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Humic Acid Buildup Increases Carbon Dioxide Emissions from Redox‐Oscillating Upland Soils while Catalyzing Iron(III) Reduction and Phosphorus Desorption
Author(s) -
Wilmoth Jared L.,
Sexstone Alan J.,
McDonald Louis M.
Publication year - 2019
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2019.01.0038
Subject(s) - chemistry , humic acid , environmental chemistry , soil water , biogeochemical cycle , redox , humin , inorganic chemistry , organic chemistry , fertilizer , soil science , environmental science
Humic substances comprise chemically and physically complex forms of organic C that are recognized to be involved in the catalytic cycling of electrons in diverse biogeochemical reactions. Recent investigations have indicated that humic substances existing in redox‐dynamic terrestrial environments, such as intermittently wet soils, may be uniquely important for regulating greenhouse gas emissions. However, many relevant studies have relied on the use of commercially available humic substances and/or synthetic humic analogs. In the present study, during a 42‐d anoxic incubation, we investigated the regulatory and catalytic effects of increasing concentrations of site‐specific (i.e., native) humic acid (HA) in an Appalachian upland soil known to experience redox‐oscillating conditions in the field. Extracted native HA was characterized using 13 C nuclear magnetic resonance ( 13 C‐NMR), Fourier transformed infrared (FT‐IR), and total CHNS elemental analyses. We found that native HA was relatively enriched in aromatic and amino acid C. Increasing concentrations of native HA added at the start of soil incubations led to higher rates of CO 2 emission and microbial Fe(III) reduction, indicating that a buildup of soil HA allows for a greater number of electrons to shuttle between Fe(III) reducers and Fe(III) electron acceptors during microbial oxidation of organic C. Treatments with native HA were also compared against a synthetic HA‐analog treatment containing 0.2 g anthraquinone‐2,6‐disulfonic acid (AQDS) kg−1 dry soil to investigate catalytic electron cycling by native HA and AQDS. Our data suggest that the buildup of native humic substances, which are relatively rich in aromatic and amino acid C, leads to globally relevant increases in CO 2 emissions, the redox cycling of Fe, and the availability of organic P after transition to anaerobic conditions in upland soils. Core Ideas Humic acid buildup increases CO 2 emissions from anaerobic soil. Humic acid buildup catalyzes increasing rates of electron transfer. Iron complexation and reduction by humic acid appear to be simultaneous.