Abundance and composition of inorganic X-ray amorphous materials in soils

“X射线无定形”固体的特征在于极小的晶体尺寸、明显的晶格畸变或原子短程有序。与结晶固体相比，“X-射线无定形”固体具有漫射X-射线衍射图案，因此在X-射线衍射图中作为升高的信号背景保持“可见”。在土壤中，无机X射线无定形材料（iXAM）以玻璃相存在，矿物的晶体具有太少的重复结构单元而不能吸收X射线（“弱结晶”或“纳米晶体”矿物），以及具有可变化学组成的固体，其仅具有原子短程有序（“类矿物”）。由于其大的比表面积和反应性表面基团，iXAM控制着重要的土壤过程，如碳周转、矿物风化以及养分和污染物的吸附反应。尽管其生态意义，土壤iXAM仍然知之甚少。特别是在其性质、总含量、化学成分和在土壤中的分布以及使用湿化学提取法的可量化性方面存在知识差距。为了填补这些知识空白，我们通过定量X射线衍射（Rietveld方法）量化了细土（<2 mm）和四种土壤类型（Cambisol，Cauczem，Luvisol，Podsol）的粒度级中的iXAM。iXAM的化学成分是根据Rietveld结果和土壤样品的化学分析通过质量平衡确定的（“平衡表法”）。在此基础上，我们探讨了常见的选择性提取方法的适用性，用于测定“X射线无定形”土壤固体定量测定其绝对含量和化学组成的土壤。此外，我们使用分析透射电子显微镜从选定的粒度级中探索iXAM（<1 µm）的性质和组成。总体而言，该研究项目提供了以下基本信息：（1）土壤中iXAM总含量、其性质和化学成分以及深度依赖性分布;（2）通过选择性提取方法对iXAM进行定量。因此，该项目构成了未来详细调查iXAMs对土壤功能和性质影响的基础。

"X-ray amorphous" solids are characterized by extremely small crystal sizes, pronounced lattice distortions or atomic short-range order. In comparison to crystalline solids, "X-ray amorphous" solids possess diffuse X-ray diffraction patterns and thus remain "visible" as elevated signal background in X-ray diffractograms. In soils, inorganic X-ray amorphous materials (iXAMs) exist as vitreous phases, minerals whose crystals have too few repeating structural units to diffract X-rays ("poorly crystalline" or "nanocrystalline" minerals), and solids of variable chemical composition possessing exclusively atomic short-range order ("mineraloids"). Due to their large specific surface areas and reactive surface groups, iXAMs control important soil processes such as carbon turnover, mineral weathering, and sorption reactions of nutrients and pollutants. Despite their ecological significance, soil iXAMs are still poorly understood. Knowledge gaps exist particularly with regard to their nature, total contents, chemical composition, and distribution in soils as well as their quantifiability using wet chemical extraction methods. To fill these knowledge gaps, we quantify iXAMs in the fine earth (<2 mm) and in particle-size fractions of four soil types (Cambisol, Chernozem, Luvisol, Podsol) by quantitative X-ray diffraction (Rietveld method). The chemical composition of the iXAMs is determined by mass balances based on the Rietveld results and chemical analyses of the soil samples ("balance sheet method"). On this basis, we investigate the suitability of common selective extraction methods for the determination of "X-ray amorphous" soil solids to quantitatively determine their absolute contents and chemical composition in soils. In addition, we explore the nature and composition of iXAMs (<1 µm) from selected particle-size fractions using analytical transmission electron microscopy. Overall, the research project provides basic information (1) on total iXAM contents in soils, their nature and chemical composition as well as depth-dependent distribution and (2) on the quantification of iXAMs by selective extraction methods. Thus, this project forms the basis for a detailed investigation of the influence of iXAMs on soil functions and properties in the future.