Unraveling the persistence of deep podzolized carbon: Insights from organic matter characterization

Ryan E Champiny; Allan R Bacon; Isabella D Brush; Amy M McKenna; Daniel J Colopietro; Yang Lin

doi:10.1016/j.scitotenv.2023.167382

Unraveling the persistence of deep podzolized carbon: Insights from organic matter characterization

Sci Total Environ. 2024 Jan 1:906:167382. doi: 10.1016/j.scitotenv.2023.167382. Epub 2023 Sep 27.

Authors

Ryan E Champiny¹, Allan R Bacon², Isabella D Brush², Amy M McKenna³, Daniel J Colopietro², Yang Lin²

Affiliations

¹ Department of Soil, Water, and Ecosystem Science, University of Florida, Gainesville, FL 32611, USA. Electronic address: ryan.champiny@ufl.edu.
² Department of Soil, Water, and Ecosystem Science, University of Florida, Gainesville, FL 32611, USA.
³ National Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA; Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523, USA.

PMID: 37774867
DOI: 10.1016/j.scitotenv.2023.167382

Abstract

Over a billion tons of terrestrial carbon (C) is stored in deep soils from the Southeastern Coastal Plain of the United States. While the size and extent of this pool, known as deep podzolized carbon (DPC), have been reported in recent studies, the stabilization mechanisms responsible for its persistence are unclear. The main hypothesis of DPC stabilization is that hydrology, specifically water table fluctuations in the phreatic zone, slow microbial degradation and promote C accumulation. This accounts for the characteristic properties and distribution of DPC and provides a mechanistic distinction between DPC and shallow podzolized C in the region's soils, however it has yet to be tested. We characterized the organic matter composition of the bulk and dissolved fractions of DPC using elemental analysis, solvent extraction, infrared spectroscopy, and high-resolution mass spectrometry. Consistent with past work, the majority of DPC organic matter was extractable by sodium pyrophosphate solution; the influence of metal association was also observable in the water extractable fraction of DPC with large species being preferentially removed and a low compound diversity compared to those from other horizons overlying DPC. Only water extractable species with low molecular mass (m/z < 375 Da) showed significant change in average nominal oxidation state of carbon (NOSC) values, indicative of oxygen-limitation influence on the processing of these species. Infrared spectroscopy revealed an increase in abundance of aliphatic (C-H:C-O) bonds relative to polysaccharide bonds with depth whereas aromatic (C=C:C-O) bonds decreased with depth in DPC relative to other subsurface horizons. Our work shows that DPC is significantly more refractory than overlying surface soil C, and yet slightly more labile than the subsoils above DPC. Together our results suggest that the maintenance of low redox conditions via persistent water saturation contributes to the stabilization and persistence of DPC.

Keywords: Deep soil; FT ICR-MS; Molecular characterization; Podzolization; Subsoil.