Technical note: Uncovering the influence of methodological variations on the extractability of iron-bound organic carbon

Abstract

Abstract. Association of organic carbon (OC) with reactive iron (FeR) represents an important mechanism by which OC is protected against remineralisation in soils and marine sediments. Recent studies indicate that the molecular structure of organic compounds and/or the identity of associated FeR phases exert a control on the ability of an OC–FeR complex to be extracted by the citrate–bicarbonate–dithionite (CBD) method. However, many variations of the CBD extraction are used, and these are often uncalibrated to each other, rendering comparisons of OC–FeR values extracted via the different methods impossible. Here, we created synthetic ferrihydrite samples coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. We altered some of the method parameters (reagent concentration, time of the extraction and sample preparation methods) and measured FeR recovery to determine which (if any) modifications affected the release of FeR from the synthetic sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method (the amount of Fe reduced by a fixed amount of dithionite) and find that the concentration of dithionite deployed can limit OC–FeR extractability for sediments with a high FeR content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing FeR. Moreover, we demonstrate that for synthetic OC–FeR samples dominated by ferrihydrite, freeze-drying samples can significantly reduce OC–FeR extractability; this appears to be less of an issue for natural marine sediments where natural ageing mechanisms may mimic the freeze-drying process for more stable Fe phases. While our study is not an all-inclusive method comparison and is not aimed at delivering the “perfect” extraction setup, our findings provide a collected summary of critical factors which influence the efficiency of the CBD extraction for OC–FeR. As such, we provide a platform from which OC–FeR values obtained under different methods can be interpreted and future studies of sediment carbon cycling can build upon.

Soil Organic Matter Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS): A Critical Review of Sample Preparation, Analysis, and Data Interpretation

“…These extractions function by either reduction, chelation, or dispersion of a mineral phase, resulting in the release of SOM stabilized by that mineral. ,,− Example mineral phases include short-range order (SRO) Fe­(III) oxides, crystalline Fe­(III) oxides, aluminum oxides, and aluminosilicate clays, among others. Selection of appropriate extraction solutions has been extensively studied in SOM–mineral or mineral cycling studies. ,,,− Not as well characterized is the potential for SOM alteration by extraction conditions or interactions between the DOM and dissolved minerals. − After dissolution, new SOM–mineral interactions can be inhibited by inclusion of an appropriate buffering or stabilizing agent, such as citrate for Fe 2+ , that can complex metal ions and prevent re-adsorption or aggregation of extracted SOM. ,, All mineral extraction will require a cleanup process to remove the extracting chemicals and stabilizing agents that can affect ionization, discussed below. Finally, mineral extractions have the potential to extract compounds that overlap with water extractions and should be noted when comparing extraction yields and spectra. ,, …”

“…Once a soil sample has been collected, it is important to perform an extraction as soon as possible or store the sample to minimize biological degradation of SOM . If extraction soon after sampling is impossible, it is recommended that soil samples be treated as similarly as possible to maintain their comparability, as no storage method can perfectly preserve the in situ conditions. , Before extraction, soil sample storage by freezing or drying can introduce bias from aggregation of the soil or SOM, ,, which can alter the structure of the soil matrix, reducing exposure to or contact with the extracting chemicals, ,, and has shown variable carbon yield , when compared to “wet” or in situ samples.…”

“…75−77 After dissolution, new SOM−mineral interactions can be inhibited by inclusion of an appropriate buffering or stabilizing agent, such as citrate for Fe 2+ , that can complex metal ions and prevent re-adsorption or aggregation of extracted SOM. 76,78,79 All mineral extraction will require a cleanup process to remove the extracting chemicals and stabilizing agents that can affect ionization, discussed below. Finally, mineral extractions have the potential to extract compounds that overlap with water extractions and should be noted when comparing extraction yields and spectra.…”

Effects of Hypoxia on Coupled Carbon and Iron Cycling Differ Between Weekly and Multiannual Timescales in Two Freshwater Reservoirs

“…(2012) and has since been adapted for freshwater lakes by Peter & Sobek, 2018. It is important to note that our measurement of Fe‐OC as the percentage of OC that is extractable using the CBD method is an operational definition (Fisher et al., 2021). CBD extractions have documented inefficiencies when extracting crystalline hematite (Adhikari & Yang, 2015; Thompson et al., 2019) and carboxyl‐rich compounds (Fisher et al., 2020).…”

“…We analyzed the amount of Fe-OC in both the whole-ecosystem and microcosm sediment samples using the citrate bicarbonate dithionite (CBD) method (Figure S3 in Supporting Information S1). This method was first described for marine systems by Lalonde et al (2012) and has since been adapted for freshwater lakes by Peter & Sobek, 2018. It is important to note that our measurement of Fe-OC as the percentage of OC that is extractable using the CBD method is an operational definition (Fisher et al, 2021). CBD extractions have documented inefficiencies when extracting crystalline hematite (Adhikari & Yang, 2015;Thompson et al, 2019) and carboxyl-rich compounds (Fisher et al, 2020).…”