Iron (Fe) (oxyhydr)oxides represent a significant phase for the organic carbon (OC) stabilization. Due to their high surface areas, short-range-ordered Fe minerals, like ferrihydrite, show a higher ability to stabilize OC than crystalline secondary minerals, like lepidocrocite, goethite, and magnetite. However, how Fe phases and their crystallinity relate to soil organic matter (SOM) composition is still not completely known. We investigated Fe solid phase speciation in two soil particle size fractions (i.e., fine sand — FSa — and fine silt and clay — FSi + Cl —), isolated from coniferous (CF) and broadleaved forest soils (BF), grassland soils (GL), and technosols (TS). All samples were characterized by Fe K-edge extended X-ray absorption fine structure (EXAFS) and X-ray diffraction, and a subset by ⁵⁷Fe Mössbauer spectroscopy. Further, ramped combustion thermal analyses (coupled differential scanning calorimetry (DSC) and CO₂ evolved gas analysis (CO₂-EGA)) were used to evaluated SOM stability. With the only exception of TS, goethite was the main Fe phase in FSa fractions, whereas less crystalline phases (i.e., ferrihydrite, based on EXAFS) dominated the FSi + Cl fractions. The proportion of goethite and ferrihydrite in both fractions decreased with increasing OC content, while that of Fe(III)-SOM in FSa increased with increasing OC content. Mössbauer and EXAFS data clearly indicated a presence of hematite in GL soils. Our data suggest that more crystalline Fe forms, like goethite and hematite, may be important for OC abundance in the FSa fraction and in soils with high OC contents, like GL. Thermal analysis showed the dominance of mineral associated organic matter in low-OC soils, and of plant residues in high-OC soils. As a whole, we posit that the FSi + Cl fractions contain Fe phases of less crystallinity because of presumed association with SOM, and that SOM in the FSi + Cl fraction is also more thermodynamically stable than in FSa, although differences are observed across land uses. Our observations suggest that the nature of Fe-SOM interactions can vary substantially with soil particle size and land use, which has important implication for SOM persistence.

铁 (Fe) (氢氧化物) 氧化物代表了有机碳 (OC) 稳定的重要相。由于它们的高表面积，短程有序 Fe 矿物（如水铁矿）显示出比结晶次生矿物（如纤铁矿、针铁矿和磁铁矿）更高的稳定 OC 的能力。然而，Fe 相及其结晶度与土壤有机质 (SOM) 组成的关系仍不完全清楚。我们研究了从针叶 (CF) 和阔叶林土壤 (BF)、草地土壤 (GL ) 和技术溶胶 (TS)。所有样品均通过 Fe K 边缘扩展 X 射线吸收精细结构 (EXAFS) 和 X 射线衍射进行表征，其中一个子集通过⁵⁷铁穆斯堡尔光谱。此外，斜坡燃烧热分析（耦合差示扫描量热法 (DSC) 和 CO ₂逸出气体分析（CO ₂-EGA)) 用于评估 SOM 稳定性。除了 TS，针铁矿是 FSa 馏分中的主要 Fe 相，而较少的结晶相（即基于 EXAFS 的水铁矿）在 FSi + Cl 馏分中占主导地位。两种馏分中针铁矿和水铁矿的比例随着OC含量的增加而降低，而FSa中Fe(III)-SOM的比例随着OC含量的增加而增加。Mössbauer 和 EXAFS 数据清楚地表明 GL 土壤中存在赤铁矿。我们的数据表明，更多的结晶铁形式，如针铁矿和赤铁矿，可能对 FSa 部分和高 OC 含量土壤（如 GL）中的 OC 丰度很重要。热分析显示低 OC 土壤中的矿物相关有机物和高 OC 土壤中的植物残留物占主导地位。总的来说，我们假设 FSi + Cl 馏分包含结晶度较低的 Fe 相，因为假定与 SOM 相关，并且 FSi + Cl 馏分中的 SOM 在热力学上也比 FSa 中更稳定，尽管在土地利用方面观察到差异。我们的观察表明，Fe-SOM 相互作用的性质会随着土壤颗粒大小和土地利用的不同而有很大差异，这对 SOM 的持久性具有重要意义。