Effect of Lupinus albus L. root activities on As and Cu mobility after addition of iron-based soil amendments

Rice rhizospheric effects and mechanism on soil cadmium bioavailability during silicon application

Exogenous Si mitigates the mobility and bioavailability of Cd in the soil, thereby alleviating its phytotoxicity. This study focused on specific Si-induced immobilisation effects within the rhizosphere (S1), near-rhizosphere (S2), and far-rhizosphere (S3) zones. Based on the rhizobox experiment, we found that applying Si significantly elevated soil pH, and the variation amplitudes in the S3 soil exceeded those in the S1 and S2 soils. Si-induced changes in the rhizosphere also included enhanced dissolved organic carbon and diminished soil Eh, particularly in the Si400 treatment. Meanwhile, the introduction of Si greatly enhanced the Fe2+ and Mn2+ concentrations in the S1 soil, but reduced them in the S2 soil. The rhizosphere effect of Si which enriched Fe2+ and Mn2+ subsequently promoted the formation of Fe and Mn oxides/hydro-oxides near the rice roots. Consequently, the addition of Si significantly reduced the available Cd concentrations in S1, surpassing the reductions in S2 and S3. Moreover, Si-treated rice exhibited increased Fe plaque generation and fixation on soil Cd, resulting in decreased Cd concentrations in rice tissues, accompanied by reduced Cd translocation from roots to shoots and shoots to grains. Structural equation modelling further highlighted that Si is essential in Cd availability in S1 and Fe plaque development, ultimately mitigating Cd accumulation in rice. Si-treated rice also exhibited higher biomass and grain yield than those of control groups. These findings provide valuable insights into Si-based strategies for addressing the Cd contamination of agricultural soils.

Phytochelatin and coumarin enrichment in root exudates of arsenic-treated white lupin

Soil contamination with toxic metalloids, such as arsenic, can represent a substantial human health and environmental risk. Some plants are thought to tolerate soil toxicity using root exudation, however, the nature of this response to arsenic remains largely unknown. Here, white lupin plants were exposed to arsenic in a semi-hydroponic system and their exudates were profiled using untargeted liquid chromatography-tandem mass spectrometry. Arsenic concentrations up to 1 ppm were tolerated and led to the accumulation of 12.9 μg As g^-1 dry weight (DW) and 411 μg As g^-1 DW in above-ground and belowground tissues, respectively. From 193 exuded metabolites, 34 were significantly differentially abundant due to 1 ppm arsenic, including depletion of glutathione disulphide and enrichment of phytochelatins and coumarins. Significant enrichment of phytochelatins in exudates of arsenic-treated plants was further confirmed using exudate sampling with strict root exclusion. The chemical tolerance toolkit in white lupin included nutrient acquisition metabolites as well as phytochelatins, the major intracellular metal-binding detoxification oligopeptides which have not been previously reported as having an extracellular role. These findings highlight the value of untargeted metabolite profiling approaches to reveal the unexpected and inform strategies to mitigate anthropogenic pollution in soils around the world.

Bridging Microbial Functional Traits With Localized Process Rates at Soil Interfaces

In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces [e.g., rhizosphere, detritusphere, (bio)-pores, and aggregate surfaces]. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment.

Effect of biochar, iron sulfate and poultry manure application on the phytotoxicity of a former tin mine

In phytomanagement approach the application of a combination of amendments is an option for remediating arsenic polluted areas and valorized biomass obtained. Various amendments can be used. Biochar has been shown to reduce metal(loid) availability, and increase soil fertility, while iron sulfate has a considerable As binding capacity, and poultry manure is a source of nutrients. A phytotoxicity test was performed by applying the three amendments (2% biochar, 0.15%, 0.30% and 0.45% iron sulfate and 0.4% poultry manure) to a former tin mine technosol, to investigate their effects on (i) soil pore water properties, (ii) metal(loid) immobilization and (iii) Phaseolus vulgaris L. growth, used as a bioindicator. Biochar addition alone did not affect soil properties or plant parameters. However, the addition of iron sulfate acidified the soil, decreased soil pore water As concentrations, and increased the ones of Fe and Pb. It also improved plant growth, and reduced As and Pb aerial and root concentrations. Finally, the addition of poultry manure had no effect on soil and plants. Based on our results, the combination of iron sulfate with biochar may be a solution for reducing soil toxicity of the Abbaretz mining technosol, improving its fertility, and thus ameliorating plant growth.Novelty statement:The work presented in this manuscript describes the effect of amendment application, i.e., biochar, chicken dung and/or iron sulfate, on soil properties, metals availability and dwarf bean growth, plant used as bioindicator.Our results showed that the combination of a low amount of iron sulfate with biochar is the strategy to reduce soil toxicity, improved its fertility and consequently authorizes plant growth.This study is one of the first describing the effects of combined amendments on a mining soil properties with focusing on metal(loid) mobility.

Contrasted tolerance of Agrostis capillaris metallicolous and non-metallicolous ecotypes in the context of a mining technosol amended by biochar, compost and iron sulfate

Metal(loid) contamination of soil, resulting from the mining activities, is a major issue worldwide, due to its negative effects on the environment and health. Therefore, these contaminated soils need to be remediated. One realistic method is the assisted phytostabilization, which aims at establishing a vegetation cover on the soil that will reduce metal(loid) bioavailability and spreading through the prevention of wind erosion and water leaching. In addition, amendments are applied to improve soil conditions and ameliorate plant growth. In this goal, biochar and compost showed good results in terms of amelioration of soil fertility and reduction in lead bioavailability. However, they usually have a negative effect on arsenic. On the contrary, iron sulfate showed capacity to reduce arsenic mobility through interaction with its iron hydroxides. Finally, the choice of the appropriate plant species is crucial for the success of assisted phytostabilization. One good option is to use endemic species, adapted to the metal(loid) stress, with a fast growth and large shoot and root systems. The aims of this study were to (1) evaluate the effects of applying biochar, compost and iron sulfate, alone or combined, to a former mine soil on the soil properties and Agrostis capillaris growth, and (2) assess the difference between two Agrostis capillaris ecotypes, an endemic metallicolous ecotype and a non-metallicolous ecotype. Results of the mesocosm experiment showed that amendment application improved soil properties, i.e., reduced soil acidity, increased nutrient availability and lower metal(loid) stress, the best being the combination biochar-compost-iron sulfate. These ameliorations allowed a better plant growth. Finally, the metallicolous ecotype performed better in terms of growth than the non-metallicolous one and could thus be used in an assisted phytostabilization process on the former mine site.

A new DGT technique comprising a hybrid sensor for the simultaneous high resolution 2-D imaging of sulfides, metallic cations, oxyanions and dissolved oxygen

A new diffusive gradients in thin films technique (HR-ZCA DGT) was developed for simultaneous two-dimensional (2-D) chemical imaging of sulfides, metallic cations and oxyanions (S, Cd, Co, Fe, Cu, Mn, Ni, Pb, Zn, As, Cr, Mo, Sb, Se, V, P and W) at the submillimeter scale, combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. A novel binding gel was prepared using a double precipitation method with AgI and zirconium oxide (Zr-oxide) deposited sequentially on a preformed Chelex-100 resin gel. A good linear relationship was observed (R²>0.99) between mass accumulation of the 17 assessed elements on the binding gel and the corresponding standardized laser ablation signals (signals of elements divided by signals of internal standard ¹³C), proving the feasibility of LA-ICP-MS analysis. Good analytical precision (RSD<12 %) was achieved for all 17 elements. A hybrid sensor comprising the novel DGT binding gel overlying an O₂ planar optrode was then tested in sediments to evaluate the dynamics of O₂ and multiple elements. Results showed that the mobility of As, P and W were controlled by precipitation/dissolution processes with Fe/Mn oxides. V, Co, Ni, Zn, Mo, Cd and Sb were released at the sediment surface with the oxidation of iron sulfides.

Variations of dissolved organic matter and Cu fractions in rhizosphere soil induced by the root activities of castor bean

The root soil interaction affects metal bioavailability in the rhizosphere, thus impacting the uptake and accumulation of metals by plants. In this study, a greenhouse experiment using a root-bag technique for castor bean plants was conducted to determine the i) rhizosphere effect on the fractions of Cu, and ii) the characteristics of dissolved organic matter (DOM) in the rhizosphere soil. Results showed that the Cu concentration in the leaves, stems, and roots was 15.41, 6.71, and 47.85 mg kg-1, respectively, in the control and reached up to 96.5, 254.9, and 3204 mg kg-1 in Cu400 treatment, respectively. After cultivating castor bean plants, the concentration of acid exchangeable Cu in rhizosphere soil was higher than that in the bulk soil for the same Cu addition, whereas the concentrations of reducible Cu, oxidizable Cu, and residual Cu in the rhizosphere soil were all lower than those in the bulk soil, respectively. In comparison to the bulk soil, the pH decreased while the total nitrogen and total carbon concentrations both increased in the rhizosphere soil. Moreover, the concentrations of total low molecular weight organic acids (LMWOAs) and total amino acids in the rhizosphere soil of the Cu treatments increased by between 15.18% to 47.17% and 36.35%-200%, respectively with respect to the control. The less complex DOM with a high LMWOAs concentration in the rhizosphere soil shifted the soil Cu from a relative stable fraction to available fractions.

Effect of biochar and redmud amendment combinations on Salix triandra growth, metal(loid) accumulation and oxidative stress response

Remediation of metal(loid) polluted soils is an important area of research nowadays. In particular, one remediation technique is much studied, phytomanagement. Phytomanagement combines amendment application and plant growth in order to reduce the risk posed by contaminants. Salicaceae plants showed tolerance towards metal(loid)s and the ability to accumulate high amounts of metal(loid)s in their tissue. Amendments are often applied to counterbalance the reduced soil fertility and high metal(loid) concentrations. Two amendments gathered attention over the last decades, biochar (product of biomass pyrolysis), which can be activated for better effects, and redmud (by-product of alumina production). Those two amendments showed ability to improve soil conditions and thus plant growth, although few studied their combined application. Moreover, since metal(loid)s are known to induce the overproduction of reactive oxygen species, it is important to measure the level of oxidative stress in the plant, to which plants respond using enzymatic and non-enzymatic systems. But no studies evaluate the response of Salicaceae plants to metal(loid) stress and amendment application at the biochemical level in a real soil condition. Therefore, a mesocosm study was set up to evaluate the effect of amending a mine soil with redmud combined to diverse biochars on the soil properties and Salix triandra growth, metal(loid) accumulation and stress marker levels. Results showed that all amendment combinations improved the soil fertility, reduced metal(loid) mobility and thus ameliorated Salix triandra growth, which accumulated metal(loid)s mainly in its roots. Moreover, among the different amendment combinations, Salix triandra plants still suffered from oxidative stress when grown on PG soil amended with redmud and chemical activated carbon, showing elevated levels of phenolic compounds and salicinoids and important antioxidant and enzymatic activities. Finally, one treatment showed levels of these stress markers similar or lower than the control, the combination of redmud with steam activated carbon. In conclusion, this treatment seemed a good solution in a phytomanagement strategy using Salix triandra, improving soil conditions and plant growth and reducing oxidative stress level in the plant roots.

Potential use of biochar, compost and iron grit associated with Trifolium repens to stabilize Pb and As on a multi-contaminated technosol

Vegetation cover can be used in the phytomanagement of polluted areas by adding value to abandoned sites and reducing the dispersion of pollutants by erosion. Appropriate amendments, that allow both efficient plant growth and the immobilization of contaminants in the soil must be chosen in order to optimize the efficiency of this process. We used a mining technosol mainly contaminated by arsenic (1068 mg kg^-1) and lead (23387 mg kg^-1) to study the effect of three amendments (biochar, compost and iron grit) on (i) physico-chemical properties of the soil and soil pore water, (ii) metal(loid) mobility, bioavailability and bioaccessibility (CaCl₂ and Simple Bioaccessibility Extraction Test (SBET)), and (iii) the capability of Trifolium repens to germinate and grow. All the amendments used increased the pH and electrical conductivity of the SPW, resulting in a 90% decrease in the concentration of lead in the soil pore water (SPW). We also demonstrated a decrease in Pb phytoavailability. The amendments allowed the establishment of a plant cover, although the addition of iron grit alone did not allow any clover germination. For the Pontgibaud technosol, the combination of the three amendments resulted in a significant decrease in As and Pb concentrations in clover tissues, mainly in the aerial organs. The amendments also made it possible for some of them to halve the phytoavailable fraction of arsenic. However, for compost, both the As concentrations in the SPW, and the bioavailable fraction of As increased. All the amendments used had contrasting effects on the bioaccessible fractions of metal(loid)s. The most efficient amendment combination was the addition of 5% biochar and 5% compost.

Spectral insight into thiosulfate-induced mercury speciation transformation in a historically polluted soil

We studied the effect of different doses (0.5%, 2% and 5% (w/w)) of ammonium thiosulfate on mercury (Hg) speciation fractionation following its addition to the soil, as well as its accumulation by oilseed rape (Brassica napus L.), corn (Zea mays L.), and sweet potato (Ipomoea batatas L.), and compared them to a non-treated control in a historically polluted soil. The oilseed rape, corn, and sweet potato were planted consecutively in the same soils on days 30, 191, and 276, respectively after the addition of thiosulfate to the soil. The key results showed that bioavailable Hg contents in the rhizosphere soils ranged from 0.18 to 2.54 μg kg^-1, 0.28 to 2.77 μg kg^-1, and 0.24 to 2.22 μg kg^-1, respectively, for the 0.5%, 2% and 5% thiosulfate treatments, which were close to the control soil (0.25 to 1.98 μg kg^-1). The Hg L₃-edge X-ray absorption near edge structure (XANES) results showed a tendency of the Hg speciation to transform from the Hg(SR)₂ (initial soil, 56%; day-191 soil, 43%; day-276 soil, 46%, and day-356 soil, 16%) to nano particulated HgS (initial soil, 26%; day-191 soil, 42%; day-276 soil, 42%, and day-356 soil, 73%) with time in the soil treated with a 5% dose of thiosulfate. The Hg contents in the tissues of the crops, except for oilseed rape, were slightly affected by the addition of thiosulfate to the soil at all dosages, compared to the control. The addition of thiosulfate did not induce the movement of bioavailable Hg to the lower layer of the soil profile. We conclude a promotion of Hg immobilization by thiosulfate in the soil for over one year, offering a promising method for in-situ Hg remediation at Hg mining regions in China.

Different dissolved organic matter (DOM) characteristics lead to diverse atrazine adsorption traits on the non-rhizosphere and rhizosphere soil of Pennisetum americanum (L.) K. Schum

Plant activities might alter the environmental behavior of organic pollutants in rhizosphere soil during phytoremediation. To further illustrate the mechanisms underlying the varying adsorption about the different adsorbing abilities of rhizosphere soil (RS) and non-rhizosphere soil (NRS) for the same pollutant, atrazine was selected to investigate the adsorption traits in the NRS and RS of Pennisetum americanum (L.) K. Schum (P. americanum), a potential phytoremediator of atrazine pollution. Furthermore, the different fluorescence spectral properties of the dissolved organic matter (DOM) extracted from RS and NRS when binding with atrazine were also investigated. RS exhibited a higher atrazine adsorption capacity than NRS, although the kinetic and isothermal properties of atrazine adsorption onto the two kinds soil were described by a pseudo second-order model and the Freundlich model. The DOM extracted from RS showed a stronger atrazine-binding ability than that extracted from NRS, as proven by the much more obvious decrease in fluorescence intensity when binding with atrazine. Although synchronous fluorescence spectra analysis suggested that both DOM types bind atrazine using a static fluorescence quenching mechanism, Fourier transform infrared spectroscopy showed that some distinct functional groups, which might liable to combine with atrazine, were found in only the DOM extracted from RS. Considering the findings mentioned above and the fact that the typical chemical characteristics of RS were different from those of NRS, we concluded that the P. americanum enhances the atrazine adsorption ability of RS by regulating the chemical characteristics and atrazine-binding ability of DOM in RS.

Aided phytostabilisation of As- and Cu-contaminated soils using white lupin and combined iron and organic amendments

An aided phytostabilisation strategy consisting of several composite amendments of iron sulphate and organic materials combined with Lupinus albus L. (white lupin) was evaluated for remediation of an As- and Cu-contaminated soil. Iron sulphate was combined with lime, paper mill sludge (PS), olive mill waste compost (OMWC) or holm oak biochar (BC) and applied to a slightly acidic soil with high concentration of As (∼2200 mg kg^-1) and Cu (∼150 mg kg^-1). White lupin was grown for 48 days in pots containing amended and non-amended soils and the effect of soil treatments on soluble and extractable trace elements, soil fertility and plant growth and composition was evaluated. The addition of the amendments raised soil pH and reduced soluble As (50-93%) and extractable As and Cu (50-89%). Despite the reduction of As- and Cu-extractable fractions, plant As and Cu uptake was not greatly affected by the amendments. Variations in soil pH and P-Olsen seemed to have influenced As dynamics in the treated soils, although they did not provoke its mobilisation with respect to the non-amended soil. Our results suggest that the freshly formed iron oxides resulting from addition of iron sulphate controlled As dynamics in the treated soils, avoiding its mobilisation due to application of organic materials. The combination of iron sulphate with OMWC and BC is shown as appropriate for aided phytostabilisation of metal(loid)s contaminated soils, as it improved soil fertility and plant nutrition while reduced As and Cu mobility.