Association between soil organic carbon and calcium in acidic grassland soils from Point Reyes National Seashore, CA

Mike C Rowley; Peter S Nico; Sharon E Bone; Matthew A Marcus; Elaine F Pegoraro; Cristina Castanha; Kyounglim Kang; Amrita Bhattacharyya; Margaret S Torn; Jasquelin Peña

doi:10.1007/s10533-023-01059-2

Association between soil organic carbon and calcium in acidic grassland soils from Point Reyes National Seashore, CA

Biogeochemistry. 2023;165(1):91-111. doi: 10.1007/s10533-023-01059-2. Epub 2023 Jul 7.

Authors

Mike C Rowley^{1

2

3}, Peter S Nico^#², Sharon E Bone⁴, Matthew A Marcus⁵, Elaine F Pegoraro², Cristina Castanha², Kyounglim Kang³, Amrita Bhattacharyya², Margaret S Torn², Jasquelin Peña^#^{2

3}

Affiliations

¹ Department of Geography, University of Zurich, Zurich, Switzerland.
² Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 USA.
³ University California Davis, Davis, USA.
⁴ Stanford Synchrotron Radiation Lightsource, Menlo Park, USA.
⁵ Advanced Light Source, Berkeley, USA.

^# Contributed equally.

Abstract

Organo-mineral and organo-metal associations play an important role in the retention and accumulation of soil organic carbon (SOC). Recent studies have demonstrated a positive correlation between calcium (Ca) and SOC content in a range of soil types. However, most of these studies have focused on soils that contain calcium carbonate (pH > 6). To assess the importance of Ca-SOC associations in lower pH soils, we investigated their physical and chemical interaction in the grassland soils of Point Reyes National Seashore (CA, USA) at a range of spatial scales. Multivariate analyses of our bulk soil characterisation dataset showed a strong correlation between exchangeable Ca (Ca_Exch; 5-8.3 c.mol_c kg^-1) and SOC (0.6-4%) content. Additionally, linear combination fitting (LCF) of bulk Ca K-edge X-ray absorption near-edge structure (XANES) spectra revealed that Ca was predominantly associated with organic carbon across all samples. Scanning transmission X-ray microscopy near-edge X-ray absorption fine structure spectroscopy (STXM C/Ca NEXAFS) showed that Ca had a strong spatial correlation with C at the microscale. The STXM C NEXAFS K-edge spectra indicated that SOC had a higher abundance of aromatic/olefinic and phenolic C functional groups when associated with Ca, relative to C associated with Fe. In regions of high Ca-C association, the STXM C NEXAFS spectra were similar to the spectrum from lignin, with moderate changes in peak intensities and positions that are consistent with oxidative C transformation. Through this association, Ca thus seems to be preferentially associated with plant-like organic matter that has undergone some oxidative transformation, at depth in acidic grassland soils of California. Our study highlights the importance of Ca-SOC complexation in acidic grassland soils and provides a conceptual model of its contribution to SOC preservation, a research area that has previously been unexplored.

Supplementary information: The online version contains supplementary material available at 10.1007/s10533-023-01059-2.

Keywords: Acidic grassland soils; Complexation; Micro-XANES; Organo-metal interactions; STXM C NEXAFS; Soil organic carbon.