Advanced spectroscopic, microscopic, and tomographic characterization techniques to study biogeochemical interfaces in soil

Abstract

Biogeochemical interfaces, the 3D association of minerals, soil organic matter, and biota, are hotspots of soil processes because they exhibit strong biological, physical, and chemical gradients. Biogeochemical interfaces have thicknesses from nanometers to micrometers and separate bulk immobile phases from mobile liquid or gaseous phases. The aim of this contribution is to review advanced microscopic and spectroscopic characterization techniques that allow for spatially resolved analysis of composition and properties of biogeochemical interfaces or their visualization. From the variety of techniques to study biogeochemical interfaces in soil, we focus on X-ray spectromicroscopy, nano-scale secondary ion mass spectrometry, atomic force microscopy, micro-X-ray tomography, and positron emission tomography. Beside an introduction into the respective method, we review published applications and give practical examples. The development of terrestrial soils involves the formation of biogeochemical interfaces as the result of the complex 3D interplay of primary and secondary minerals, soil organic matter together with soil biota. X-ray microscopy allows for the visualization of structures down to range of 10–30 nm and for the determination of binding states of elements. Nano-scale secondary ion mass spectrometry is capable of simultaneously analyzing up to seven secondary ion species to give the elemental and isotopic composition down to 50–150 nm. Atomic force microscopy enables to study the topography and mechanical properties (softness, elasticity, plasticity, deformability) of soil particle surfaces down to the nm scale. X-ray micro-tomography has been shown to visualize the interior of materials at the sub-micrometer scale successfully. Introducing and adapting the discussed methods in soil science has increased the understanding of formation, properties, and functioning of biogeochemical interfaces in soil. A further challenging task is to utilize further promising techniques, e.g., advanced Raman techniques or atomic probe tomography with the highest spatial resolution for 3D compositional information of any microscopy technique.