Advances in the use of synchrotron-based spatially resolved imaging and spectroscopy to speciate phosphorus in soils
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Phosphorus (P) is an essential plant nutrient. Unfortunately, P loss to aquatic ecosystems can be detrimental to aquatic life and water quality due to eutrophication caused by nutrient enrichment. By determining the solid-phase P species present in soils, we will be able to better understand P availability to plants and the environmental susceptibility to nutrient enrichment of aquatic ecosystems. Although P in soil has been extensively researched, direct evidence of the solid phase species is limited to only a few studies due to limitations in direct speciation techniques. Recent advances in synchrotron-based light sources are now allowing us to advance direct solid phase soil P speciation with higher precision and accuracy. I used high resolution, micro-X-ray fluorescence (-XRF) mapping of two agricultural soils located on the Delmarva Peninsula (Delaware, USA) with high total P (excess of 800 mg/kg). Using these samples, µ-XRF maps of the same soil thin section and powder mount locations were completed at the National Synchrotron Light Source II (NSLS-II) using beamlines 8-BM (TES, 2-6 keV) and 4-BM (XFM, 5-20 keV). By utilizing the two beamlines, we were able to successfully demonstrate the spatial distribution and co-location of P with Al, Si, S (8-BM), Fe, Ca, Mn, and As (4-BM). Following µ-XRF mapping, we used beamline 8-BM to probe P hotspots using P K-edge micro-X-ray absorption near edge structure (µ-XANES) to determine P speciation at the micron scale. To compliment direct solid phase P speciation, this research utilized multiple tools to indirectly investigate the chemical forms and stability of legacy soil P. The indirect methods include batch desorption kinetics and Hedley sequential extraction to investigate P contributions from different pools. Using the bulk methods paired with examining P speciation at the micron scale will help give a more complete, high resolution picture of fundamental soil P chemistry. A better understanding of the microsite P speciation may help determine P susceptibility to dissolution and may therefore eventually allow for improved fertilizer recommendations, updated best management practices, and informed remediation strategies to prevent P losses to aquatic environments.
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Keywords
Micro-spectroscopy, Phosphorus, Synchrotron, XANES