Influence of iron redox cycling on organo-mineral associations in Arctic tundra soil

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Herndon, Elizabeth, AlBashaireh, Amineh, Singer, David, Chowdhury, Taniya Roy, Gu, Baohua, Graham, David
• Language: unknown
• Published: 2023
• Subjects: 54 ENVIRONMENTAL SCIENCES; 59 BASIC BIOLOGICAL SCIENCES; Arctic; north slope; Tundra; Alaska
• Online Access: http://www.osti.gov/servlets/purl/1356942; https://www.osti.gov/biblio/1356942; https://doi.org/10.1016/j.gca.2017.02.034

Arctic tundra stores large quantities of soil organic matter under varying redox conditions. As the climate warms, these carbon reservoirs are susceptible to increased rates of decomposition and release to the atmosphere as the greenhouse gases carbon dioxide (CO 2 ) and methane (CH 4 ). Geochemical interactions between soil organic matter and minerals influence decomposition in many environments but remain poorly understood in Arctic tundra systems and are not considered in decomposition models. The accumulation of iron (Fe) oxyhydroxides and organo- iron precipitates at redox interfaces may be particularly important for carbon cycling given that ferric iron [Fe(III)] species can enhance decomposition by serving as terminal electron acceptors in anoxic soils or inhibit microbial decomposition by binding organic molecules. Here in this paper, we examine chemical properties of solid-phase Fe and organic matter in organic and mineral horizons within the seasonally thawed active layer of Arctic tundra on the North Slope of Alaska. Spectroscopic techniques, including micro-X-ray fluorescence ( XRF) mapping, micro-X-ray absorption near-edge structure ( XANES) spectroscopy, and Fourier transform infrared spectroscopy (FTIR), were coupled with chemical sequential extractions and physical density fractionations to evaluate the spatial distribution and speciation of Fe-bearing phases and associated organic matter in soils. Organic horizons were enriched in poorly crystalline and crystalline iron oxides, and approximately 60% of total Fe stored in organic horizons was calculated to derive from upward translocation from anoxic mineral horizons. Ferrihydrite and goethite were present as coatings on mineral grains and plant debris, and in aggregates with clays and particulate organic matter. Minor amounts of ferrous iron [Fe(II)] were present in iron sulfides (i.e., pyrite and greigite) in mineral horizon soils and iron phosphates (vivianite) in organic horizons. Concentrations of organic carbon in the organic horizons (28 ± ...