We found that in a tropical soil from the Luquillo Critical Zone Observatory in Puerto Rico, different types of iron-bearing minerals appear to be associated with organic matter with differing chemical composition. A laboratory sorption experiment also found that the amount of dissolved organic matter sorbed to soil mineral matrices depleted or enriched in iron minerals was correlated to the surface area of the mineral matrix, which in turn was correlated with the concentration of these iron minerals. These findings suggest that iron-carbon associations in soils may control both the amount and the chemical composition of organic matter retained in the soil or released to ground- or steam-water.
Fe-bearing mineral phases contribute substantially to adsorption and stabilization of soil organic matter (SOM), due largely to their high specific surface area (SSA) and reactivity. While the importance of adsorption onto mineral surfaces has been well-elucidated, selectivity of various mineral and organic phases remains poorly understood. The goals of this work were to: 1) quantify the contributions of Fe-minerals of varying crystallinity to dissolved organic matter (DOM) sorption, and 2) characterize the molecular fractionation of DOM induced by reactions at the mineral interface, using a highly-weathered Oxisol from the Luquillo Critical Zone Observatory (LCZO). Three selective dissolution experiments targeting Fe-mineral phases were followed by specific surface area (SSA) analysis of the residues and characterization of extracted DOM by high resolution mass spectrometry (FT-ICR-MS). Fe-depleted extraction residue samples, untreated control soil samples, and Fe-enriched ferrihydrite-coated soil samples were then subjected to a batch sorption experiment with litter-derived DOM. Results of selective dissolution experiments indicated that a substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-MS analysis of extracted DOM revealed distinct chemical signatures. Sorbed C concentrations were well correlated with Fe contents induced by treatments, and thus SSA. Molecular characterization of the DOM post-sorption indicated that poorly crystalline Fe phases preferentially adsorbed highly unsaturated aromatic compounds, and higher-crystallinity Fe phases were associated with more aliphatic compounds. These findings suggests that molecular fractionation via organomineral complexation may act as a physicochemical filter of DOM released to the critical zone.
Plante, Alain F., Elizabeth Coward, Tsutomu Ohno, Aaron Thompson (2017): Differential chemical fractionation of dissolved organic matter during sorption by Fe mineral phases in a tropical soil from the Luquillo Critical Zone Observatory. American Geophysical Union 2017 Fall Meeting, New Orleans, Louisiana, 11-15 December 2017.
This Paper/Book acknowledges NSF CZO grant support.