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

• Main Authors: B. J. Fisher, J. C. Faust, O. W. Moore, C. L. Peacock, C. März
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-06-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/3409/2021/bg-18-3409-2021.pdf

<p>Association of organic carbon (OC) with reactive iron (Fe<span class="inline-formula">_R</span>) 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 Fe<span class="inline-formula">_R</span> phases exert a control on the ability of an OC–Fe<span class="inline-formula">_R</span> 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–Fe<span class="inline-formula">_R</span> 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 Fe<span class="inline-formula">_R</span> recovery to determine which (if any) modifications affected the release of Fe<span class="inline-formula">_R</span> 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–Fe<span class="inline-formula">_R</span> extractability for sediments with a high Fe<span class="inline-formula">_R</span> content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing Fe<span class="inline-formula">_R</span>. Moreover, we demonstrate that for synthetic OC–Fe<span class="inline-formula">_R</span> samples dominated by ferrihydrite, freeze-drying samples can significantly reduce OC–Fe<span class="inline-formula">_R</span> 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–Fe<span class="inline-formula">_R</span>. As such, we provide a platform from which OC–Fe<span class="inline-formula">_R</span> values obtained under different methods can be interpreted and future studies of sediment carbon cycling can build upon.</p>