Unveiling the Importance of Calcite Dissolution for Stabilizing Soil Organic Matter

The Science  

Soils that contain highly reactive minerals, such as calcite that can easily dissolve, accumulate substantially higher levels of soil organic matter (SOM) compared to soils with lower levels of these minerals. A multi-institutional team of researchers investigated the chemical composition, coordination, and stability of calcium–organic associations that form when calcite mineral is exposed to organic matter. The results demonstrate that these associations are formed in the presence of dissolving calcite and organic matter components such as lignin and amino acids. The research team also found solution-derived (bi)carbonate in Ca–organic associations, indicating for the first time that an inorganic anion, such as (bi)carbonate, may be part of these associations. This research revealed that calcite dissolution and subsequent release of calcium promotes calcium–(bi)carbonate–organic associations with greater chemical stability and, possibly, higher environmental stability compared to non-calcium-bound organic matter. 

The Impact 

Carbon reservoirs play an important role in the Earth’s carbon cycle. These reservoirs take many forms, from fossil fuels to the carbon stored in ocean waters and sediments. SOM represents the Earth’s largest terrestrial carbon reservoir. Maintaining the stability of this carbon reservoir is essential for ensuring that carbon dioxide emissions into the atmosphere do not increase. The formation of calcium–(bi)carbonate–organic associations resulting from the interaction of organic matter components and calcite contributes to enhancing SOM's stability. This study reveals valuable details about the chemical composition, coordination, and stability of calcium–(bi)carbonate–organic associations, providing critical insight into the long-term stability of SOM. 

Summary 

A multi-institutional team of researchers exposed calcite to organic matter components, i.e., a mixture of lipids, amino acid, carbohydrate, and lignin, to form calcium–organic complexes. The chemical composition of these complexes was investigated with advanced spectro-microscopic techniques. Scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy, were used to interrogate the chemical composition and elemental spatial distribution of the complexes. These analyses revealed significant calcium content as well as bicarbonate and organic carbon within the aggregate complexes. The samples were then probed using infrared scattering scanning nearfield optical microscopy at the Environmental Molecular Sciences Laboratory, a Department of Energy (DOE), Office of Science user facility. The combined techniques provided molecular-level evidence that calcium ions were associated with organic and inorganic (bi)carbonate molecules and information on the chemical nature of the organic functional groups in these associations. The results indicate that SOM stabilized through the formation of calcium–(bi)carbonate–organic associations has a higher stability than SOM that is not bound by calcite. This stabilization is critical to assuring that atmospheric emissions of carbon dioxide from soil organic carbon reservoirs do not increase. 

Contact 

Odeta Qafoku 
​​Environmental Molecular Sciences Laboratory​​ 
odeta.qafoku@pnnl.gov 

Funding 

This research was performed on a project award from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Biological and Environmental Research program. This research also used resources at the Advanced Light Source, also a DOE Office of Science user facility. 

Publication

Qafoku, O., et al. (2023). Chemical composition, coordination, and stability of Ca–organic associations in the presence of dissolving calcite. Environmental Science: Nano Vol. 10, Issue 5, pp. 1504–1517. Royal Society of Chemistry (RSC). [DOI: 10.1039/d2en01143c]