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Importance of Calcium for Stabilizing Carbon in Soil Organic Matter

Differences in carbon and calcium chemistry have been identified in calcium-organic matter complexes based on the relative location of organic matter to calcium minerals. 

A composite image showing various microscopy and elemental maps of a material sample. The top and bottom left images are grayscale microscopy views with a highlighted area. The remaining panels are color-coded elemental maps labeled C(K), O(K), Ca(K), N(K), Fe(K), and Si(K), each showing the distribution of these elements in the sample.

High-angle annular dark-field microscopy images and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy elemental maps collected on a select region near the mineral CaCO3. (Image courtesy of Tamas Varga | Environmental Molecular Sciences Laboratory)

The Science  

Soil organic matter (SOM) is a key component of terrestrial ecosystems with significant implications for soil health, agricultural productivity, water quality, biodiversity, and ecosystem resilience. Thus, it is critical to understand how SOM, and the organic carbon found within it, is retained for overall soil health (especially in marginal soils such as Aridisols, which are found in arid environments). Within this study, performed by a multi-institutional team of researchers, the chemical state of calcium (Ca) and its role in SOM stabilization was investigated in an alkaline or calcareous soil using a suite of X-ray diffraction, spectroscopy, and microscopy techniques. Ca-/calcite-organic matter (OM) associations were found to be ubiquitous in this system and appear to be critical for understanding SOM stabilization/degradation in alkaline soils.  

The Impact 

While OM-mineral interactions have been extensively studied for their ability to retain SOM, an in-depth investigation of nanoscale interactions of Ca, specifically the nature of Ca-OM complexes and calcite in alkali soils, has been lacking. Such information is essential for understanding the development and degradation of SOM, particularly in alkaline soils. Results from this study provide a mechanistic understanding of how factors such as OM chemical composition, spatial elemental structure, and interactions between OM and colloidal mineral phases control SOM properties and potentially carbon stability in arid environments, and results reveal important insights for maintaining and promoting soil health for agriculture, water quality, and ecosystem resilience. 

Summary 

Research on mineral and OM interactions thus far have primarily centered on acidic soils rich in iron (Fe) and aluminum (Al)-oxides. OM associations with common Ca minerals, especially calcite, are not as well-characterized. A multi-institutional team explored these interactions within an alkaline or calcareous Aridisol from Prosser, Washington. Recent spectroscopic and modeling studies using aqueous Ca ions or synthetic calcite with various organic compounds revealed that Ca-OM formations tend to be closely linked with calcite at the microscopic level. To gain further insights into the influence of Ca and Fe on SOM stabilization in a soil system, a team of scientists used X-ray diffraction, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, all available from the Environmental Molecular Sciences Laboratory (EMSL), a Department of Energy Office of Science user facility located at Pacific Northwest National Laboratory. Additionally, the team used synchrotron-based chemical imaging methods like scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure. The results showed that Ca- and calcite-organic associations in soil systems are ubiquitous, more complex than in model calcite systems, and crucial for understanding the stabilization or breakdown of SOM in alkaline soils. Investigations into mineralogy, speciation, and Ca-OM bridging revealed variations in carbon and Ca chemistry related to the proximity of OM to calcite. From the three different environments studied, OM near calcite crystals was rich in lipids and proteins. Ca-OM near iron oxides had significant aromatic compound content. On microbial surfaces, Ca-OM coexisted with carbonate. This research advances understanding of Ca mineral-OM interactions in alkaline soils, offering valuable insights for enhancing soil management practices. 

Contacts 

Tamas Varga 

Environmental Molecular Sciences Laboratory

tamas.varga@pnnl.gov 

 

Ravi Kukkadapu 

Environmental Molecular Sciences Laboratory

ravi.kukkadapu@pnnl.gov  

Funding 

This research was performed under the EMSL Extracellular Strategic Science Area initiative at Pacific Northwest National Laboratory. It was carried out at EMSL, a Department of Energy (DOE) Office of Science user facility sponsored by the Biological and Environmental Research program. This research also involved the use of resources from the Advanced Light Source, also a DOE Office of Science user facility. 

Publications 

T. Varga, et al. “The role of Ca-bridged organic matter in an alkaline soil, as revealed by multimodal chemical imaging.” Geoderma 456, 117256 (2025). [DOI: 10.1016/j.geoderma.2025.117256]