The role and distribution of iron (Fe) species in physical soil fractions have received remarkably little attention in field-scale systems. Here, we identify and quantify the Fe phases into two fractions (fine sand, FSa, and fine silt and clay, FSi + Cl), isolated from an agricultural soil unamended and amended with different organic materials, by Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The linear combination fitting and wavelet transform of EXAFS data revealed noticeable differences between unamended FSa and FSi + Cl fractions. Specifically, the FSi + Cl fraction was mainly characterized by ferrihydrite (48%) and Fe(III)-soil organic matter (SOM) complexes (37%), whereas in the FSa fraction, ferrihydrite still represented a major phase (44%), with a lower contribution from Fe(III)-SOM (18%). In the FSa fraction, the addition of the organic amendments resulted in an increase of Fe-SOM complexes (31-35%) and a decrease of ferrihydrite (28-29%). By contrast, in the amended FSi + Cl fractions, the added organic matter led to negligible changes in percent ferrihydrite. Therefore, regardless of the amendment type, the addition of organic matter to soil increased the capability of the coarse fraction (FSa) to stabilize organic carbon, thus pointing out that the role of FSa in carbon sequestration in agricultural soils at a global scale may be overlooked.

中文翻译：

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从生物炭和有机肥料改良过的土壤中分离出的有机物组分中的铁形态。

铁（Fe）物种在物理土壤部分中的作用和分布在田间规模系统中很少受到关注。在这里，我们将Fe相分为两部分（细砂，FSa，细粉砂和粘土，FSi + Cl），并将其定量，并通过Fe K边扩展X-射线吸收精细结构（EXAFS）光谱。EXAFS数据的线性组合拟合和小波变换揭示了未修正的FSa和FSi + Cl分数之间的显着差异。具体而言，FSi + Cl馏分的主要特征是水铁矿（48％）和Fe（III）-土壤有机物（SOM）络合物（37％），而在FSa馏分中，水铁矿仍是主要相（44％） ，其中Fe（III）-SOM的贡献较低（18％）。在FSa分数中，有机改性剂的添加导致Fe-SOM配合物的增加（31-35％）和亚铁水合物的减少（28-29％）。相比之下，在修正的FSi + Cl馏分中，添加的有机物导致亚铁水百分比的变化可忽略不计。因此，无论修改类型如何，向土壤中添加有机物都会提高粗级分（FSa）稳定有机碳的能力，因此指出FSa在全球范围内农业土壤碳固存中的作用可能是被忽略了。