Active Iron Phases Regulate the Abiotic Transformation of Organic Carbon during Redox Fluctuation Cycles of Paddy Soil

Abstract

Iron (Fe) phases are tightly linked to the preservation rather than the loss of organic carbon (OC) in soil; however, during redox fluctuations, OC may be lost due to Fe phase-mediated abiotic processes. This study examined the role of Fe phases in driving hydroxyl radical (•OH) formation and OC transformation during redox cycles in paddy soils. Chemical probes, sequential extraction, and Mössbauer analyses showed that the active Fe species, such as exchangeable and surface-bound Fe and Fe in low-crystalline minerals (e.g., green rust-like Fe phases), predominantly regulated •OH formation during redox cycles. The •OH oxidation strongly induced the oxidative transformation of OC, which accounted for 15.1–30.8% of CO2 production during oxygenation. Microbial processes contributed 7.3–12.1% of CO2 production, as estimated by chemical quenching and γ-irradiation experiments. After five redox cycles, 30.1–71.9% of the OC associated with active Fe species was released, whereas 5.2–7.1% was stabilized by high-crystalline Fe phases due to the irreversible transformation of these active Fe species during redox cycles. Collectively, our findings might unveil the under-appreciated role of active Fe phases in driving more loss than conservation of OC in soil redox fluctuation events.

Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils

“…Aliquots (900 μL) were collected from the supernatant at designed time points and immediately mixed with methanol (100 μL) to scavenge • OH production in subsequent processes. 35 Samples were stored in a heat-insulated box filled with an ice chest and analyzed within 48 h.…”

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation

“…•− ), 1 hydrogen peroxide (H 2 O 2 ), 1−4 and hydroxyl radicals (•OH) 5−7 has been increasingly recognized at oxic−anoxic interfaces in subsurface environments. Particularly, •OH (standard reduction potential: 2.8 V 8 ) is the most powerful ROS in natural systems which plays an important role in biogeochemical element cycles, such as greenhouse gas emission (e.g., CO 2 and CH 4 ) 5,9,10 and natural attenuation of contaminants. 6,11−13 Generally, ferrous iron (Fe(II)) and reduced natural organic matter (NOM) are considered to be the main contributors to O 2 reduction and •OH formation in the subsurface.…”

Iron (hydr)oxide formation in Andosols under extreme climate conditions

“…1b). We address this relationship to a decrease in iron associated with labile OM compounds related to changes in redox state (Fe 2+/3+ ) and increasing Fe-(hydr)oxide crystallinity 10,39,40 , sharply contrasting with the behavior of aluminum. The spatial distribution of (sub)micrometer-scale Fe-(hydr)oxides within the soil aggregates can be attributed to this increase in crystallinity.…”

“…Recent studies highlight that redox (trans)formations of iron in soils are controlled by a complex cascade of abiotic and biotic processes 2,6,11 . The reduction and oxidation of iron by microorganisms should be inseparably linked to the amount of organic ligands (OL) and DOC released from intense OM turnover 12,13,35,40 as well as to the cycling of speci c nutrients (especially N, P and S) 10,43 . This suggests an e cient interaction of the iron redox cycling with the simultaneously formed Al-humus complexes and the dominant oxyanions adsorbed, e.g., NO x − , PO 4 3− , SO 4 2− .…”