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

“x射线无定形”固体的特点是晶体尺寸极小，晶格畸变明显或原子短程有序。与结晶固体相比，“x射线无定形”固体具有漫射x射线衍射模式，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（iXAMs）以玻璃相的形式存在，其晶体中重复结构单元太少而无法衍射x射线的矿物（“低结晶”或“纳米晶”矿物），以及化学成分可变的固体，只具有原子短程秩序（“类矿物”）。由于其较大的比表面积和活性表面基团，ixam控制着重要的土壤过程，如碳周转、矿物风化、营养物质和污染物的吸附反应。尽管它们具有重要的生态意义，但人们对土壤ixam的了解仍然很少。特别是在它们的性质、总含量、化学成分和在土壤中的分布以及它们使用湿化学提取方法的可量化性方面存在知识空白。为了填补这些知识空白，我们通过定量x射线衍射（Rietveld法）量化了细土（<2 mm）和四种土壤类型（Cambisol, Chernozem, Luvisol, Podsol）颗粒级的iXAMs。iXAMs的化学成分由基于Rietveld结果和土壤样品化学分析（“资产负债表法”）的质量平衡确定。在此基础上，研究了常用的选择性提取方法对“x射线无定形”土壤固体含量测定的适用性，以定量测定其在土壤中的绝对含量和化学成分。此外，我们利用分析透射电子显微镜从选定的粒径分数中探索了iXAMs （<1 μ m）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的总含量、性质和化学组成以及随深度的分布；(2)通过选择性提取方法定量分析iXAM。因此，该项目为今后详细研究ixam对土壤功能和性质的影响奠定了基础。

"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.