High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe(III) reduction by electron shuttling

Abstract

Abstract. Soil organic matter (SOM) is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals, thus serving as an electron shuttle. The standard procedure to isolate organic matter (OM) from soil involves the use of alkaline and acidic solutions and the separation of humic acids (HAs) and fulvic acids (FAs). This process potentially leads to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron-shuttling properties of SOM extracts, we prepared HA, FA, and water-extractable organic matter (OM) extracts, applying either a combination of 0.1 M NaOH and 6 M HCl or ultrapure water (pH 7), from soil samples collected from the subsoil (0–15 cm, A horizon, pH 6.5–6.8) in Schönbuch forest, Baden-Württemberg, Germany. Both chemical extractions (NaOH∕HCl) and water extractions were done in separate experiments under either oxic or anoxic conditions. Furthermore, we applied the NaOH∕HCl treatment to a subsample of the water-extractable OM to separate HA and FA from the water-extractable OM. When comparing the amount of carbon extracted from soil by different extraction methods, we found that FA and HA chemically extracted from the soil can make up to 34 %–40 % of the soil organic carbon pool while the water-extractable OM only represents 0.41 %–2.74 % of the total soil organic carbon. The higher extraction efficiency of the chemical extraction is probably due to the deprotonation of carboxyl and phenol functional groups under high pH. Anoxic extraction conditions also led to more extracted carbon. For water-extractable OM, 7 times more C was extracted under anoxic conditions compared to oxic conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the anoxic water extraction and thus the concomitant release of Fe(III) mineral-bound organic matter. To compare the redox activity of different SOM extracts, the electron-exchange capacity (EEC) of all extracted HA, FA, and water-extractable OM was analyzed and our results showed that, under anoxic extraction conditions, the HA chemically isolated from the water-extractable OM had 2 times higher EEC values compare to the water-extractable OM itself, suggesting the potential formation of redox-active aromatic functional groups during the extraction with NaOH under anoxic conditions by condensation reactions between amino acids, aldehydes, and hydroxyl- and catechol-containing molecules. We also performed a microbial Fe(III) reduction experiment with all extracts and found that higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e., faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that OM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.

Electron Hopping Enables Rapid Electron Transfer between Quinone-/Hydroquinone-Containing Organic Molecules in Microbial Iron(III) Mineral Reduction

“…With 5 mmol L –1 , 10 mmol L –1 , 25 mmol L –1 , and 50 mmol L –1 AQDS immobilized in agar, the average ferrihydrite reduction rates were 1.25 ± 0.05 mmol L –1 Fe­(II) d –1 , 1.52 ± 0.16 mmol L –1 Fe­(II) d –1 , 1.76 ± 0.07 mmol L –1 Fe­(II) d –1 , and 1.88 ± 0.11 mmol L –1 Fe­(II) d –1 , respectively (SI Figure S3). During our previous study, we showed that the diffusion coefficient of AQDS was 10 –7 cm 2 s –1 in agar, which is about ten times lower than that in water (10 –6 cm 2 s –1 ). , However, the ferrihydrite reduction rate of the incubations with AQDS immobilized in agar was comparable and in the same order of magnitude than in water (2.36 ± 1.07 mmol L –1 Fe­(II) d –1 ) . These results indicated that other electron-transfer mechanisms existed in addition to diffusion, which enhanced the overall AQDS electron-transfer rate in agar.…”

“…32,33 However, the ferrihydrite reduction rate of the incubations with AQDS immobilized in agar was comparable and in the same order of magnitude than in water (2.36 ± 1.07 mmol L −1 Fe(II) d −1 ). 34 These results indicated that other electrontransfer mechanisms existed in addition to diffusion, which enhanced the overall AQDS electron-transfer rate in agar.…”

Visible-Light Enhanced Catalytic Wet Peroxide Oxidation of Natural Organic Matter in the Presence of Al/Fe-Pillared Clay

“…Bauer et al, 2009 [56] also claimed the electron donation of NOM to be capable of reducing crystalline minerals such as goethite or hematite phases containing Fe 3+ , where specific interactions between Fe 3+ on the mineral's surface and NOM influences the electron transfer process. In addition, it has been extensively reported [57][58][59] that NOM and iron can form complexes, and the reduction of iron can occur even if the metal is part of the complex; NOM can act as a ligand towards Fe 3+ , and NOM's composition, concentration, aromatic character and presence of complex compounds highly impact the reactivity of the Fe-organic associations [53].…”

Long-Term Warming Decreases Redox Capacity of Soil Organic Matter

“…33−36 We focused our analysis on WEOM extracts as water soluble compounds represent the most bioavailable and dynamic pool of C in soil 37 and reflect bulk SOM composition better than other, more exhaustive extractions, 38,39 which have been shown to bias electrochemical analysis through the formation of redox-active functional groups. 40 We hypothesized that warming-induced changes in WEOM aromaticity would co-vary with overall electron exchange capacity (EEC), while changes in the abundance of phenols and condensed aromatics would correlate with EDC and EAC, respectively.…”

“…Long-term warming at the site resulted in a decline of fine root biomass, microbial and fungal biomass, ,, and SOM stocks (−60%). , While SOM stocks declined the most in the organic surface horizon, as evident by visible thinning, SOM composition changed most significantly in the underlying mineral horizons . To assess whether these warming-induced changes in SOM composition altered its redox capacity, we analyzed the WEOM from both heated and control soils by electrochemical flow injection analysis ,, and high resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). − We focused our analysis on WEOM extracts as water soluble compounds represent the most bioavailable and dynamic pool of C in soil and reflect bulk SOM composition better than other, more exhaustive extractions, , which have been shown to bias electrochemical analysis through the formation of redox-active functional groups . We hypothesized that warming-induced changes in WEOM aromaticity would co-vary with overall electron exchange capacity (EEC), while changes in the abundance of phenols and condensed aromatics would correlate with EDC and EAC, respectively.…”