J.B. et al. “Calcareous organic matter coatings sequester siderophores in alkaline soils.” Science of the Total Environment 274, 138250 (2020). [DOI:10.1016/j.scitotenv.2020.138250]