Evaluating Cr behaviour in two different polluted soils: Mechanisms and implications for soil functionality.

This work investigates the mechanisms determining Cr speciation and availability in two different soils polluted with two chromium sources (an industrial sludge, highly polluted with Cr, and Cr(VI) solution) and the influence of these parameters on the recovery of the soil functions related with biological quality and plant growth. The experiment was carried out in greenhouse conditions using 36 pots of 17 kg for the growth of Silene vulgaris for 21 months. Logistic Regression Model using Lasso estimator shows that soil organic matter (SOM) and pH control Cr availability in studied soils. In soils treated with the sludge, X ray Absorption spectroscopy showed that Cr was present as Cr(III), biological quality indicators increased and plants were able to grow. However, in soils polluted with Cr(VI), Cr availability was significantly different in the two soils. In the alkaline and poor in organic matter soil, 12% of Cr(VI) remained in the soil leading to the decrease of soil quality indicators and the total inhibition of plant growth. In the neutral soil, Cr(VI) was totally reduced to Cr(III) by soil organic matter (SOM), quality indicators were not affected and plants grown properly. Infrared Spectroscopy showed that different functional groups reacted with Cr in the two soils. This study highlights the importance to understand the mechanisms underlaying Cr redox and adsorption reactions in Cr polluted soils as they determine the potential recovery of the functions related with biological quality indicators and plant growth. The methodology proposed allows this study in complex soil samples at realistic concentrations and may be useful for risk assessment and for the planning of managing strategies in Cr polluted soils.

Phytoavailability of Cr in Silene vulgaris: The role of soil, plant genotype and bacterial rhizobiome.

Understanding the metal behavior at the soil-root interface is of utmost significance for a successful implementation of phytoremediation. In this study, we investigated the differences in chromium (Cr) uptake, chemical changes in soil solution and the shifts in rhizosphere bacterial communities of two genotypes of Silene vulgaris (SV21, SV38) with different tolerance to Cr. A greenhouse experiment was performed in two soils that differed on pH and organic matter (OM) content. An industrial sludge with high content in Cr was used as pollution source. The soil solution in the rhizosphere was sample by Rhizon Soil Moisture Samplers. The total concentration of Cr reached the highest values in soil solution samplers from calcareous soils with poor contents in OM. Plants grown in this soil also increased the Cr uptake in roots of both genotypes, but the concentration was higher in genotype SV-38 than in SV21. The clustering analysis of denaturing gradient gel electrophoresis (DGGE) of 16S rRNA fragments revealed major differences in bacterial community structure related to Cr pollution, followed by soil type and finally, plant genotype. Diversity indices based on DGGE profiles were the highest in alkaline soil, and between genotypes, values were significantly greater in SV38. Canonical correspondence analysis (CCA) showed that changes in bacterial community structure of rhizosphere were highly correlated with total Cr concentration and soil solution pH. The isolation and identification of S. vulgaris bacterial rhizosphere revealed a different composition according to soil type and plant genotype. Results suggested the potential role of Pseudomonas fluorescens on Cr mobilization and therefore, on enhanced metal bioavailability and may provide a starting point for further studies aimed at the combined use of tolerant plants and selected metal mobilizing rhizobacteria, in the microbial-assisted phytoremediation of Cr-polluted soils.

Soil properties determine the impact of nZVI on Lactuca sativa L and its rhizosphere.

Nanoscale zero-valent iron (nZVI) is a promising material tool for the remediation of metal(loid)-contaminated soils since it reduces metal(loid) availability and plant uptake, thereby enhancing the development of the plants. However, the effects of nZVI as nanoparticles on soil properties, plants, and the microbial rhizosphere in unpolluted soils are poorly understood. Here we tested the impact of nZVI at different doses (0.5 and 5% of commercial suspension) on soil properties, lettuce plants, and their microbial rhizosphere in two non-contaminated soils with distinct physico-chemical properties (alkaline versus acidic soil). To this end, a pot experiment was performed with lettuce plants in a growth chamber for a month. Both soils showed an increase in of pH and available Fe after nZVI application. However, these effects were more marked in the acidic soil. In this regard, the plants in this soil showed increased biomass and Fe content. TEM analysis revealed that although the roots and leaves of plants grown in the alkaline soil showed better cell integrity than those in acidic soil-an observation that was consistent with the visual appearance of the plants-the former were more affected by the nZVI treatment. Regarding the microbial rhizosphere, in general, nZVI enhanced enzyme activity regardless of the soil type. Microbial functional diversity showed a significant decline in response to nZVI in alkaline soil. In contrast, the 0.5% nZVI treatment had a positive effect on this parameter in acidic soil. Bacterial genetic diversity was less affected by the presence of nZVI than fungal diversity, which was higher in nZVI-treated acidic soils. In addition, alterations of bacterial and fungal communities were associated with available Fe in acidic soil. In conclusion, soil properties play a key role in determining the effects of nZVI on lettuce plants and their rhizosphere.

Selecting efficient methodologies for estimation of As and Hg availability in a brownfield.

The determination of soil metal(loid) availability presents controversy and there is no consensus or uniformity on used analytical methods. In this study nine single extraction methods (H2O, CaCl2, NaNO3, NH4NO3, DTPA, EDTA, HCl, LMWOA, TCLP) and four sequential extraction procedures (Tessier, BCR, Wenzel and Fernández-Martínez) have been compared to estimate the availability of As and Hg in two soils from a highly polluted brownfield, especially with As. The metal(loid) concentrations were also determined in three native plant species (Lotus corniculatus, Betula celtiberica and Dactylis glomerata) collected in the habitat under study. Each single extractant showed a particular capacity of As/Hg extraction because they do not extract the same forms of each element. The availability of As and Hg depended on the element characteristics, soil properties, type of extractant and degree of pollution, thus the use of a single extraction procedure provides limited information of metal(loid) availability and to reach general conclusions is difficult. Regarding the sequential extractions, each procedure showed a specific pattern for As and Hg regardless of the soil. Thus, the choice of one or other method depends on the environmental conditions, metal(loid) and soil properties. In risk assessment studies it would be recommendable to select one of the more aggressive extractants, so as not to underestimate the environmental risk. In this regard, the sequential extraction procedures render more detailed information about metal(loid) potential availability in relation to soil properties. The analysis of native plant species showed higher metal(loid) concentrations in roots than in aerial parts and differences were observed depending on the metal(loid) and the species. In general, plants showed a higher BCFs for Hg than As even though the total and available As concentrations were higher than those found for Hg, which highlights the influence of plant species on the metal(loid) uptake.

Different genotypes of Silene vulgaris (Moench) Garcke grown on chromium-contaminated soils influence root organic acid composition and rhizosphere bacterial communities.

Plant-microbe interactions are considered to be important processes determining the efficiency of phytoremediation of heavy metal-contaminated soils. However, relatively little is known about how these interactions are influenced by chromium (Cr) contamination. The effect of Cr stress on metal uptake, root organic acid composition, and rhizosphere bacterial communities was studied using two genotypes of the metallophyte Silene vulgaris, which have shown different tolerance to Cr(VI). The results indicated that root biomass and shoot biomass were not significantly influenced by Cr treatment, but metal uptake in shoots and roots was significantly impacted by the genotype. Principal component analyses (PCA) showed that variation in organic acids oxalic, citric, malic, formic, lactic, acetic, and succinic differed between genotypes. Changes in root organic acid contents in response to Cr revealed a significant increase of oxalic acid in genotype SV-21. The denaturing gradient gel electrophoresis (DGGE) cluster analysis showed that the community structure (determined by PCR-DGGE) was affected by plant genotype and, to a lesser extent, by Cr contamination, the first being the most influential factor shaping the rhizosphere microbiome. Under Cr pollution, a shift in the relative abundance of specific taxa was found and dominant phylotypes were identified as Variovorax in SV-21 and Chitinophaga niastensis, Pontibacter sp., and Ramlibacter sp. in SV-38. These results provided the basis for further studies aimed at the combined use of plants and soil microorganisms in the remediation of Cr-polluted soils.