Organo-mineral associations and sequestration mechanisms impacting carbon cycling in diverse terrestrial and aquatic systems
Date
2019
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Sequestration of organic matter (OM) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OM sequestration and stability in soils. In the past 20 years, research exploring the sequestration of OM to Fe oxides has intensified. Poorly crystalline Fe oxides, such as ferrihydrite, demonstrate a high affinity for OM in binary systems. Calcium commonly co-associates with OM and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the coassociation for OM sequestration remain unresolved. The biogeochemical cycling of OM in systems containing biogenically-produced Fe oxides and in permafrost conditions is also poorly understood. In an effort to gain an environmentally comprehensive understanding of C cycling in chemically heterogeneous systems, we explored the effect of calcium (Ca2+), biogenic Fe oxides, and permafrost conditions on the cycling of OM.
Extensive batch and advanced spectroscopic experiments were used to investigate the occurrence and importance of Fe, Ca, OM ternary associations. OC sorption extent to ferrihydrite in the presence of Ca2+ increased across all tested pH values, especially at pH >7. Sorbed OC concentration at pH 9 increased from 8.72 to 13.30 mmol OC g-1 ferrihydrite between treatments of no added Ca2+ and 30 mM Ca2+ addition. Batch experiments were paired with spectroscopic studies to probe OC speciation and mechanism of sorption complexes. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. STXM-NEXAFS was used to spatially resolve Fe, Ca, and OC relationships and to probe the effect of Ca on sorbed OC speciation. Organic carbon was found to highly associate with Ca (R2 = 0.91). Carboxylic acid moieties were dominantly sequestered; however, Ca facilitated the additional sequestration of aromatic and phenolic moieties.
Bacteriogenic iron (oxyhydr)oxides (BIOs), common to quiescent waterways and soil redox transitions, possess a high affinity for oxyanions (i.e., arsenate and chromate); therefore, we investigated BIOs reactivity for OM due to similar interactions with carboxylic acids. Using adsorption and desorption batch reactions, paired with Fourier transform infrared spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry, this work demonstrates that BIOs are capable of sorbing leaf litter-extracted DOM and Suwannee River Humic/Fulvic Acid (SRHA/SRFA) and have sorptive preference for distinct organic carbon compound classes at the biomineral interface. BIOs were found to sorb DOM and SRFA to half the extent of 2-line ferrihydrite per mass of sorbent and were resilient to desorption at high ionic strength and in the presence of a competitive ligand.
Similarly, we performed advanced spectroscopic experiments for permafrost collected across a chronosequence gradient in Fairbanks Alaska. We observed spatially heterogenous OM speciation in permafrost OM that ranges in age from 19,000 to 36,000 years old. Permafrost stores approximately 50% of global soil carbon and this research indicates that modeling Fe cycling with fluctuating global temperatures may greatly dictate the transfer of terrestrial OM.