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

• Applicants: Dr. Reiner Dohrmann; Professor Dr. Christian Mikutta
• Subject Area: Soil Sciences
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 460744281

"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.

• DFG Programme: Research Grants