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

Abstract

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