Jelle Mertens

The Red Mud Accident in Ajka (Hungary): Plant Toxicity and Trace Metal Bioavailability in Red Mud Contaminated Soil

The red mud accident of October 4, 2010, in Ajka (Hungary) contaminated a vast area with caustic, saline red mud (pH 12) that contains several toxic trace metals above soil limits. Red mud was characterized and its toxicity for plants was measured to evaluate the soil contamination risks. Red mud radioactivity (e.g., (238)U) is about 10-fold above soil background and previous assessments revealed that radiation risk is limited to indoor radon. The plant toxicity and trace metal availability was tested with mixtures of this red mud and a local noncontaminated soil up to a 16% dry weight fraction. Increasing red mud applications increased soil pH to maximally 8.3 and soil solution EC to 12 dS m(-1). Shoot yield of barley seedlings was affected by 25% at 5% red mud in soil and above. Red mud increased shoot Cu, Cr, Fe, and Ni concentrations; however, none of these exceed toxic limits reported elsewhere. Moreover, NaOH amended reference treatments showed similar yield reductions and similar changes in shoot composition. Foliar diagnostics suggest that Na (>1% in affected plants) is the prime cause of growth effects in red mud and in corresponding NaOH amended soils. Shoot Cd and Pb concentrations decreased by increasing applications or were unaffected. Leaching amended soils (3 pore volumes) did not completely remove the Na injury, likely because soil structure was deteriorated. The foliar composition and the NaOH reference experiment allow concluding that the Na salinity, not the trace metal contamination, is the main concern for this red mud in soil.

Bacteria, not archaea, restore nitrification in a zinc-contaminated soil

Biological ammonia oxidation had long been thought to be mediated solely by discrete clades of β- and γ-proteobacteria (ammonia-oxidizing bacteria; AOB). However, ammonia-oxidizing Crenarchaeota (ammonia-oxidizing archaea; AOA) have recently been identified and proposed to be the dominant agents of ammonia oxidation in soils. Nevertheless, the dynamics of AOB versus AOA, and their relative contribution to soil ammonia oxidation and ecosystem functioning on stress and environmental perturbation, remain unknown. Using a 3-year longitudinal field study and the amoA gene as a molecular marker, we demonstrate that AOB, but not AOA, mediate recovery of nitrification after zinc (Zn) contamination. Pristine soils showed approximately equal amoA gene copy numbers and transcript levels for AOB and AOA. At an intermediate Zn dose (33.7 mmol Zn per kg), ammonia oxidation was completely inhibited, and the numbers of AOB and AOA amoA gene copies and gene transcripts were reduced. After 2 years, ammonia oxidation in the field soils was fully restored to preexposure levels, and this restoration of function was concomitant with an increase of AOB amoA gene copy and gene transcript numbers. Analysis of the restored community revealed domination by a phylogenetically distinct Zn-tolerant Nitrosospira sp. community. In contrast, the numbers of AOA amoA gene copies and gene transcripts remained 3- and 104-fold lower than recovered AOB values, respectively. Thus, although recent findings have emphasized a dominant role of archaea in soil-borne ammonia oxidation, we demonstrate that a phylogenetic shift within the AOB community drives recovery of nitrification from Zn contamination in this soil.

Toxicity of heavy metals in soil assessed with various soil microbial and plant growth assays: A comparative study

Abstract-Elevated metal concentrations in soils can disturb the soil ecosystem; thus, researchers strive to identify the most sensitive assay for detection of the early signs of toxicity. The purpose of the present study was to compare eight different ecotoxicological endpoints on the same set of metal-contaminated soils that were collected from seven series of soils sampled during field trials. The endpoints are based on three microbial assays (potential nitrification rate [PNR], substrate-induced respiration [SIR], and basal respiration [BR]) and two plant growth tests, one of which included symbiotic N fixation. The overall sensitivity of the endpoints to detect statistically significant adverse effects ranked as follows: PNR > SIR (lag time) > plant yield and N fixation > SIR (respiration after 24 and 48 h) > BR. The lowest adverse effect concentrations were found with the PNR at 7 mg kg(-1) of Cd and 107 mg kg(-1) of Zn. The variability of these endpoints among different uncontaminated soils was additionally assessed on 14 soil samples. That variability showed a strong correlation with sensitivity scores, illustrating that metal-sensitive endpoints have a large natural variability. We question the ecological relevance of highly sensitive microbial assays, because they tend to have a large natural variability. The identification of toxicity in the field requires endpoints that are highly sensitive and that do not vary greatly among soils (i.e., robust); however, no such endpoint was found in the present study. The endpoints that combined average sensitivity and robustness were SIR (lag time), clover yield, and N fixation in clover.

Extent of copper tolerance and consequences for functional stability of the ammonia-oxidizing community in long-term copper-contaminated soils.

Adaptation of soil microbial communities to elevated copper (Cu) concentrations has been well documented. However, effects of long-term Cu exposure on adaptation responses associated with functional stability and structural composition within the nitrifying community are still unknown. Soils were sampled in three field sites (Denmark, Thailand, and Australia) where Cu gradients had been established from 3 to 80 years prior to sampling. In each field site, the potential nitrification rate (PNR) decreased by over 50% with increasing soil Cu, irrespective of a 20 to >200-fold increase in Cu tolerance (at the highest soil Cu) among the nitrifying communities. This increased tolerance was associated with decreasing numbers (15-120-fold) of ammonia-oxidizing bacteria (AOB), except in the oldest contaminated field site, decreasing numbers of ammonia-oxidizing archaea (AOA; 10-130-fold) and differences in the operational taxonomic unit (OTU) composition of the AOB and, to a lesser extent, AOA communities. The sensitivity of nitrifying communities, previously under long-term Cu exposure, to additional stresses was assessed. Nitrification in soils from the three field sites was measured following acidification, pesticide addition, freeze-thaw cycles, and dry-rewetting cycles. Functional stability of the nitrification process was assessed immediately after stress application (resistance) and after an additional three weeks of incubation (resilience). No indications were found that long-term Cu exposure affected the sensitivity to the selected stressors, suggesting that resistance and resilience were unaffected. It was concluded that the nitrifying community changed structurally in all long-term Cu-exposed field sites and that these changes were associated with increased Cu tolerance but not with a loss of functional stability.

Copper toxicity to bioluminescent Nitrosomonas europaea in soil is explained by the free metal ion activity in pore water.

The terrestrial biotic ligand model (BLM) for metal toxicity in soil postulates that metal toxicity depends on the free metal ion activity in solution and on ions competing for metal sorption to the biotic ligand. Unequivocal evidence for the BLM assumptions is most difficult to obtain for native soil microorganisms because the abiotic and biotic compartments cannot be experimentally separated. Here, we report copper (Cu) toxicity to a bioluminescent Nitrosomonas europaea reporter strain that was used in a solid phase-contact assay and in corresponding soil extracts and artificial soil solutions. The Cu(2+) ion activities that halve bioluminescence (EC50) in artificial solutions ranged 10(-5) to 10(-7) M and increased with increasing activities of H(+), Ca(2+) and Mg(2+) according to the BLM concept. The solution based Cu(2+) EC50 values of N. europaea in six contaminated soils ranged 2 × 10(-6) to 2 × 10(-9) M and these thresholds for both solid phase or soil extract based assays were well predicted by the ion competition model fitted to artificial solution data. In addition, solution based Cu(2+) EC50 of the solid phase-contact assay were never smaller than corresponding values in soil extracts suggesting no additional solid phase toxic route. By restricting the analysis to the same added species, we show that the Cu(2+) in solution represents the toxic species to this bacterium.

Toxicity of the molybdate anion in soil is partially explained by effects of the accompanying cation or by soil pH

Previous studies have shown that toxicity of cationic trace metals in soil is partially confounded by effects of the accompanying anions. A similar assessment is reported here for toxicity of an oxyanion, i.e., molybdate (MoO(4) (2-)), the soil toxicity of which is relatively unexplored. Solubility and toxicity were compared between the soluble sodium molybdate (Na(2)MoO(4)) and the sparingly soluble molybdenum trioxide (MoO(3)). Confounding effects of salinity were excluded by referencing the Na(2)MoO(4) effect to that of sodium chloride (NaCl). The pH decrease from the acid MoO(3) amendment was equally referenced to a hydrochloric (HCl) treatment or a lime-controlled MoO(3) treatment. The concentrations of molybdenum (Mo) in soil solution or calcium chloride (CaCl(2)) 0.01 M extracts were only marginally affected by either MoO(3) or Na(2)MoO(4) as an Mo source after 10 to 13 days of equilibration. Effects of Mo on soil nitrification were fully confounded by associated changes in salinity or pH. Effects of Mo on growth of wheat seedlings (Triticum aestivum L) were more pronounced than those on nitrification, and toxicity thresholds were unaffected by the form of added Mo. The Mo thresholds for wheat growth were not confounded by pH or salinity at incipient toxicity. It is concluded that oxyanion toxicity might be confounded in relatively insensitive tests for which reference treatments should be included.

Chapter 14 – Emerging Tools in the Assessment of Metals: Current Applicability

Abstract In addition to the approaches and tools currently used for the risk management of complex inorganic materials, new tools are being developed or further adapted towards the specificities of inorganic materials. Quantitative ion–character activity relationships and adverse outcome pathway analysis are promising tools to predict toxic effects or mode of actions for less data rich substances and can help to prioritise critical substances or constituents and endpoints. Promising models for predicting the combined toxicity of inorganics are being developed and can support decision-making for constituent-based risk assessment of complex inorganic materials. Life-cycle assessment approaches are a valuable tool to assess and compare impacts from substances or processes on a range of endpoints. However, such approaches need further standardisation and implementation of the inorganics’ specifics to allow a sound comparison of impacts from different types of (inorganic) materials. When further developed and accepted, all these tools will add to the risk management of complex inorganic materials.

Transformation‐dissolution reactions partially explain adverse effects of metallic silver nanoparticles to soil nitrification in different soils

Risk assessment of metallic nanoparticles (NPs) is critically affected by the concern that toxicity goes beyond that of the metallic ion. The present study addressed this concern for soils with silver nanoparticles (AgNPs) using the Ag-sensitive nitrification assay. Three agricultural soils (A, B, and C) were spiked with equivalent doses of either AgNP (diameter = 13 nm) or AgNO3 . Soil solution was isolated and monitored over 97 d with due attention to accurate Ag fractionation at low (∼10 μg L-1 ) Ag concentrations. Truly dissolved (<1 kDa) Ag in the AgNO3 -amended soils decreased with reaction half-lives of 4 to 22 d depending on the soil, denoting important Ag-aging reactions. In contrast, truly dissolved Ag in AgNP-amended soils first increased by dissolution and subsequently decreased by aging, the concentration never exceeding that in the AgNO3 -amended soils. The half-lives of AgNP transformation-dissolution were approximately 4 d (soils A and B) and 36 d (soil C). The Ag toxic thresholds (10% effect concentrations, milligrams of Ag per kilogram of soil) of nitrification, evaluated at 21 or 35 d after spiking, were similar between the 2 Ag forms (soils A and B) but were factors of 3 to 8 lower for AgNO3 than for AgNP (soil C), largely corroborating dissolution differences. This fate and bioassay showed that AgNPs are not more toxic than AgNO3 at equivalent total soil Ag concentrations and that differences in Ag dissolution at least partially explain toxicity differences between the forms and among soils. Environ Toxicol Chem 2018;37:2123-2131.