Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils

Abstract

Mountain floodplain soils often show spatiotemporal variations in redox conditions that arise due to changing hydrology and resulting biogeochemistry. Under oxygen-depleted conditions, solid phase terminal electron acceptors (TEAs) can be used in anaerobic respiration. However, it remains unclear to what degree the redox properties of solid phases limit respiration rates and hence organic matter degradation. Here, we assess such limitations in soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified CO 2 production and microbial Fe(III) reduction, the main microbial metabolic pathway, as well as the reactivity of whole-soil solid phase TEAs toward mediated electrochemical reduction. Fe(III) reduction occurred together with CO 2 production in native oxic soils, while neither Fe(II) nor CO 2 production was observed in native anoxic soils. Initial CO 2 production rates increased with increasing TEA redox reactivity toward mediated electrochemical reduction across all soil depths. Low TEA redox reactivity appears to be caused by elevated Fe(II) concentrations rather than crystallinity of Fe(III) phases. Our findings illustrate that the buildup of Fe(II) in systems with long residence times limits the thermodynamic viability of dissimilatory Fe(III) reduction and thereby limits the mineralization of organic carbon.

Manganese Oxidation States in Volcanic Soils across Annual Rainfall Gradients

“…With increasing MAP from 1.5 to 5 m, the median redox condition of the Maui soils gradually shifts from oxic to suboxic (slightly oxidizing and slightly reducing) and eventually reaches anoxic conditions (Figure a). Also, the subsurface soils were more oxidizing than the surface soils (Figure S4), consistent with a recent study . Although the soils are presumably not at thermodynamic equilibrium, the changes of Mn oxidation states with mean soil E h follow the general pattern that Mn­(IV) and Mn­(II) prevail in oxidizing and reducing conditions, respectively.…”

“…Also, the subsurface soils were more oxidizing than the surface soils (Figure S4), consistent with a recent study. 48 Although the soils are presumably not at thermodynamic equilibrium, the changes of Mn oxidation states with mean soil E h follow the general pattern that Mn(IV) and Mn(II) prevail in oxidizing and reducing conditions, respectively. The abrupt increase of the Mn(II) fraction is likely caused by the switch of median E h from suboxic (E h = 400−300 mV) to anoxic (E h < 300 mV), where the occurrence of neither Mn(III) nor Mn(IV) is favorable.…”

Seasonal Oxygenation of Contaminated Floodplain Soil Releases Zn to Porewater

“…The latter can be attributed to the pH along the soil profile (5.78−6.85), which is slightly below the Zn adsorption edge for Fe(III)-(hydr)oxides, 10,40 and by the high porewater Fe(II) concentrations, which likely suppress dissimilatory Fe(III) reduction and the coupled release of Zn from goethite surfaces. 41 The low reactivity of Zn-HIV and unfavorable thermodynamics for Zn adsorption on Fe(III)-(hydr)oxides together limit repartitioning of oxidatively dissolved Zn to the solid phase.…”

“…Minimal variation in the proportion of Zn-HIV along the soil profile suggests a sparingly reactive Zn-HIV phase, consistent with the use of phyllosilicates for Zn sequestration. , Similarly, minor variation in the proportion of Zn-Gt indicates that adsorption to Fe­(III)-(hydr)­oxides and dissolution/precipitation of Fe­(III)-(hydr)­oxides do not strongly affect porewater Zn concentrations. The latter can be attributed to the pH along the soil profile (5.78–6.85), which is slightly below the Zn adsorption edge for Fe­(III)-(hydr)­oxides, , and by the high porewater Fe­(II) concentrations, which likely suppress dissimilatory Fe­(III) reduction and the coupled release of Zn from goethite surfaces . The low reactivity of Zn-HIV and unfavorable thermodynamics for Zn adsorption on Fe­(III)-(hydr)­oxides together limit repartitioning of oxidatively dissolved Zn to the solid phase.…”

Selective Removal of Organic Pollutants in Groundwater and Surface Water by Persulfate-Assisted Advanced Oxidation: The Role of Electron-Donating Capacity

“…The pore water samples were collected and quickly flushed with N 2 and sealed. In the field, we stored the sampling bottles at temperatures Prior to electrochemical examination, 26,27 the samples underwent N 2 sparging for 15 min. The terms of pore water and groundwater can be used interchangeably.…”