Walter J. Fitz

Arsenic transformations in the soil–rhizosphere–plant system: fundamentals and potential application to phytoremediation

This paper reviews major processes that potentially affect the fate of arsenic in the rhizosphere of plants. Rhizosphere interactions are deemed to play a key role in controlling bioavailability to crop plants and for a better understanding and improvement of phytoremediation technologies. Substantial progress has been made towards an understanding of As transformation processes in soils. However, virtually no information is available that directly addresses the fate of As in the rhizosphere. We are proposing a conceptual model of the fate of As in the soil-rhizosphere-plant system by integrating the state-of-the art knowledge available in the contributing disciplines. Using this model and recent studies on hyperaccumulation of As, we discuss research needs and the potential application of rhizosphere processes to the development of phytoremediation technologies for As-polluted soils.

In situ fixation of metals in soils using bauxite residue: chemical assessment

Contamination of soils with heavy metals and metalloids is a widespread problem all over the world. Low cost, non-invasive, in situ technologies are required for remediation processes. We investigated the efficiency of a bauxite residue (red mud) to fix heavy metals in two soils, one contaminated by industrial activities (French soil), and one by sewage sludge applications (UK soil). This Fe-oxide rich material was compared with lime, or beringite, a modified aluminosilicate that has been used for in situ fixation processes. Four different crop species were successively grown in pots. Metal concentrations in the soil pore waters were analyzed during the growing cycles. At the end of the experiment fluxes of heavy metals were measured using a diffusive gradient in thin film technique (DGT). Furthermore, a sequential extraction procedure (SEP) and an acidification test were performed to investigate the mechanisms of metal fixation by different soil amendments. In both soils, the concentrations of metals in the soil pore water and metal fluxes were greatly decreased by the amendments. An application of 2% red mud performed as well as beringite applied at 5%. Increasing soil pH was a common mechanism of action for all the amendments. However, the red mud amendment shifted metals from the exchangeable to the Fe-oxide fraction, and decreased acid extractability of metals. The results suggest that specific chemisorption, and possibly metal diffusion into oxide particles could also be the mechanisms responsible for the fixation of metals by red mud.

Novel rhizobox design to assess rhizosphere characteristics at high spatial resolution

Available tools to study rhizosphere characteristics at a sub-mm spatial resolution suffer from a number of shortfalls, including geometrically and physiologically ill-defined root layers containing soil or other growth medium. Such designs may result in over- or underestimation of root-induced changes in the rhizosphere. We present a novel rhizobox design that overcomes these shortfalls. Plants are pre-grown in a soil–root compartment with an opening slit at the bottom. As plants reach the targeted physiological stage, this compartment is transferred on top of a rhizosphere soil compartment attached to a vertical root-only compartment. The latter is made up of a membrane (pore size 7 μm to restrict root hair growth into the rhizosphere compartment or 30 μm to restrict only root growth) and a transparent acrylic window which is gently pressed against the membrane and rhizosphere soil compartment using an adjustable screw. This design allows roots to penetrate from the upper soil–root compartment through the slit into the root-only compartment. Root growth and distribution can be monitored through the acrylic window using digital camera equipment. Upon termination of the experiment, the rhizosphere compartment is removed and frozen prior to separation of sub-mm soil layers using microtome techniques. In a test experiment, canola (Brassica napus L. cv. Sprinter) developed a fairly dense root monolayer within 8 days. Using measurement of soil characteristics at 0.5–1-mm increments across the rhizosphere we demonstrate that the proposed rhizobox design is yielding reproducible data. Due to exudation of LMWOC, we found a statistically significant increase of DOC towards the root plane, whereas more stable soil characteristics were not affected by root activity. Limitations and further extensions of this rhizobox design, including the use of micro suction cups and microsensors for pH and redox potential to measure spatial and temporal changes in a non-destructive manner are discussed along with potential applications such as validation of rhizosphere models.

Ectomycorrhizal communities associated with Populus tremula growing on a heavy metal contaminated site

European aspen is one of the most widely distributed trees in Central Europe and is a typical early colonizer of poor and disturbed soils. However, little is known about ectomycorrhizal (ECM) fungi in these ecosystems. We examined the ECM community of European aspen growing on a heavily contaminated site in southern Austria by analysing ECM roots, sorting them into morphotypes, subjecting them to DNA extraction, PCR, and DNA sequencing. ECM root symbionts were sampled two times in 2004. During this time, the below-ground community structure was relatively stable; we found no evidence of taxa adapted to summer or autumn conditions and only two species varied widely in occurrence between soil horizons. The ECM fungal community was diverse (54 species), rich in Basidiomycota (43 species), and dominated by Cenococcum geophilum and fungi with corticoid basidiomes (e.g. Thelephoraceae).

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

The worlds 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.

Zinc accumulation potential and toxicity threshold determined for a metal-accumulating Populus canescens clone in a dose-response study.

The effect of increasing soil Zn concentrations on growth and Zn tissue concentrations of a metal-accumulating aspen clone was examined in a dose-response study. Plants were grown in a soil with a low native Zn content which was spiked with Zn salt solutions and subsequently aged. Plant growth was not affected by NH(4)NO(3)-extractable soil Zn concentrations up to 60 microg Zn g(-1) soil, but it was completely inhibited at extractable concentrations above 90 microg Zn g(-1) soil. From these data an effective concentration of 68.5 microg extractable Zn g(-1) soil was calculated at which plant growth was reduced by 50%. The obtained information on toxicity threshold concentrations, and the relation between plant Zn accumulation and extractable soil Zn concentrations may be used to assess the suitability of the investigated Populus canescens clone for various phytoremediation strategies. The potential risk of metal transfer into food webs associated with P. canescens stands on Zn-polluted sites may also be estimated.

Microtome sectioning causes artifacts in rhizobox experiments

Experimental data on rhizosphere characteristics at high spatial resolution are required to improve our knowledge on phytoavailability of nutrients and pollutants. In numerous studies, sectioning using refrigerated microtomes has been employed to obtain thin soil layers at defined distances from the root surface. In this study, we assessed the effect of thin slicing and freezing on soil chemical characteristics. Two experimental soils were frozen at −20 °C and sliced using a refrigerated microtome. In general, chemical changes relative to the non-sliced control were more pronounced as the trim thickness (thickness of a single slice) decreased. Maximum increases in pH and electrical conductivity (EC) for the smallest trim thickness used (20 μm) were 0.9 units and 50%, respectively. Extractable fractions of P (0.5 M NaHCO3) K, Mg, Mn, Na and Si (1 M NH4NO3) increased up to 40, 91, 19, 621, 50 and 100%, respectively. Based on these results, we suggest to use a trim thickness of ≥ 200 μm. Apart from slicing, freezing (a prerequisite for the microtome technique) was found to bias soil chemical parameters. To circumvent microtome-related artifacts we present a home-made slicing device as a cost-effective alternative, which allows sectioning of non-frozen rhizosphere soil employing one single slice.

CE-ICP-SFMS and HPIC-ICP-SFMS for arsenic speciation in soil solution and soil water extractsPresented at the 2002 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, USA, January 6???12, 2002.

Arsenic speciation in soil solutions and soil water extracts was assessed using two different hyphenated techniques, i.e. ion chromatography inductively coupled plasma sector field mass spectrometry (HPIC-ICP-SFMS) and capillary electrophoresis (CE)-ICP-SFMS. Arsenobetaine (AB), dimethylarsinate (DMA), arsenite AsIII, monomethylarsonate (MMA) and arsenate AsV were separated using an anion exchange resin. The CE separation protocol allowed the determination of one more species, namely arsenocholine (AC). A functional make-up flow interface design based on pneumatic nebulization was used for both techniques, allowing stable and dependable operation. The separation time of CE-ICP-SFMS could be significantly reduced by application of hydrodynamic pressure. Addition of isopropanol to the sheath revealed improved sensitivity (factor 3). Excellent detection limits for the arsenic species were in the range of 0.04–0.08 ng g−1 for HPIC-ICP-MS. The detection limits of CE-ICP-SFMS were found to be 2 orders of magnitude higher. Soil solutions and soil water extracts obtained by rhizobox experiments of the hyperaccumulating plant Pteris vittata were investigated. The measurements showed only AsV ranging at concentration levels of 0.3–0.1 µg g−1 and 0.3–0.5 µg g−1 in soil solution and water extracts, respectively.

Bioconcentration of zinc and cadmium in ectomycorrhizal fungi and associated aspen trees as affected by level of pollution.

Concentrations of Zn and Cd were measured in fruitbodies of ectomycorrhizal (ECM) fungi and leaves of co-occurring accumulator aspen. Samples were taken on three metal-polluted sites and one control site. Fungal bioconcentration factors (BCF = fruitbody concentration: soil concentration) were calculated on the basis of total metal concentrations in surface soil horizons (BCF(tot)) and NH(4)NO(3)-extractable metal concentrations in mineral soil (BCF(lab)). When plotted on log-log scale, values of BCF decreased linearly with increasing soil metal concentrations. BCF(lab) for both Zn and Cd described the data more closely than BCF(tot). Fungal genera differed in ZnBCF but not in CdBCF. The information on differences between fungi with respect to their predominant occurrence in different soil horizons did not improve relations of BCF with soil metal concentrations. Aspen trees accumulated Zn and Cd to similar concentrations as the ECM fungi. Apparently, the fungi did not act as an effective barrier against aspen metal uptake by retaining the metals.

Ectomycorrhizal impact on Zn accumulation of Populus tremula L. grown in metalliferous soil with increasing levels of Zn concentration

AimsOur study aimed at characterizing the Zn phytoextraction potential of a metal tolerant Populus tremula accession in symbiosis with a community of ectomycorrhizal fungi from metal-contaminated soil that is naturally forming mycorrhizae with the experimental plant. Effects of the fungal community on P. tremula development, metal translocation and accumulation properties were tested under variable Zn bioavailability.MethodsIn a pot experiment, P. tremula seedlings were grown for 88 days in a substrate composed of metalliferous soil from an aspen stand and non-contaminated agricultural soil spiked with ZnSO4 to yield total Zn additions from 0 to 80 mg kg-1 substrate. The substrate contained the inherent mycorrhizal community of P. tremula (Nat-Myc) or was γ-irradiated to eliminate living microbial propagules (Irr-NM treatment).Resultsγ-Irradiation efficiently inhibited the formation of functional ectomycorrhizae in the control treatments. It increased dissolved organic carbon (DOC) in the substrate and increased the extractability of Zn and Cd by 1M NH4NO3. We found three times larger biomass and more than four times increased root lengths in the Irr-NM compared to the Nat-Myc treatments which may be explained by the doubled DOC concentrations and related Fe mobilization due to formation of labile complexes in the irradiation treatment and the absence of microbial competitors for (nutrient) resources. Our results indicate an imbalance of the normally mutualistic symbiosis between mycorrhizal fungi and the host at early growth stage, possibly further exacerbated by the high fragility and low nutrient reserves of the P. tremula seedlings obtained from a contaminated site. Foliar Zn concentrations were generally larger in the Nat-Myc treatments and exceeded those reported for numerous Salix and Populus species. While the Zn concentrations increased with increasing Zn additions, Zn translocation to shoots was inhibited at high Zn levels in the Nat-Myc treatments, indicating a barrier function of the mycorrhizal community.ConclusionsThe observed barrier properties in the mycorrhizal treatments suggest that mycorrhizal inoculation of P. tremula may be a promising strategy to enhance revegetation and phytostabilization of metal-polluted sites. However, early-stage growth of P. tremula may be limited by imbalances between the fungal and plant partner in such nutrient-deficient, toxic environments.