An Arsenic-Hyperaccumulating Fern Mobilizes Arsenic in Rhizosphere Soil Through Processes Known to Release Phosphorus, and Prefers a Low Phosphorus Environment
Abstract
Rhizosphere processes mobilizing nutrients could also increase availability of toxic trace elements. Arsenic (As) in particular can be mobilized through processes releasing phosphorus (P) and iron (Fe), including ion exchange and reductive dissolution. To survive in As-enriched soils, the As-hyperaccumulating fern Pteris vittata takes up As and sequesters it in fern fronds. Because P. vittata nutrient and As acquisition remain poorly understood, efforts to increase As uptake rates through nutrient application in order to use P. vittata for soil remediation could have unintended consequences, including As leaching from soil.
We combined synchrotron-based X-ray absorption spectroscopy with a soil column study to determine the effects of P or mycorrhizal fungi inoculant addition to soil on As uptake and leaching in the P. vittata soil-plant system. Ferns were planted in columns packed with sandy loam soil (114 mg As/kg) treated with P or mycorrhizal fungi inoculant. Synthetic rain was eluted through columns for 22 weeks. Column porewater, effluent, and plant tissue analyses provided insight into bulk elemental cycling. To determine As and Fe speciation in rhizosphere and bulk soil, we used micro-focused As and Fe K-edge X-ray absorption spectroscopy and X-ray fluorescence elemental and chemical mapping of roots with soil grains attached (frozen whole samples and thin sections) and thin sectioned soil aggregates. Measurements were complemented with bulk X-ray absorption near edge structure (XANES) data on powdered samples. Fern As uptake outpaced loss to leaching, and was attributed mainly to As mobilization directly at the root-soil interface, not porewater transport of As. Arsenic leached from all columns, with the highest concentrations and total amount lost from P-treated soil. P treatment decreased As uptake in the fern. Arsenic and Fe XANES speciation indicated less than 30% As(III) and Fe(II) species in rhizoplane soil, but up to 80% As(III) in roots. We conclude phytoextraction can cause As leaching from bulk soil but is limited to rhizosphere soil, with As mobilized primarily via ion exchange processes that could also mobilize P, and subsequently reduced within the root.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B21L2307M
- Keywords:
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- 0409 Bioavailability: chemical speciation and complexation;
- BIOGEOSCIENCES;
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0452 Instruments and techniques;
- BIOGEOSCIENCES;
- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES