Advances in Using Soft X-Ray Spectroscopy for Measurement of Soil Biogeochemical Processes

Light elements are particularly important in biogeochemical processes. These include organic matter components and macronutrients (C, N, O, S, P), micronutrients (Na, Mg, K, Mg), mineral elements (Si, Al), and transition metals. Determining the chemical speciation of these light elements in environmental samples is important for understanding bioavailability, decomposition, contamination mobility, and nutrient cycling. Soft X-ray absorption spectroscopy is a useful tool available to probe the chemistry of atoms important in biogeochemical processes. X-ray absorption spectroscopy (XAS) probes the local bonding and coordination environment of these elements in whole samples. Bulk XAS techniques permit for high throughput, the study of whole soils, and high sampling density. These analyses are complementary to X-ray transmission microscopy techniques which are limited by low throughput, thin particles (<100 nm), and low sampling density. In many projects, these bulk XAS measurements may be essential to understanding large-scale processes in soils such as the global C cycle. Despite these important applications, bulk soft XAS has not been extensively applied to environmental samples until recently. The primary reasons for this gap is the lack of beamline endstations that are suitable for "dirty" samples and the technical challenges related to acquiring and normalizing spectra from dilute samples. Many of these technical challenges have now been overcome through the development of energy-resolving detectors, proper detector positioning, and development of liquid cell applications. Technical developments and recent applications will be presented, showing how bulk soft X-ray XAS is now positioned to contribute significantly to advancing the characterization of soils and environmental samples.