A Larger than Expected Role for Calcium in Binding Soil Organic Carbon in Acidic Soils: Results from the Belowground Biogeochemistry Science Focus Area


Mike C. Rowley1,2,3 ([email protected]), Peter S. Nico1*, Sharon Bone4, Matthew A. Marcus1, Elaine Pegoraro1, Cristina Castanha1, Kyounglim Kang2, Amrita Bhattacharyya1, Margaret Torn1, Jasquelin Peña1,2*


1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of California–Davis, Davis, CA; 3Swiss National Science Foundation, Bern, Switzerland; 4Stanford Synchrotron Radiation Lightsource, Menlo Park, CA



Past research on the retention and accumulation of soil organic carbon (SOC) has focused on its biogeochemical interactions with iron (Fe) or aluminum (Al), largely overlooking a role of calcium (Ca). Recent studies have demonstrated a strong link between Ca and SOC in a range of soil types (Rasmussen et al. 2018; Yang et al. 2020), but the current conceptual model limits the role of Ca on SOC to soils with carbonates and a basic soil pH (pH > 6; Rowley et al., 2018). Working with the whole-soil warming sites of the Belowground Biogeochemistry SFA, researchers combined classical characterization methods with synchrotron-based spectromicroscopy to analyze Ca-SOC interactions in soil samples taken from the Point Reyes National Seashore and Blodgett Experimental Forest warming sites (soil pH 3.8–5.3) in California.

Both the Ca and SOC content were high in samples at Point Reyes and were correlated in multivariate analyses of the project’s classic characterization dataset. Calcium K-edge X-ray absorption near-edge structure data revealed that Ca was predominantly associated with organic matter at the site. Additionally, scanning transmission x-ray microscopy analysis from both Blodgett Forest and Point Reyes showed that Ca had a strong spatial correlation with carbon (C). Characterization of the SOC associated with Ca revealed spectral features consistent with C that had a higher proportion of aromatic and phenolic C, relative to C associated with Fe. These features in the C near-edge X-ray absorption fine structure spectra were observed in samples obtained from up to 70 cm depths at both sites. This work thus suggests that the Ca-C association observed from bulk soil analyses arises from the preservation of more plant-like products, even at depth in acidic grassland and forest soils developed on different parent material. These findings now challenge existing paradigms that these Ca-mediated processes are only found in soils with a pH > 6.5. Incubations are underway to connect the Ca-association processes to impacts on total respiration.


Rasmussen, C., et al. 2018. “Beyond Clay: Towards an Improved Set of Variables for Predicting Soil Organic Matter Content,” Biogeochemistry 137(3), 297–306. DOI:10.1007/s10533-018-0424-3.

Rowley, M. C., et al. 2018. “Calcium-Mediated Stabilisation of Soil Organic Carbon,” Biogeochemistry 137(1), 27-49. DOI:10.1007/s10533-017-0410-1.

Yang, S., et al. 2020. “Lithology Controlled Soil Organic Carbon Stabilization in an Alpine Grassland of the Peruvian Andes,” Environmental Earth Sciences 79(2), 66. DOI:10.1007/s12665-019-8796-9.