The persistence of organic carbon in soil is an essential ecosystem property, impacting global carbon cycling, microbial access to energy resources, and soil health. The oldest carbon in soils is associated with soil minerals, with minerals providing physical and chemical protection from microbial degradation. Thus, elucidating the associations between organic carbon and soil minerals is essential to understanding carbon residence time and persistence in soil. However, the fundamental mechanisms that control carbon storage in soils are unknown. The objective of our study was to investigate how the persistence of carbon-mineral bonds varies by mineral type and carbon substrate type in soil. We characterize these carbon-mineral associations through chemical analysis of a series of sequential extractions of mineral-associated carbon. Three mineral types – quartz, kaolinite, and ferrihydrite – were incubated in soil with growing Avena barbata plants in labeling chambers with 99 atom% 13CO2. Progressively stronger extraction reagents were used to characterize the types of carbon-mineral bonds present. Subsamples of the minerals were taken after each extraction, and analyzed for total C and 13C by Isotope Ratio Mass Spectrometry (IRMS). The extracted solutions are undergoing analysis for Total Inorganic Carbon and Total Organic Carbon (TIC/TOC) and for organometal complexes through Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
After 2.5 months of incubation in the complex experimental system, carbon accumulated on minerals to 0.34 mgC-g-1 mineral (±0.04 SE) for quartz, 0.48 mgC-g-1 mineral (±0.15 SE) for kaolinite, and 0.66 mgC-g-1 mineral (±0.15 SE) for ferrihydrite. As determined by a 13C mixing model, up to 30% of the carbon sorbed to the minerals originated from the growing plant root, and the remainder originated from the bulk soil. Ongoing work using elemental analysis (EA), Total Inorganic Carbon and Total Organic Carbon (TIC/TOC), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) will show how the presence of iron and aluminum affect these associations either present on the carbon in an organometal complex, or bound onto or mixed with the minerals themselves. This study provides a chemical characterization of carbon-mineral associations towards understanding the persistence of carbon in soils. Characterizing the nature of organic carbon bonds with mineral surfaces is critical for assessing carbon vulnerability to microbial degradation, how that mineral organic carbon contributes to soil health, and, ultimately, the basis of carbon persistence in soil.