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 exerts a control on the ability of an OC-FeR complex to be extracted by the citrate-bicarbonate-dithionite (CBD) method. While many variations of this method exist in the literature, these are often uncalibrated to each other, rendering comparisons of OC-FeR values extracted by different method iterations impossible. Here, we created a synthetic ferrihdyrite sample coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. Method parameters (reagent concentration, time of the extraction and sample preparation methods) were altered and FeR recovery measured to determine which (if any) modifications resulted in the greatest release of FeR from the sediment sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method 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. Finally, we demonstrate that for synthetic OC-FeR samples, the almost universal technique of freeze drying samples can significantly reduce OC-FeR extractability and we offer insight into how this may translate to environmental samples using Arctic Ocean sediments. These results provide a valuable perspective on how the efficiency of this extraction could be improved to provide a more accurate assessment of sediment OC-FeR content. Accurate determinations of OC-FeR in sediments and soils represents an important step in improving our understanding of, and ability to model, the global carbon cycle.

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).…”