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Technical note: Uncovering the influence of methodological variations on the extractability of iron-bound organic carbon
Biogeosciences ( IF 4.9 ) Pub Date : 2021-06-07 , DOI: 10.5194/bg-18-3409-2021
Ben J. Fisher , Johan C. Faust , Oliver W. Moore , Caroline L. Peacock , Christian März

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.

中文翻译:

技术说明:揭示方法变化对铁结合有机碳的可提取性的影响

有机碳 (OC) 与活性铁 (Fe R ) 的结合代表了保护 OC 免受土壤和海洋沉积物再矿化的重要机制。最近的研究表明,有机化合物的分子结构和/或相关 Fe R 相的特性对通过柠檬酸盐-碳酸氢盐-连二亚硫酸盐 (CBD) 方法提取OC-Fe R络合物的能力施加控制。但是,使用了 CBD 提取的许多变体,并且这些变体通常彼此未校准,从而可以比较 OC-Fe R通过不同的方法提取的值是不可能的。在这里,我们创建了与简单有机结构共沉淀的合成水铁矿样品,并对最常见的 CBD 方法进行了修改。我们更改了一些方法参数(试剂浓度、提取时间和样品制备方法)并测量了 Fe R回收率,以确定哪些(如果有)修改影响了合成样品中 Fe R的释放。我们评估了 CBD 方法中 Na 连二亚硫酸盐的还原能力(固定量的连二亚硫酸盐减少的 Fe 量),发现部署的连二亚硫酸盐浓度会限制 高 Fe R沉积物的OC-Fe R可提取性内容。此外,我们表明,延长任何 CBD 提取的长度对去除 Fe R没有任何好处。此外,我们证明对于 以水铁矿为主的合成 OC-Fe R样品,冷冻干燥样品可以显着降低 OC-Fe R 的可萃取性;对于天然海洋沉积物来说,这似乎不是什么问题,因为自然老化机制可以模拟冷冻干燥过程以获得更稳定的 Fe 相。虽然我们的研究不是一个包罗万象的方法比较,也不是旨在提供“完美”的提取设置,但我们的研究结果提供了影响 CBD 提取 OC-Fe R效率的关键因素的汇总摘要。因此,我们提供了一个平台,从中 OC-Fe R 可以解释在不同方法下获得的值,并且可以在此基础上进一步研究沉积物碳循环。
更新日期:2021-06-07
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