Pteris vittata (PV) and Pteris quadriaurita (PQ) are reported to hyperaccumulate arsenic (As) when grown in Asrich soil. Yet, little is known about the impact of their unique As accumulation mechanisms on As transformations and cycling at the soil-root interface. Using a combined approach of two-dimensional (2D), sub-mm scale solute imaging of arsenite (As III ), arsenate (As V ), phosphorus (P), manganese (Mn), iron (Fe) and oxygen (O 2 ), we found localized patterns of As III /As V redox transformations in the PV rhizosphere (As III /As V ratio of 0.57) compared to bulk soil (As III /As V ratio of ≤0.04). Our data indicate that the high As root uptake, translocation and accumulation from the As-rich experimental soil (2080 mg kg -1 ) to PV fronds (6986 mg kg -1 ) induced As detoxification via As V reduction and As III root efflux, leading to As III accumulation and re-oxidation to As V in the rhizosphere porewater. This As cycling mechanism is linked to the reduction of O2 and Mn III/IV (oxyhydr)oxides resulting in decreased O2 levels and increased Mn solubilization along roots. Compared to PV, we found 4-fold lower As translocation to PQ fronds (1611 mg kg -1 ), 2-fold lower As V depletion in the PQ rhizosphere, and no As III efflux from PQ roots, suggesting that PQ efficiently controls As uptake to avoid toxic As levels in roots. Analysis of root exudates obtained from soil-grown PV showed that As acquisition by PV roots was not associated with phytic acid release. Our study demonstrates that two closely-related As-accumulating ferns have distinct mechanisms for As uptake modulating As cycling in As-rich environments.

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Synchrotron-based X-ray fluorescence microscopy (XFM) analysis is a powerful technique that can be used to visualize elemental distributions across a broad range of sample types. Compared to conventional mapping techniques such as laser ablation inductively coupled plasma mass spectrometry or benchtop XFM, synchrotron-based XFM provides faster and more sensitive analyses. However, access to synchrotron XFM beamlines is highly competitive, and as a result, these beamlines are often oversubscribed. Therefore, XFM experiments that require many large samples to be scanned can penalize beamline throughput. Our study was largely driven by the need to scan large gels (170 cm 2 ) using XFM without decreasing beamline throughput. We describe a novel approach for acquiring two sets of XFM data using two fluorescence detectors in tandem; essentially performing two separate experiments simultaneously. We measured the effects of tandem scanning on beam quality by analyzing a range of contrasting samples downstream while simultaneously scanning different gel materials upstream. The upstream gels were thin (<200 μm) diffusive gradients in thin-film (DGT) binding gels. DGTs are passive samplers that are deployed in water, soil, and sediment to measure the concentration and distribution of potentially bioavailable nutrients and contaminants. When deployed on soil, DGTs are typically small (2.5 cm 2 ), so we developed large DGTs (170 cm 2 ), which can be used to provide extensive maps to visualize the diffusion of fertilizers in soil. Of the DGT gel materials tested ( bis -acrylamide, polyacrylamide, and polyurethane), polyurethane gels were most suitable for XFM analysis, having favorable handling, drying, and analytical properties. This gel type enabled quantitative (>99%) transmittance with minimal (<3%) flux variation during raster scanning, whereas the other gels had a substantial effect on the beam focus. For the first time, we have (1) used XFM for mapping analytes in large DGTs and (2) developed a tandem probe analysis mode for synchrotron-based XFM, effectively doubling throughput. The novel tandem probe analysis mode described here is of broad applicability across many XFM beamlines as it could be used for future experiments where any uniform, highly transmissive sample could be analyzed upstream in the “background” of downstream samples.

A method using diffusive gradients in thin films (DGT) for the accurate quantification of trace-level (μg L–1) Sr and Pb concentrations and isotope ratios [δ SRM 987(87Sr/86Sr) and δ SRM 981(207Pb/206Pb)] in labile, bioavailable element fractions in soils is reported. The method is based on a novel poly­(tetrafluoroethylene) (PTFE) membrane binding layer with combined di­(2-ethyl-hexyl)­phosphoric acid (HDEHP) and 4,4′(5′)-bis-t-butylcyclohexano-18-crown-6 (crown-ether) functionality with high selectivity for Sr and Pb (TK100 membrane). Laboratory evaluation of the TK100 DGT showed linear uptake of Sr over time (2–24 h) up to very high Sr mass loadings on TK100 membranes (288 μg cm–2) and effective performance in the range of pH (3.9–8.2), ionic strength (0.001–0.1 mol L–1), and cation competition (50–160 mg L–1 Ca in a synthetic soil solution matrix) of environmental interest. Selective three-step elution of TK100 membranes using hydrochloric acid allowed us to obtain purified Sr and Pb fractions with adequate (≥75%) recovery and quantitative (≥96%) matrix reduction. Neither DGT-based sampling itself nor selective elution or mass loading effects caused significant isotopic fractionation. Application of TK100 DGT in natural soils and comparison with conventional approaches of bioavailability assessment demonstrated the method’s unique capability to obtain information on Sr and Pb resupply dynamics and isotopic variations with low combined uncertainty within a single sampling step.