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15 October 2020

Root induced soil deformation influences Fe, S and P: rhizosphere chemistry investigated using synchrotron XRF and XANES

Root
Top left: Sulfur map, showing the distribution of sulfur in the mixed phase/pore space and the stark enrichment inside the root (highlighted in yellow circle). Top right: Mineral composite map showing the complex nature of the chemical distribution within the soil (Fe – yellow, S – blue, P – green, Si – grey, Al – red, Mg – cyan). Bottom left: XANES spectra of some of the point location. Bottom right: Linear combination fits (LCF’s) of the collected XANES with distance from the root surface. The LCF’s clearly show the gradual changes in speciation with increasing distance from the root surface. Both images were collected at the PHOENIX beamline of the Swiss Light Source. Scalebar = 500 µm.

Soil resources are usually inaccessible to plants, because of the tightly bound nature of nutrients to soil minerals and organic matter. Roots can overcome this by engineering their direct environment, the rhizosphere. Roots significantly augment rhizosphere soil structure that is beneficial, such as bulk density and porosity. However, the chemical alterations have not been extensively investigated in situ. This study combines structural information, previously obtained with synchrotron X‐ray computed tomography (XCT), with XRF microscopy and XANES spectroscopy to unravel chemical changes induced by plant roots. Our results suggest that iron (Fe) and sulfur (S) increase notably in the direct vicinity of the root via solubilization and microbial activity. XANES further shows that Fe is slightly reduced, S is increasingly transformed into sulfate (SO42−) and phosphorus (P) is increasingly adsorbed to humic substances in this enrichment zone. Moreover, the ferrihydrite fraction decreases drastically, suggesting the preferential dissolution and the formation of more stable Fe oxides. Additionally, the increased transformation of organic S to sulfate indicates that the microbial activity in this zone is increased. These changes in soil chemistry correspond to the soil compaction zone as previously measured via XCT. The fact that these changes are co-located near the root and the compaction zone suggests that decreased permeability as a result of soil structural changes acts as a barrier creating a zone with increased rhizosphere chemical interactions via surface‐mediated processes, microbial activity and acidification. The measurements have been taken at the PHOENIX beamline of the Swiss Light source and beamline I18 of the Diamond Light Source.

Contact

Dr. Arjen van Veelen
Los Alamos National Laboratory
SLAC National Accelerator Laboratory
Telephone: +1 505 695 6589
E-mail: arjen.vanveelen@lanl.gov
 
Prof. Tiina Roose
University of Southampton
E-mail: t.roose@soton.ac.uk
 
Dr. Thomas Huthwelker
Swiss Light Source
Paul Scherrer Institut
Telephone: +41 56 310 5314
E-mail: thomas.huthwelker@psi.ch
 

Original Publication

Root‐induced soil deformation influences Fe, S and P: rhizosphere chemistry investigated using synchrotron XRF and XANES.
Arjen van Veelen, Nicolai Koebernick, Callum S. Scotson, Daniel McKay‐Fletcher, Thomas Huthwelker, Camelia N. Borca, J. Fred W. Mosselmans, Tiina Roose.
New Phytol, 2020, 225: 1476-1490
DOI: 10.1111/nph.16242

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