Tandem Probe Analysis Mode for Synchrotron XFM: Doubling Throughput Capacity.

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 cm2) 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 cm2), so we developed large DGTs (170 cm2), 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.

Arsenic redox transformations and cycling in the rhizosphere of Pteris vittata and Pteris quadriaurita.

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 (AsIII), arsenate (AsV), phosphorus (P), manganese (Mn), iron (Fe) and oxygen (O2), we found localized patterns of AsIII/AsV redox transformations in the PV rhizosphere (AsIII/AsV ratio of 0.57) compared to bulk soil (AsIII/AsV 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 AsV reduction and AsIII root efflux, leading to AsIII accumulation and re-oxidation to AsV in the rhizosphere porewater. This As cycling mechanism is linked to the reduction of O2 and MnIII/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 AsV depletion in the PQ rhizosphere, and no AsIII 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.

Functional Recycling of Biobased, Borate-Stabilized Insulation Materials As B Fertilizer.

Boron is a finite resource, which has been listed as a critical raw material in the EU since 2014. Glass, frits and ceramics production, as well as fertilizers are among the major uses of B. Moreover, about 50 000 t B have been applied as fire retardant and pest repellent in cellulose fiber insulation (CFI) in Europe since the 1980s. Here we propose the end-of-life utilization of borated CFI as B fertilizer, to decrease societal B consumption and to avoid costly and potentially environmentally harmful CFI incineration and deposition in landfills. In a case study, we show that CFI biochar can provide substantial amounts of B to rapeseed and sunflower, with the B plant-availability being comparable to sodium tetraborate, a conventional B fertilizer. The annual B fertilizer consumption of the EU is estimated at ∼4000 t B yr-1, which could be sustained by the B currently installed as CFI for >10 years. In addition, the annual use of B in CFI of 1100 t B yr-1 could cover ∼25% of the annual B fertilizer demand of the EU. Hence, conversion of CFI to B fertilizer provides a meaningful end-of-life strategy, which would contribute to a more resource-efficient and sustainable economy and to several of the UN Sustainable Development Goals.

Silicon Availability from Chemically Diverse Fertilizers and Secondary Raw Materials.

Crops may require Si fertilization to sustain yields. Potential Si fertilizers include industrial byproducts (e.g., steel slags), mined minerals (CaSiO3), fused Ca-Mg-phosphates, biochar, ash, diatomaceous earth, and municipal sewage sludge. To date, no extraction method was shown to accurately predict plant availability of Si from such chemically diverse Si fertilizers. We tested a wide range of products in greenhouse experiments and related the plant Si content to Si extracted by several common Si fertilizer tests: 5-day extraction in Na2CO3-NH4NO3, 0.5 mol L-1 HCl, and Resin extraction. In addition, we tested a novel sink extraction approach for Si(OH)40 that utilizes a dialysis membrane filled with ferrihydrite ("Iron Bag"). Wheat straw biochars and ash exhibited equivalent or marginally higher Si solubility and availability compared to wheat straw. Thermo-chemically treated municipal sewage sludge, as well as diatomaceous earth, did not release substantial amounts of Si. The Resin and the Iron Bag extraction methods gave the best results to predict plant availability of Si. These methods better reproduce the conditions of fertilizer dissolution in soil and around the root by (1) buffering the pH close to neutral and (2) extracting the dissolved Si(OH)40 with ferrihydrite (Iron Bag method) for maximum quantitative extraction.

Diffusive gradients in thin films measurement of sulfur stable isotope variations in labile soil sulfate.

A diffusive gradient in thin films (DGT) technique, based on a strongly basic anion exchange resin (Amberlite IRA-400), was successfully tested for 34S/32S analysis in labile soil sulfate. Separation of matrix elements (Na, K, and Ca) that potentially cause non-spectral interferences in 34S/32S analysis by MC ICP-MS (multi-collector inductively coupled plasma-mass spectrometry) during sampling of sulfate was demonstrated. No isotopic fractionation caused by diffusion or elution of sulfate was observed below a resin gel disc loading of ≤79 µg S. Above this threshold, fractionation towards 34S was observed. The method was applied to 11 different topsoils and one mineral soil profile (0-100 cm depth) and compared with soil sulfate extraction by water. The S amount and isotopic ratio in DGT-S and water-extractable sulfate correlated significantly (r 2 = 0.89 and r 2 = 0.74 for the 11 topsoils, respectively). The systematically lower 34S/32S isotope ratios of the DGT-S were ascribed to mineralization of organic S.

Numerical evaluation of lateral diffusion inside diffusive gradients in thin films samplers.

Using numerical simulation of diffusion inside diffusive gradients in thin films (DGT) samplers, we show that the effect of lateral diffusion inside the sampler on the solute flux into the sampler is a nonlinear function of the diffusion layer thickness and the physical sampling window size. In contrast, earlier work concluded that this effect was constant irrespective of parameters of the sampler geometry. The flux increase caused by lateral diffusion inside the sampler was determined to be ∼8.8% for standard samplers, which is considerably lower than the previous estimate of ∼20%. Lateral diffusion is also propagated to the diffusive boundary layer (DBL), where it leads to a slightly stronger decrease in the mass uptake than suggested by the common 1D diffusion model that is applied for evaluating DGT results. We introduce a simple correction procedure for lateral diffusion and demonstrate how the effect of lateral diffusion on diffusion in the DBL can be accounted for. These corrections often result in better estimates of the DBL thickness (δ) and the DGT-measured concentration than earlier approaches and will contribute to more accurate concentration measurements in solute monitoring in waters.

Localized metal solubilization in the rhizosphere of Salix smithiana upon sulfur application.

A metal-accumulating willow was grown under greenhouse conditions on a Zn/Cd-polluted soil to investigate the effects of sulfur (S(0)) application on metal solubility and plant uptake. Soil porewater samples were analyzed 8 times during 61 days of growth, while DGT-measured metal flux and O2 were chemically mapped at selected times. Sulfur oxidation resulted in soil acidification and related mobilization of Mn, Zn, and Cd, more pronounced in the rooted compared to bulk soil. Chemical imaging revealed increased DGT-measured Zn and Cd flux at the root-soil interface. Our findings indicated sustained microbial S(0) oxidation and associated metal mobilization close to root surfaces. The localized depletion of O2 along single roots upon S(0) addition indicated the contribution of reductive Mn (oxy)hydoxide dissolution with Mn eventually becoming a terminal electron acceptor after depletion of O2 and NO3(-). The S(0) treatments increased the foliar metal concentrations (mg kg(-1) dwt) up to 10-fold for Mn, (5810 ± 593), 3.3-fold for Zn (3850 ± 87.0), and 1.7-fold for Cd (36.9 ± 3.35), but had no significant influence on biomass production. Lower metal solubilization in the bulk soils should translate into reduced leaching, offering opportunities for using S(0) as environmentally favorable amendment for phytoextraction of metal-polluted soils.

Uncertainty evaluation of the diffusive gradients in thin films technique.

Although the analytical performance of the diffusive gradients in thin films (DGT) technique is well investigated, there is no systematic analysis of the DGT measurement uncertainty and its sources. In this study we determine the uncertainties of bulk DGT measurements (not considering labile complexes) and of DGT-based chemical imaging using laser ablation - inductively coupled plasma mass spectrometry. We show that under well-controlled experimental conditions the relative combined uncertainties of bulk DGT measurements are ∼10% at a confidence interval of 95%. While several factors considerably contribute to the uncertainty of bulk DGT, the uncertainty of DGT LA-ICP-MS mainly depends on the signal variability of the ablation analysis. The combined uncertainties determined in this study support the use of DGT as a monitoring instrument. It is expected that the analytical requirements of legal frameworks, for example, the EU Drinking Water Directive, are met by DGT sampling.

Root exudation of phytosiderophores from soil-grown wheat.

For the first time, phytosiderophore (PS) release of wheat (Triticum aestivum cv Tamaro) grown on a calcareous soil was repeatedly and nondestructively sampled using rhizoboxes combined with a recently developed root exudate collecting tool. As in nutrient solution culture, we observed a distinct diurnal release rhythm; however, the measured PS efflux was c. 50 times lower than PS exudation from the same cultivar grown in zero iron (Fe)-hydroponic culture. Phytosiderophore rhizosphere soil solution concentrations and PS release of the Tamaro cultivar were soil-dependent, suggesting complex interactions of soil characteristics (salinity, trace metal availability) and the physiological status of the plant and the related regulation (amount and timing) of PS release. Our results demonstrate that carbon and energy investment into Fe acquisition under natural growth conditions is significantly smaller than previously derived from zero Fe-hydroponic studies. Based on experimental data, we calculated that during the investigated period (21-47 d after germination), PS release initially exceeded Fe plant uptake 10-fold, but significantly declined after c. 5 wk after germination. Phytosiderophore exudation observed under natural growth conditions is a prerequisite for a more accurate and realistic assessment of Fe mobilization processes in the rhizosphere using both experimental and modeling approaches.

Gel for simultaneous chemical imaging of anionic and cationic solutes using diffusive gradients in thin films.

We report on a novel gel based on diffusive gradients in thin films (DGT) for the simultaneous measurement of cations and anions and its suitability for high resolution chemical imaging by using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The new high resolution mixed binding gel (HR-MBG) is based on zirconium-hydroxide and suspended particulate reagent-iminodiacetate (SPR-IDA) as resin materials which are embedded in an ether-based urethane polymer hydrogel. The use of this polymer hydrogel material allows the production of ultrathin, highly stable and tear-proof resin gel layers with superior handling properties compared to existing ultrathin polyacrylamide gels. The gel was characterized regarding its uptake kinetics, the anion and cation capacities, and the effects of pH, ionic strength, and aging on the performance of the HR-MBG. Our results demonstrate the capability of this novel gel for concomitant sampling of anions and cations. The suitability of this new gel type for DGT chemical imaging at submm spatial resolution in soils using LA-ICPMS is shown. 2D images of P, As, Co, Cu, Mn, and Zn distributions around roots of Zea mays L. demonstrate the new opportunities offered by the HR-MBG for high-resolution mapping of solute dynamics in soil and sediment hotspots, such as the rhizosphere, by simultaneous observation of anionic and cationic solute species.

The role of plant-associated bacteria in the mobilization and phytoextraction of trace elements in contaminated soils.

Phytoextraction makes use of trace element-accumulating plants that concentrate the pollutants in their tissues. Pollutants can be then removed by harvesting plants. The success of phytoextraction depends on trace element availability to the roots and the ability of the plant to intercept, take up, and accumulate trace elements in shoots. Current phytoextraction practises either employ hyperaccumulators or fast-growing high biomass plants; the phytoextraction process may be enhanced by soil amendments that increase trace element availability in the soil. This review will focus on the role of plant-associated bacteria to enhance trace element availability in the rhizosphere. We report on the kind of bacteria typically found in association with trace element - tolerating or - accumulating plants and discuss how they can contribute to improve trace element uptake by plants and thus the efficiency and rate of phytoextraction. This enhanced trace element uptake can be attributed to a microbial modification of the absorptive properties of the roots such as increasing the root length and surface area and numbers of root hairs, or by increasing the plant availability of trace elements in the rhizosphere and the subsequent translocation to shoots via beneficial effects on plant growth, trace element complexation and alleviation of phytotoxicity. An analysis of data from literature shows that effects of bacterial inoculation on phytoextraction efficiency are currently inconsistent. Some key processes in plant-bacteria interactions and colonization by inoculated strains still need to be unravelled more in detail to allow full-scale application of bacteria assisted phytoremediation of trace element contaminated soils.

Pedogenic controls of soil organic carbon stocks and stability beneath montane Norway spruce forests along a precipitation gradient.

Reliable data on SOC stocks in forest soils is required in the context of climate change and soil health assessments but still limited by input data availability (e.g., bulk density) and methods used for stock calculation. Relatively few studies have investigated the stability of SOC in forest soils. We investigated SOC stocks and fractionation in soils beneath Norway spruce forests and grasslands in the montane zone along a gradient of mean annual precipitation (MAP). We sampled soil cores volumetrically to 40 cm depth and measured SOC in the fractions <2 mm (fine earth), >200 µm and 200-20 µm (coarse and fine POM), and <20 µm (MAOM) along with potential pedogenic controls. Total SOC stocks beneath forests in the study region, calculated by the equivalent soil mass (ESM) approach to 40 cm depth, amount to 79.0 ± 29.9 (mean ± standard deviation) Mg ha-1 (n = 20) in the mineral soil, and to 92.9 ± 30.6 Mg ha-1 including the litter layer, with a share of 55 % associated with POM. MAOM makes up ∼41 % of SOC in the uppermost mineral layer (0-5 cm) and increases to 71 % in the subsoil (20-40 cm). Multiple regression models show that MAOM is largely controlled by ammonium oxalate extractable Al (Alo) in the forest subsoils (20-40 cm), and increases with MAP in the topsoil layers (0-20 cm). Soils on carbonate rock stand out with ∼80-100 % larger shares of MAOM in the uppermost soil layers (0-10 cm) which is likely connected to higher soil pH and MAP, supporting microbial transformation and subsequent stabilisation of organic matter, which is reflected in narrower C:N ratios in MAOM and SOC. Including the litter layers, ESM-based total SOC stocks in forest soils tend to exceed those beneath grassland (80.2 ± 21.9 Mg ha-1; n = 31) by 16 %, but only by 6.4 % if calculated by the conventional fixed-depth (FD) approach. In contrast to the forest soils, SOC stocks beneath grasslands are dominated by MAOM (75.6 %). We conclude that (coniferous) forest soils are a poor reference for establishing sequestration potentials for stable SOC. The observed large proportion of POM in forest topsoils and its increase with declining MAP (indicating water availability) suggests a risk of SOC losses in response to increasing droughts due to climate change.

Chemical imaging reveals environmental risk of minor tungsten and lead shotgun pellet constituents during weathering in soil.

Tungsten (W)-based shots are considered more environmentally safe than lead (Pb)-based shots, but knowledge about the W-shot fate in the soil environment is still limited, especially in terms of minor constituents such as iron, copper, and nickel (Ni). Contaminant behaviour in soil strongly depends on pH; in turn, the corrosion of metal composites may affect the pH locally. The aim of this study was to compare Pb- and W-shot weathering dynamics in soil (silt loam, pH 6.3) and reveal the interplay of shot weathering-induced pH-changes on the mobility of elements using in situ chemical imaging (Diffusive gradients in thin films for labile elements, planar optodes for soil pH) and batch incubation experiments over time (16 months). Despite our expectation to find acidification due to W oxidation, we observed a pH increase by 0.2 units in extracted soil solutions and by 0.6 units in the soil around W-shots as Ni dissolved from the binder phase of the shot. After 10 weeks, release of labile Ni was 3-times higher compared to W despite the low Ni content in the shot (7 %, m/m). Pb-shot oxidation increased soil solution pH by 0.5 units which likely supported mobility of Pb-shot-derived antimony (Sb). Steep gradients of labile W and Pb and soil solution concentrations <0.8 µmol L-1 indicated that transfer from shot to soil was low. Contrastingly, labile Ni and Sb were found up to ~4 mm from the shot surface and in higher soil solution concentrations as suggested by the shot constitution, indicating higher mobility of minor as compared to major shot constituents. After 16 months, 36 % of total Ni were dissolved in the soil solution highlighting the environmental relevance of minor shot constituents in Pb-shot alternatives after short term weathering in soil.

First observation of diffusion-limited plant root phosphorus uptake from nutrient solution.

Diffusion towards the root surface has recently been shown to control the uptake of metal ions from solutions. The uptake flux of phosphorus (P) from solutions often approaches the maximal diffusion flux at low external concentrations, suggesting diffusion-controlled uptake also for P. Potential diffusion limitation in P uptake from nutrient solutions was investigated by measuring P uptake of Brassica napus from solutions using P-loaded Al(2) O(3) nanoparticles as mobile P buffer. At constant, low free phosphate concentration, plant P uptake increased up to eightfold and that of passive, diffusion-based samplers up to 40-fold. This study represents the first experimental evidence of diffusion-limited P uptake by plant roots from nutrient solution. The Michaelis constant of the free phosphate ion obtained in unbuffered solutions (K(m) = 10.4 µmol L(-1) ) was 20-fold larger than in the buffered system (K(m) ∼0.5 µmol L(-1) ), indicating that K(m) s determined in unbuffered solutions do not represent the transporter affinity. Increases in the P uptake efficiency of plants by increasing the carrier affinity are therefore unlikely, while increased root surface area or exudation of P-solubilizing compounds are more likely to enhance P uptake. Furthermore, our results highlight the important role natural nanoparticles may have in plant P nutrition.

Improving the prediction of fertilizer phosphorus availability to plants with simple, but non-standardized extraction techniques.

In the framework of the circular economy, new P fertilizers produced from diverse secondary raw materials are being developed using various technologies. Standard extraction methods (neutral ammonium citrate (NAC) and H2O) provide limited information about the agronomic efficiency of these often heterogenous new products. Here, we compared these extractions with two alternative methods: 0.5 mol L-1 NaHCO3 and a sink extraction driven by phosphate adsorption onto ferrihydrite ("Iron Bag") on 79 recycled and mineral reference fertilizers. We compared their capacity to predict shoot biomass and P content of rye (S. cereale L.) grown in a greenhouse on three soils of contrasting pH with a subset of 42 fertilizers. The median extracted P (% of total P) was H2O (1%) < NaHCO3 (25%) < Iron Bag (67%) < NAC (85%). The NaHCO3 extraction stood out as a cost-effective and reliable method to predict plant shoot biomass and P content (R2 ranging between 0.65 and 0.86 in the slightly acidic and alkaline soil). Notwithstanding, the other methods provide complementary information for a more detailed characterization of how P solubility may be impacted by e.g. soil pH, granulation, or time. The implications of this work are therefore significant for fertilizer production, regulation, and use.

In situ spatiotemporal solute imaging of metal corrosion on the example of magnesium.

Visualization and quantification of corrosion processes is essential in materials research. Here we present a new approach for 2D spatiotemporal imaging of metal corrosion dynamics in situ. The approach combines time-integrated Mg2+ flux imaging by diffusive gradients in thin films laser ablation inductively coupled plasma mass spectrometry (DGT LA-ICP-MS) and near real-time pH imaging by planar optodes. The parallel assessment of Mg2+ flux and pH distributions on a fine-structured, bare Mg alloy (b-WE43) showed intense Mg dissolution with Mg2+ flux maxima up to 11.9 ng cm-2 s-1 and pH increase >9 during initial corrosion (≤15 min) in aqueous NaNO3 solution (c = 0.01 mol L-1). The techniques visualized the lower initial corrosion rate in buffered synthetic body fluid (Hank's balanced salt solution; pH 7.6) compared to unbuffered NaNO3 (pH 6.0), but precise localization of Mg corrosion remains challenging under these conditions. To further demonstrate the capability of DGT LA-ICP-MS for spatiotemporal metal flux imaging at the microscale, a coated Mg alloy (c-WE43) with lower reactivity was deployed for ≤120 min. The high spatial resolution (∼10 µm × 80 µm) and low limits of detection (≤0.04 ng cm-2 s-1, t = 60 min) enabled accurate in situ localization and quantification (Urel = 20%, k = 2) of distinct Mg2+ flux increase, showing micro-confined release of Mg2+ from surface coating defects on c-WE43 samples. The presented approach can be extended to other metal species and applied to other materials to better understand corrosion processes and improve material design in technological engineering.

Differentiation of metallicolous and non-metallicolous Salix caprea populations based on phenotypic characteristics and nuclear microsatellite (SSR) markers.

The Salicaceae family comprises a large number of high-biomass species with remarkable genetic variability and adaptation to ecological niches. Salix caprea survives in heavy metal contaminated areas, translocates and accumulates Zn/Cd in leaves. To reveal potential selective effects of long-term heavy metal contaminations on the genetic structure and Zn/Cd accumulation capacity, 170 S. caprea isolates of four metal-contaminated and three non-contaminated middle European sites were analysed with microsatellite markers using Wright's F statistics. The differentiation of populations North of the Alps are more pronounced compared to the Southern ones. By grouping the isolates based on their contamination status, a weak but significant differentiation was calculated between Northern metallicolous and non-metallicolous populations. To quantify if the contamination and genetic status of the populations correlate with Zn/Cd tolerance and the accumulation capacity, the S. caprea isolates were exposed to elevated Cd/Zn concentrations in perlite-based cultures. Consistent with the genetic data nested anova analyses for the physiological traits find a significant difference in the Cd accumulation capacity between the Northern and Southern populations. Our data suggest that natural populations are a profitable source to uncover genetic mechanisms of heavy metal accumulation and biomass production, traits that are essential for improving phytoextraction strategies.

Disposal of coal combustion residues in terrestrial systems: contamination and risk management.

The world's ever-growing energy demand will lead to the installation of new coal-fired power plants. At least part of the coal combustion residue (CCR) generated in the coming years will be disposed of, adding to the large number of CCR disposal sites generated in the past and reinforcing the need for sound assessment and management of associated risks. Physical and chemical composition of CCR varies considerably depending on the quality of the feed coal, the combustion technology, fraction considered, and the method of disposal. Related risk pathways include (i) aerial routes, i.e., dust resuspension (Cr(VI)), emanation of radioactivity (Rn associated with U and Th series), and Hg volatilization threatening animal and human health; (ii) phytoaccumulation (B, Se, Mo, As) and plant toxicity (B) with subsequent effects on animals (e.g., Mo-induced hypocuprosis, As and Se toxicity) and humans (e.g., selenosis; food chain); and (iii) effluent discharge and percolation to groundwater and rivers (suspended solids, unfavorable pH, high Se, B, Hg, and As(III) concentrations). Recent and projected changes of CCR composition due to emerging clean coal technologies require close monitoring as the concentration of volatile elements such as Hg and Se, solubility (Hg, Cd, Cu) and volatilization (Hg, NH(3)) of some pollutants are likely to increase because of higher retention in certain fractions of CCRs and concurrent changes in pH (e.g., by mineral carbonation) and NH(3) content. These changes require additional research efforts to explore the implications for CCR quality, use, and management of risk associated with disposal sites.

Metal accumulation and rhizosphere characteristics of Noccaea rotundifolia ssp. cepaeifolia.

This work aimed to investigate the metal accumulation characteristics as well as biogeochemical changes in the rhizosphere and root foraging strategies of this plant species. Previous reports suggested that Noccaea rotundifolia ssp. cepaeifolia is a Zn, Cd and Pb hyperaccumulator. We used hydroponic, rhizobox and split-pot experiments for studying metal accumulation and related rhizosphere processes. Although this species accumulated up to 1250 mg Pb kg-1 and 27,000 mg Zn kg-1 in shoots, translocation factors <1 do not meet the hyperaccumulation criteria. Substantial increases in Ca(NO3)2-extractable metals in the N. rotundifolia rhizosphere of a metal-spiked soil can be explained by proton release from N. rotundifolia roots to maintain the charge balance during excessive metal uptake; this was not observed for the non-spiked, moderately contaminated control soil. Specific rhizosphere mechanisms targeting the alleviation of metal toxicity in N. rotundifolia rhizosphere were not detected. Generally, N. rotundifolia had larger total root and shoot mass in soils with heterogeneous distribution of Zn and Pb relative to homogeneous treatments, associated with less root mass placed in metal-enriched patches. However, the avoidance strategy was not reflected by low shoot metal concentrations. Metal accumulation rates and translocation factors do not meet the criteria for hyperaccumulation. Changes of pH and DOC in N. rotundifolia rhizosphere were apparently not involved in targeted immobilisation or detoxification of Pb, Zn and Cd. Avoidance of metal-rich patches in soil is a major tolerance strategy of N. rotundifolia.