Soil cadmium mobilisation by dissolved organic matter from soil amendments

Cadmium inhibits carbon and nitrogen cycling through soil microbial biomass and reduces soil nitrogen availability

Microbial mediated carbon and nitrogen cycling response to cadmium are often observed in soil; however, a unified framework of this response has not yet been established. By analyzing 1232 observations from 166 publications, we found that cadmium decreased microbial biomass carbon (-16 %) and nitrogen (-21 %), dissolved organic nitrogen (-27 %), nitrification rate (-17 %), microbial respiration rate (-12 %), and β-1,4-glucosidase (-21 %) and urease (-16 %) activities, but increased microbial metabolic quotient (+11 %) and fungal-to-bacterial ratio (+39 %). The cadmium impact was concentration-dependent, becoming more pronounced at higher concentrations. Increasing cadmium concentration reduced soil N mineralization rate and total N content, but increased microbial biomass carbon-to-nitrogen ratio. These results indicate that cadmium reduced carbon and nitrogen assimilation into microbial biomass and limited soil inorganic nitrogen production. Soil bulk density drove soil microbial biomass and nitrogen availability response to cadmium. Lower soil bulk density and higher initial carbon and clay contents and soil pH reduced the negative impact of cadmium on microbial biomass and nitrogen availability, suggesting that anthropogenic activities that enhance soil quality may mitigate the inhibitory effect of cadmium on soil carbon and nitrogen cycling. Our analysis provides critical implications for improving our understanding of the ecological consequences of cadmium on soil carbon and nitrogen cycling.

Experimental Evaluation of Zinc and Copper Terrestrial Ecotoxicity Prediction by Life Cycle Assessment in Agricultural Recycling of Livestock Effluent

The agronomic benefits of organic waste application on farmland can be overshadowed by the potential health and environmental impacts associated with trace element emissions, especially copper (Cu) and zinc (Zn). This has prompted a need for life cycle assessment (LCA) models to predict their terrestrial comparative toxicity potential (CTP) and impact score. We compared the LCA (reference method) prediction ability with that obtained with an experimental data set obtained from a month-long incubation experiment (experimental method) on a soil amended or not with 31 livestock effluents mimicking agronomically relevant scenarios. The impact scores for Cu and Zn differed between the reference and experimental methods due to CTP discrepancies between the two methods. The application of 31 livestock effluents on the soil resulted in variability in the experimentally determined CTP for Zn and Cu (0.6 and 1.9 log10 units) comparable to the CTP variability estimated by the reference method for several hundred worldwide soils. These results could be explained by the inadequate prediction of the dissolved organic carbon concentration and Cu and Zn concentration and speciation in soil solution. We propose refinements to the multilinear regressions used for the CTP computation that take the soil property patterns following livestock effluent application on soils into account.

Dolomite dissolution, pH neutralization, and potentially toxic element immobilization in stormwater bioretention beds

The importance of pH in stormwater bioretention beds cannot be overstated since it impacts plant and microbial populations and removal of potentially toxic elements (PTEs) from stormwater runoff. This study investigated the effects of dolomite amendment on pH neutralization and subsequent PTE immobilization in bioretention media. To assess dolomite dissolution, pH neutralization, and PTE immobilization, engineered bioretention media was amended with different dolomite ratios and samples of dolomite-amended media were collected from two bioretention beds, one and two months after installation. The effects of inflow conditions and operational time on dolomite dissolution and PTE immobilization were investigated through a column study. Laboratory batch experiments revealed that pH neutralization was fast and reached the recommended pH range of 5 to 8 within 5 min. Among the 1-D column conditions, intermittent inflow had the highest porewater Mg and Ca concentrations, indicating greater dolomite dissolution. Batch experiments on field-collected media showed that pH neutralization was substantial within 1 month, and continued during the second month, due to dolomite dissolution. Dissolved (batch experiments) and column porewater Mg and Ca concentrations supported instantaneous Ca and a relatively slow Mg dissociation during dolomite dissolution. Among the monitored PTEs, dissolved and porewater concentrations (μg/L) were found to decrease with time, in the order Mn > Fe > Zn > Cu > Pb > Cd, with Cr mainly undetected in all experimental conditions. However, as pH became neutralized to slightly basic (pH ~ 8), dissolved and porewater As concentrations increased. The study suggests that pH neutralization and PTE immobilization depend on the soil-to-dolomite ratio and hydrological properties such as inflow rate, dry-wet cycle, and soil-water contact time. Dolomite amendment to engineered media can be an effective measure for intercepting contaminants, improving ecosystem health, and enhancing biochemical contaminant breakdown, though consideration should be given to bioretention bed design features and system hydrology.

Mycorrhizal fungi mitigate cadmium leaching losses by decreasing the inorganic cadmium proportion in soil solutions

Arbuscular mycorrhizal fungi (AMF) are common in cadmium (Cd)-contaminated soil. However, the effects of AMF on Cd migration in contaminated soil are still poorly understood. A pot experiment involving a control without AMF inoculation (CK), inoculation with AMF (AMF), and bacterial filtrate of the AMF inoculant (LY) was conducted in the present study. AMF caused an increase in total glomalin-related soil protein (T-GRSP) of 12.2 % and in the exudation of low-molecular-weight organic acids (LMWOAs), such as citric acid, malic acid, oxalic acid, and free amino acids, by maize roots of 30.4 %-200.0 % but caused a decrease in the Cd contents of the maize shoots and roots by 53.2 % and 47.2 %, respectively compared those of the CK. Moreover, AMF decreased the Cd concentration in the leachate by 36.0 %-76.3 % by reducing the proportion of Cd2 + and inorganic Cd complexes in the soil solution and leachate by 26.2 %-85.7 %, and increasing the proportion of organic Cd complexes. Structural equation analysis revealed that the GRSP and LMWOAs were the primary factors driving the potential of AMF to reduce Cd leaching loss from polluted soil. The adsorption of Cd ions by quartz sand increased with the application of GRSP and LMWOAs, which resulted in a 27.6 %-69.5 % reduction in Cd leaching loss in the sand column. In the soil with AMF, the proportion of organic-bound Cd increased and the proportion of inorganic-bound Cd decreased by promoting the secretion of LMWOAs via mycelium, thus reducing Cd leaching loss.

New insights into the stages of cadmium remediation in ryegrass enhanced by kitchen compost-derived dissolved organic matter: Activation, absorption, and storage

Dissolved organic matter (DOM) regulates plant behavior in both agricultural and environmental fields. However, the regulatory mechanisms by which DOM influences soil-plant system interactions during the phytoremediation of Cd-contaminated soils remain unclear. Therefore, this study investigated the enhanced effect of kitchen compost-derived DOM on the Cd remediation capability of ryegrass across three phases of phytoremediation. The main pathways and mechanisms of DOM-assisted phytoremediation were identified through the analysis of changes in soil microbial communities and metabolism functions. The results revealed that DOM increased the bioavailability of soil Cd and significantly enhanced the Cd enrichment capacity of ryegrass, regardless of the application rate. The application of 20 % DOM to soil with a 20 mg/kg Cd content increased the bioconcentration factors of ryegrass roots and shoots by up to 38.19 and 11.08 times, respectively, compared with the control group. The direct or indirect optimizing effects of DOM on Cd fraction transformation, microbial communities, and their metabolism functions significantly enhanced the Cd enrichment capacity of ryegrass. Notably, DOM exhibited dual effects on ryegrass growth, mainly influenced by changes in soil physicochemical properties, optimization of microbial communities, and alterations in nitrogen metabolic functions. Additionally, the Cd reserves in ryegrass, which serve as a vital indicator of phytoremediation, exhibited a positive response to DOM. This study provides insights into the various reinforcing roles of kitchen compost-derived DOM in Cd-contaminated soil phytoremediation. These findings support the development of effective agronomic strategies for precise Cd regulation.

Leveraging machine learning for sustainable cultivation of Zn-enriched crops in Cd-contaminated karst regions

Karst soils often exhibit elevated zinc (Zn) levels, providing an opportunity to cultivate Zn-enriched crops. (meanwhile) However, these soils also frequently contain high background levels of toxic metals, particularly cadmium (Cd), posing potential health risks. Understanding the bioaccumulation of Cd and Zn and the related drivers in a high geochemical background area can provide important insights for the safe development of Zn-enriched crops. Traditional models often struggle to accurately predict metal levels in crop systems grown on soils with high geochemical background. This study employed machine learning models, including Random Forest (RF), Support Vector Machine (SVM), and Extreme Gradient Boosting (XGBoost), to explore effective strategies for sustainable cultivation of Zn-enriched crops in karst regions, focusing on bioaccumulation factors (BAF). A total of 10,986 topsoil samples and 181 paired rhizosphere soil-crop samples, including early rice, late rice, and maize, were collected from a karst region in Guangxi. The SVM and XGBoost models demonstrated superior performance, achieving R2 values of 0.84 and 0.60 for estimating the BAFs of Zn and Cd, respectively. Key determinants of the BAFs were identified, including soil iron and manganese contents, pH level, and the interaction between Zn and Cd. By integrating these soil properties with machine learning, a framework for the safe cultivation of Zn-enriched crops was developed. This research contributes to the development of strategies for mitigating Zn deficiency in crops grown on Cd-contaminated soils.

Cadmium availability in rhizosphere and non-rhizosphere soils in cacao farms in Santander, Colombia

Current research has highlighted the need to understand the factors influencing cadmium (Cd) availability in cacao-growing soils to elucidate its presence in cacao beans (the raw material for chocolate). Although literature about this topic is increasing, few report the importance of rhizosphere soils on Cd dynamics. This study aimed to understand the changes in available Cd and its association with soil properties (pH, pseudo-total Cd, available Cd (Cd-DTPA), Ca, Mg, K, Na, soil organic carbon, P, Zn, urease activity, exchangeable acidity, and cation exchange capacity) considering rhizosphere and non-rhizosphere soils. Both soil types (51 samples of each, 102 in total) were collected from two Colombian cacao farms. The medians of pseudo-total Cd (1.86 mg kg-1) and Cd-DTPA (0.76 mg kg-1) were, respectively, about threefold and fourfold higher in rhizosphere compared to non-rhizosphere soils. Principal component analysis showed a clear distinction between rhizosphere and non-rhizosphere soils based on differences in soil properties, which explained the observed changes in available Cd when comparing both soil types. Soil organic carbon and Zn were important drivers of available Cd in rhizosphere soils. Spatial distribution analysis revealed a tendency of available Cd to cluster in rhizosphere soils, and indicated hotspots within each farm. These findings highlight the importance of rhizosphere soils for Cd diagnosis and monitoring, and for improving knowledge about Cd dynamics in the soil-Theobroma cacao L. system.

In-situ remediation of cadmium contamination in paddy fields: from rhizosphere soil to rice kernel

Cadmium (Cd) has become an important heavy metal pollutant because of its strong migration and high toxicity. The industrial production process aggravated the Cd pollution in rice fields. Human exposure to Cd through rice can cause kidney damage, emphysema, and various cardiovascular and metabolic diseases, posing a grave threat to health. As modern technology develops, the Cd accumulation model in rice and in-situ remediation of Cd pollution in cornfields have been extensively studied and applied, so it is necessary to sort out and summarize them systematically. Therefore, this paper reviewed the primary in-situ methods for addressing heavy metal contamination in rice paddies, including chemical remediation (inorganic-organic fertilizer remediation, nanomaterials, and composite remediation), biological remediation (phytoremediation and microbial remediation), and crop management remediation technologies. The factors that affect Cd transformation in soil and Cd migration in crops, the advantages and disadvantages of remediation techniques, remediation mechanisms, and the long-term stability of remediation were discussed. The shortcomings and future research directions of in situ remediation strategies for heavily polluted paddy fields and genetic improvement strategies for low-cadmium rice varieties were critically proposed. To sum up, this review aims to enhance understanding and serve as a reference for the appropriate selection and advancement of remediation technologies for rice fields contaminated with heavy metals.

Reduction of the exchangeable cadmium content in soil by appropriately increasing the maturity degree of organic fertilizers

To enhance the remediation effect of heavy metal pollution, organic fertilizers with different maturity levels were added to cadmium-contaminated soil. The remediation effect was determined by evaluating the form transformation and bioavailability of cadmium in heavy metal-contaminated soil. -Results showed that when the maturity was 50%, although the soil humus (HS) content increased, it didn't contribute to reducing the bioavailability of soil Cd. Appropriately increasing the maturity (GI ≥ 80%), the HS increased by 113.95%∼157.96%, and reduced significantly the bioavailability of soil Cd, among the exchangeable Cd decreased by 16.04%∼33.51% (P < 0.01). The structural equation modeling (SEM) revealed that HS content is a critical factor influencing the transformation of Cd forms and the reduction of exchangeable Cd accumulation; the HS and residual Cd content were positively correlated with the maturity (P < 0.01), while exchangeable Cd content was negatively correlated with maturity (P < 0.01), and the correlation increased with increasing maturity. In summary, appropriately increasing the maturity (GI ≥ 80%) can increase significantly HS, promote the transformation of exchangeable Cd into residual Cd, and ultimately enhance the effectiveness of organic fertilizers in the remediation of soil Cd pollution. These results provide a new insight into the remediation of Cd-contaminated soil through organic fertilizer as soil amendment in Cd-contaminated soil.

Mechanisms insights into Cd passivation in soil by lignin biochar: Transition from flooding to natural air-drying

Biochar shows great potential in soil cadmium pollution treatment, however, the effect and mechanisms of biochar on cadmium passivation (CP) during the long-term process of soil from flooding to natural air-drying are not clear. In this study, a 300-day experiment was conducted to keep the flooded water level constant for the first 100 days and then dried naturally. Mechanisms of CP by lignin biochar (LBC) were analyzed through chemical analysis, FTIR-2D-COS, EEMs-PARAFAC, ultraviolet spectroscopy characterizations, and microbial community distribution of soil. Results showed that application of LBC results in rapid CP ratio in soil within 35 days, mainly in the residual and Fe-Mn bound states (total 72.80%). CP ratio further increased to 90.89% with water evaporation. The CP mechanisms include precipitation, electrostatic effect, humus complexation, and microbial remediation by promoting the propagation of fungi such as Penicillium and Trichoderma. Evaporation of water promoted the colonization of aerobic microorganisms and then increased the degree of soil humification and aromatization, thereby enhancing the cadmium passivation. Simultaneously, the biochar could reduce the relative abundance of plant pathogens in soil from 1.8% to 0.03% and the freshness index (β/α) from 0.64 to 0.16, favoring crop growth and promoting carbon sequestration and emission reduction.

Protaetia brevitarsis larvae produce frass that can be used as an additive to immobilize Cd and improve fertility in alkaline soils

Bioconversion of agricultural waste by Protaetia brevitarsis larvae (PBL) holds significant promise for producing high-quality frass organic amendments. However, the effects and mechanisms of PBL frass on Cd immobilization in an alkaline environment remain poorly understood. In this study, three types of frass, namely maize straw frass (MF), rice straw frass (RF), and sawdust frass (SF), were produced by feeding PBL. The Cd immobilization efficiencies of three frass in alkaline solutions and soils were investigated through batch sorption and incubation experiments, and spectroscopic techniques were employed to elucidate the sorption mechanisms of Cd onto different frass at the molecular level. The results showed that MF proved to be an efficient sorbent for Cd in alkaline solutions (176.67-227.27 mg g-1). X-ray absorption near-edge structure (XANES) spectroscopy indicated that Cd immobilization in frass is primarily attributed to the association with organic matter (OM-Cd, 78-90%). And MF had more oxygen-containing functional groups than the other frass. In weakly alkaline soils, MF application (0.5-1.5%) significantly decreased Cd bioavailability (5.65-18.48%) and concurrently improved soil nutrients (2.21-56.79%). Redundancy analysis (RDA) unveiled that pH, CEC, and available P were important factors controlling Cd fractions. Path analysis demonstrated that MF application affected Cd bioavailability directly and indirectly by influencing soil chemical properties and nutrients. In summary, MF, the product of PBL-mediated conversion maize straw, demonstrated promise as an effective organic amendment for Cd immobilization and fertility improvement in alkaline soils.

Effects of Biochar and Straw Amendment on Soil Fertility and Microbial Communities in Paddy Soils

Straw and biochar, two commonly used soil amendments, have been shown to enhance soil fertility and the composition of microbial communities. To compare the effects of straw and biochar on soil fertility, particularly focusing on soil dissolved organic matter (DOM) components, and the physiochemical properties of soil and microbial communities, a combination of high-throughput sequencing and three-dimensional fluorescence mapping technology was employed. In our study, we set up four treatments, i.e., without biochar and straw (B0S0); biochar only (B1S0); straw returning only (B0S1); and biochar and straw (B1S1). Our results demonstrate that soil organic matter (SOM), available nitrogen (AN), and available potassium (AK) were increased by 34.71%, 22.96%, and 61.68%, respectively, under the B1S1 treatment compared to the B0S0 treatment. In addition, microbial carbon (MBC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were significantly increased with the B1S1 treatment, by 55.13%, 15.59%, and 125.46%, respectively. The results also show an enhancement in microbial diversity, the composition of microbial communities, and the degree of soil humification with the application of biochar and straw. Moreover, by comparing the differences in soil fertility, DOM components, and other indicators under different treatments, the combined treatments of biochar and straw had a more significant positive impact on paddy soil fertility compared to biochar. In conclusion, our study revealed the combination of straw incorporation and biochar application has significant impacts and is considered an effective approach to improving soil fertility.

Organic amendment application affects the release behaviour, bioavailability, and speciation of heavy metals in zinc smelting slag: Insight into dissolved organic matter

Organic amendments are commonly used in assisted phytostabilization of mine wastes by improving their physicochemical and biological properties. These amendments are susceptible to leaching and degradation, resulting in the generation of dissolved organic matter (DOM), which significantly influences the geochemical behaviour of heavy metals (HMs). However, the geochemical behaviour of HMs in metal smelting slag driven by organic amendment-derived DOM remains unclear. In this study, we investigated the impact of cow manure-derived DOM on the release behaviour, bioavailability, and speciation of HMs (Cu, Pb, Zn, and Cd) in zinc smelting slag using a multidisciplinary approach. The results showed that DOM enhanced the weathering of the slag, with a minimal impact on the slag's mineral phases, except for causing gypsum dissolution. The DOM addition resulted in a slight increase in HM release from the slag during the initial inoculation period, followed by a reduction in HM release during the later period. Furthermore, the DOM addition increased the diversity and relative abundance of the bacterial community. This, in turn, led to a decrease in the dissolved organic carbon (DOC) content and enhanced the transformation of labile DOM compounds into recalcitrant compounds. The variation in HM release during various inoculation periods can be attributed to the bacterial decomposition and transformation of DOM, which further enhanced the transformation of HM fractions. Specifically, during the later period, DOM promoted the conversion of a portion of the reducible and oxidizable fractions of Cu, Pb, and Zn into the acid-soluble and residual fractions. Moreover, it partially transformed the reducible, oxidizable, and residual fractions of Cd into the acid-soluble fraction. Overall, this study provides new insights into the geochemical behaviour of HMs in slag governed by the coupling effect of DOM and the bacterial community. These findings have implications for the use of organic amendments in assisted phytostabilization of metal smelting slag. ENVIRONMENTAL IMPLICATION: Metal smelting slag is hazardous due to its high levels of HMs, and its improper disposal has serious consequences for the ecosystem. Organic amendments are employed in assisted phytostabilization of the slag site by improving its microecological properties. However, the impact of organic amendment-derived DOM on HM migration and transformation in slag remains unclear. This study indicated that the coupling effects of DOM and microbes governed the geochemical behaviour of HMs in slag. These findings provide new insights into how organic amendments impact the geochemical behaviour of HMs in slag, contributing to the development of phytostabilization technology.

Machine learning-based prediction of cadmium bioaccumulation capacity and associated analysis of driving factors in tobacco grown in Zunyi City, China

Tobacco grown in areas with high-geochemical backgrounds exhibits considerably different cadmium (Cd) bioaccumulation abilities due to regional disparities and environmental changes. However, the impact of key factors on the Cd bioaccumulation ability of tobacco grown in the karst regions with high selenium (Se) geochemical backgrounds is unclear. Herein, 365 paired rhizospheric soil-grown tobacco samples and 321 topsoil samples were collected from typical karst tobacco-growing soil in southwestern China and analyzed for Cd and Se. XGBoost was used to predict and evaluate the Cd bioaccumulation ability of tobacco and potential influencing factors. Results showed that regional geochemical characteristics, such as soil Cd and Se contents, soil type, and lithology, have the highest influence on the Cd bioaccumulation ability of tobacco, accounting for 46.5% of the overall variation. Moreover, soil Se contents in high-geochemical background areas considerably affect Cd bioaccumulation in tobacco, with a threshold for the mutual suppression effects of Cd and Se at a soil Se content of 0.8 mg/kg. According to the results of bivariate local indicators of spatial association analysis, tobacco cultivated in the central, northeast, and southeast regions of Zunyi City carries a lower risk of soil Cd contamination. This study provides new insights for managing tobacco cultivation in karst regions.

Ability of equilibrium and non-equilibrium models to simulate the effects of vermicompost and hydraulic conditions on nitrate and DOC leaching

ABSTRACTThis study aims to model the effects of saturated-unsaturated flow rates and initial moisture content on nitrate and dissolved organic carbon (DOC) leaching in soils amended and unamended with vermicompost using equilibrium and non-equilibrium models. Flow rates ranging from 0.4 to 5.1 cm3/min were applied to the columns filled with the soils under initial saturated and air-dried conditions. The leaching of nitrate and DOC was simulated using a one-dimensional advection-dispersion model coupled with the equilibrium and non-equilibrium models. The accuracy of equilibrium without distribution coefficient (KD), equilibrium with KD, one-site, two-site and dual porosity models for modelling the nitrate leaching was 21.8, 33.6, 67.5, 82.2 and 83.9%, respectively, indicating the higher accuracy of dual porosity and two-site models compared to the other models. According to the results of the two-site model, the kinetic release was the most dominant process in all leaching experiments due to the fractions of equilibrium soil sites (F) < 0.5. Vermicompost decreased the diffusion coefficient (D0), distribution coefficient (KD), first-order rate constant (β) and retardation factor (RF). In comparison to the air-dried condition, the initial saturated condition compared to the air-dried condition resulted in less F and D0, higher KD and RF lower β for nitrate and lower KD and RF and higher β for DOC. Leaching using a desaturation flow rate of 0.4 cm3/min was more time-dependent, which reduced RF values from 22.6 to 1.09 and 21.5 to 3.68 for nitrate and DOC, respectively. Moreover, the desaturation flow rate reduced D0 and KD and increased β.

Transport of dissolved organic matters derived from biomass-pyrogenic smoke (SDOMs) and their effects on mobility of heavy metal ions in saturated porous media

Biomass-pyrogenic smoke-derived dissolved organic matter (SDOMs) percolating into the underground environment profoundly impacts the transport and fate of environmental pollutants in groundwater systems. Herein, SDOMs were produced by pyrolyzing wheat straw at 300-900 °C to explore their transport properties and effects on Cu²⁺ mobility in quartz sand porous media. The results indicated that SDOMs exhibited high mobility in saturated sand. Meanwhile, the mobility of SDOMs was enhanced at a higher pyrolysis temperature due to the decrease in their molecular sizes and the declined H-bonding interactions between SDOM molecules and sand grains. Furthermore, the transport of SDOMs was elevated as pH values were raised from 5.0 to 9.0, which resulted from the strengthened electrostatic repulsion between SDOMs and quartz sand particles. More importantly, SDOMs could facilitate Cu²⁺ transport in the quartz sand, which stemmed from forming soluble Cu-SDOM complexes. Intriguingly, the promotional function of SDOMs for the mobility of Cu²⁺ was strongly dependent on the pyrolysis temperature. Generally, SDOMs generated at higher temperatures exhibited superior effects. The phenomenon was mainly due to the differences in the Cu-binding capacities of various SDOMs (e.g., cation-π attractive interactions). Our findings highlight that the high-mobility SDOM can considerably affect heavy metal ions' environmental fate and transport.

Past, present and future trends in the remediation of heavy-metal contaminated soil - Remediation techniques applied in real soil-contamination events

Most worldwide policy frameworks, including the United Nations Sustainable Development Goals, highlight soil as a key non-renewable natural resource which should be rigorously preserved to achieve long-term global sustainability. Although some soil is naturally enriched with heavy metals (HMs), a series of anthropogenic activities are known to contribute to their redistribution, which may entail potentially harmful environmental and/or human health effects if certain concentrations are exceeded. If this occurs, the implementation of rehabilitation strategies is highly recommended. Although there are many publications dealing with the elimination of HMs using different methodologies, most of those works have been done in laboratories and there are not many comprehensive reviews about the results obtained under field conditions. Throughout this review, we examine the different methodologies that have been used in real scenarios and, based on representative case studies, we present the evolution and outcomes of the remediation strategies applied in real soil-contamination events where legacies of past metal mining activities or mine spills have posed a serious threat for soil conservation. So far, the best efficiencies at field-scale have been reported when using combined strategies such as physical containment and assisted-phytoremediation. We have also introduced the emerging problem of the heavy metal contamination of agricultural soils and the different strategies implemented to tackle this problem. Although remediation techniques used in real scenarios have not changed much in the last decades, there are also encouraging facts for the advances in this field. Thus, a growing number of mining companies publicise in their webpages their soil remediation strategies and efforts; moreover, the number of scientific publications about innovative highly-efficient and environmental-friendly methods is also increasing. In any case, better cooperation between scientists and other soil-related stakeholders is still required to improve remediation performance.

Initial soil moisture conditions affect the responses of colloid mobilisation and associated cadmium transport in opposite directions

The effects of initial soil moisture on colloid-associated transport are still poorly understood given the well-recognized significance of colloid-facilitated transport of strongly-sorbing contaminants. In this study, Cd leaching was sequentially conducted in an intact soil column under three initial moisture conditions (near saturation, field capacity and dryness). Soil colloids were always the dominant carriers for Cd. However, upon the lowering of initial soil moisture, increased transport of colloids (96.2→101.0→168.2 mg) was observed, surprisingly, along with decreased transport of colloid-associated Cd (C-Cd) (23.9→10.7→8.2 µg) and enrichment factor (248.4→105.9→48.8 mg/kg) of Cd on colloids, resulting from pH reduction which increased Cd desorption and colloid size increase and/or ζ-potential decrease that showed lower affinity for Cd. Correlation, redundancy analysis and structural equation modelling revealed the dominantly positive role of colloids, EC plus cations (Ca²⁺ and Mg²⁺) in the release of C-Cd and dissolved Cd (D-Cd), respectively, under initial moistures of near saturation and field capacity. Under initially dry conditions, soil water potential showed dominantly negative effects on the transport of both C-Cd and D-Cd. These findings highlighted the critical role of initial moisture conditions in modulating colloid-facilitated Cd mobilisation, providing insights into the environmental risk assessment of heavy metals in other leaching scenarios.

A Study on the Behavior of Cadmium in the Soil Solution-Plant System by the Lysimeter Method Using the 109Cd Radioactive Tracer

In soils, cadmium (Cd) and its compounds, originating from industrial activities, differ both in mobility as well as in their ability to permeate the soil solution from naturally occurring cadmium compounds (native Cd). Therefore, the determination of the parameters of cadmium mobility in soils and its accumulation by plants in the soil-soil solution-plant system is very important from both scientific and practical viewpoints. ¹⁰⁹Cd was used as a radioactive tracer to study the processes of the transition of Cd into the aqueous phase and its uptake by plants over the course of a vegetative lysimeter experiment. Using sequential extraction according to the Tessier-Förstner procedure and modified BCR schemes, certain patterns were determined in the distribution of Cd/¹⁰⁹Cd among their forms in various compounds in the soil, along with the coefficients of the enrichment of native stable Cd with radioactive ¹⁰⁹Cd. It was shown that the labile pool of stable Cd compounds (29%) was significantly smaller than that of radioactive ¹⁰⁹Cd (69%). The key parameters characterizing the migration capacity of Cd in the soil-soil solution-plant system were determined. It was found that the distribution coefficient of native Cd between the soil and the quasi-equilibrium lysimeter solution exceeded the similar value for the ¹⁰⁹Cd radionuclide by 2.2 times, and the concentration coefficients of Cd and ¹⁰⁹Cd in the barley roots were 9 times higher than in its vegetative parts. During the experiment, the average removal of Cd (¹⁰⁹Cd) from the soil by each barley plant was insignificant: 0.002 (0.004)%. Based on the results of ¹³C nuclear magnetic resonance (NMR) spectroscopy of a lyophilized sample of the high-molecular-weight dissolved organic matter (HMWDOM) of the soil solution, its components were determined. It transpired that the isolated lyophilized samples of HMWDOM with different molecular weights had an identical structural and functional composition. The selective sorption parameters of the HMWDOM and humic acid (HA) with respect to Cd²⁺ ions were determined by the isotope dilution method.

Facile synthesis of waste-based CdS-loaded hierarchically porous geopolymer for adsorption-photocatalysis of organic contamination and its environmental risks

In order to obtain an adsorbent-photocatalyst with low-cost, strong stability and great reusability/recyclability, a waste-based and CdS-loaded hierarchically porous geopolymer (HPG) was prepared by facile synthesis. The adsorption-photocatalysis ability, reusability, and stability of HPG under different conditions were determined. Results indicated that HPG showed better adsorption-photocatalysis performance for organic dyes under alkaline environment, and it remained a high adsorption-photocatalysis efficiency after used for five times. Furthermore, HPG was stable in different environment conditions (strong acidic, acid raining, neutral, high salinity, and high alkali environment). The mass loss of HPG were around 3.22-6.68% (7 days extraction), and the immobilization rates of Cd²⁺ in neutral, high salinity, and high alkali environments were higher than 99.99%. Under visible light irradiation, HPG effectively photo-degraded the organic substances in overlying water of polluted sediments. After 330 min irradiation, the concentrations of COD and TOC were decreased from 47.52 mg/L and 20.9 mg/L to 16.58 mg/L and 11.19 mg/L, respectively. The humic-like and fulvic-like substances were transformed to protein-like substances under photo-degradation effect. This study confirmed that HPG possesses advantages in cost, chemical stability, and reusability, and it has a great potential to be used as in-situ remediation environmental functional material for organic contaminants in lake.

Distinct response of arsenic speciation and bioavailability to different exogenous organic matter in paddy soil

Land application of organic waste has been increasingly encouraged since it could sequester carbon to mitigate climate change. Considering the susceptibility of arsenic (As) bioavailability in soils to organic matter, understanding the influence of different exogenous organic matter on As biogeochemical behavior in rice-soil system is crucial to reasonably recycle organic waste on soils and ensure the food safety. In this study, impacts of two typical organic matter amendments, rice straw and humic substance, on the As speciation and bioavailability in paddy soil were investigated. Results showed that addition of both rice straw and humic substance could increase the dissolved organic carbon (DOC) content in soil solution by 16.4%-34.4% and 21.7%-53.2%, respectively, but the response of As speciation and bioavailability was quite different, showing the decoupling between As release and DOC. Rice straw addition increased As release to porewater by 28.0%-28.4%, particularly at the initial 0-18 days after the soil was flooded, but humic substance presented the opposite effect, decreasing As release by 27.4%-43.1% which was mainly attributable to the As^III immobilization. This study suggests that the organic matter with high contents of labile heteroaliphatic/aliphatic carbon, being easily to be biodegraded, should not be applied on As contaminated soils.

Continuously applying compost for three years alleviated soil acidity and heavy metal bioavailability in a soil-asparagus lettuce system

Soil acidification and heavy metal pollution are two common barrier factors threatening plant growth and agro-product quality. Applying manure compost is promising to alleviate soil acidity, while it may increase heavy metal accumulation in soil. In a 3-year field experiment, compost was applied for 12 consecutive harvest seasons at 15, 30, and 45 t ha^-1 in a slightly acidic soil. Samples were taken at the twelfth season to examine the changes of soil properties, vegetable productivity, heavy metal accumulation and bioavailability in the soil-asparagus lettuce system. The results showed that the pH values of the topsoil were increased by 0.49-0.75 units in compost added soils compared with no compost control, soil organic matter (SOM) contents and cation exchange capacity (CEC) were increased by 34-101% and 43-44%, respectively. The soil nutrient contents were also increased in compost treatments. Continuously applying compost increased Cd, Cu, and Zn concentrations in topsoil by up to 32, 20, and 22% and decreased Pb by 10%, while soil available Cd and Zn concentrations were reduced by up to 54 and 86%, and available Cu was increased by 19-63%. The biomass of asparagus lettuce was increased by 30-59% in compost treatments, with Cd and Zn concentrations in the plant tissues reduced by 28-50% and 14-67%. Cu concentrations in the lettuce shoots were increased by 20-39%. The concentration factor and total uptake of Cd and Zn in lettuce were effectively reduced in compost treatments. Cd was more prone to be taken up, translocated and accumulated from soil to the lettuce plant than the other heavy metals. Continuously applying compost over 3 years increased soil pH, SOM, CEC, nutrient contents, and lettuce productivity, decreased Cd and Zn bioavailability in the soil-lettuce system, while posing a risk of increasing heavy metal accumulation in topsoil.

Field-scale fluorescence fingerprints of biochar-derived dissolved organic matter (DOM) provide an effective way to trace biochar migration and the downward co-migration of Pb, Cu and As in soil

Although the benefits of biochar amendment for heavy metal(loid) immobilization in soil have been widely recognized, its migration in soil and the resultant effects on the risk of downward migration of metal(loid)s are still poorly understood. In this study, based on biochar derived dissolved organic matter (DOM), excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) technique was employed to trace biochar migration within one year in 0-100 cm soil profiles in the field. The vertical co-migration of Pb, Cu and As was also analyzed. With biochar amended, DOM, humification index (HIX) and biological index (BIX) in 0-60 cm soil profiles increased significantly, while pH only increased in the topsoil. The identified water-extracted DOM components showed that biochar could enhance the content of fulvic acids and humic acids in soil DOM and biochar might migrate downward to 60 cm soil profiles. Furthermore, toluene/methanol-extracted DOM also confirmed the migration extent of biochar, which was more suitable to trace biochar migration because of its high resistance to the long-term ageing in the field. Moreover, we found that biochar reduced the content of Cu in 0-60 cm soil profiles, but increased the available Pb and As in the 20-40 cm soil layers. The Pearson's correlation study confirmed a strong correlation (0.568**≤R ≤ 0.803**) between the content of heavy metal(loid)s and humic-like components of soil DOM, which suggested that biochar co-migrated with Pb, Cu and As, and the potential environmental risks of biochar should be fully evaluated while it was applied for soil metal(loid) remediation.

A quick and simple spectrophotometric method to determine total carbon concentrations in root exudate samples of grass species

Purpose

Root exudates are key components driving belowground interaction between plant, microbes and soil. High-end analytical approaches provide advanced insights into exudate metabolite diversity, however, the amount of total carbon (C) released by roots should always be determined as the most basic parameter when characterizing root exudation as it (i) provides quantitative information of C exuded into the surrounding soil and (ii) allows to relate the abundance of individual exudate compounds to total C released. Here we propose a simple and quick, spectrophotometry-based method to quantify total dissolved organic carbon (DOC) concentration in exudation samples that is based on measuring the absorption of a pre-filtered but otherwise untreated exudate sample at 260 nm (DOC260).

Method

Exudate samples collected from different grass genotypes (Zea mays, Oryza sativa, Hordeum vulgare) grown in various experimental settings (soil, hydroponic) were analysed with the DOC260 assay and results were compared with C concentrations obtained by liquid TOC-analyser.

Conclusion

We demonstrated that the DOC260 method allowed for quick and inexpensive measurements of total dissolved organic carbon concentrations in exudate samples from grass species grown under nutrient sufficient as well as under P deficient conditions. Interestingly, DOC260 failed to predict DOC concentrations in exudate samples from plants grown under Zn and Fe deficiency suggesting a strong shift in metabolite composition under micronutrient deficiency. Even though the applicability of the DOC260 method remains to be tested on exudate samples originating from dicots and plants exposed to other environmental stresses (e.g. pathogen attack, heavy metal stress, etc), it will help to increase our understanding of root exudation and related rhizosphere processes in the future.

A comprehensive study on aquatic chemistry, health risk and remediation techniques of cadmium in groundwater

Cadmium (Cd), a non-essential trace element, it's intrusion in groundwater has ubiquitous implications on the environment and human health. This review is an approach to comprehensively emphasize on i) chemistry and occurrence of Cd in groundwater and its concomitant response on human health ii) sustainable Cd remediation techniques, iii) and associated costs. Current study is depending on meta-analysis of Cd contaminations in groundwater and discusses its distributions around the globe. Literature review primarily comprises from the last three decades online electronic published database, which mainly includes i) research literatures, ii) government reports. On the basis of meta-data, it was concluded that Cd mobility depends on multiple factors: such as pH, redox state, and ionic strength, dissolved organic (DOC) and inorganic carbon (DIC). A substantially high Cd concentration has been reported in Lagos, Nigeria (0.130 mg/L). In India, groundwater is continuing to be contaminated by Cd in the proximity of industrial, agricultural areas, high concentrations (>8.20 mg/L) were reported in Tamil Nadu and Maharashtra. Depending on chemical behavior and ionic radius cadmium disseminate into the food chain and ultimately cause health hazard that can be measured by various index-based assessment tools. Instead of chemical adsorbents, nanoparticles, phytoextraction, and bioremediation techniques can be very useful in the remediation and management of Cd polluted groundwater at a low-cost. For Cd pollution, the development of a comprehensive framework that links the hydro-geological, bio-geochemical processes to public health is important and need to be further studied.

The spectral characteristics and cadmium complexation of soil dissolved organic matter in a wide range of forest lands

The quality and quantity of dissolved organic matter (DOM) greatly controls the fate of heavy metals. The characteristics of DOM and its interaction with metals are essential for the metal ecological risk assessment of soils. In this study, the DOM spectral characteristics of representative forest soils and the complex capacities between fluorescent DOM components and cadmium (Cd) were analyzed. Functional groups, such as carboxylic acids, alcohols and phenols, were determined by FT-IR analysis. Chromophoric DOM, fluorescent DOM and dissolved organic carbon (DOC) concentrations exhibited strong correlations with each other, indicating that variations of DOC could be well explained by Chromophoric DOM or fluorescent DOM due to high correlation coefficients. The spectral slope ratio was in the range of 0.85-5.90, implying an abundance of heavy macromolecular humic acids, peptides, and polycondensates. The absorbance spectral at 254 nm (SUVA₂₅₄) strongly correlated with SUVA₂₆₀ (r = 0.992, P < 0.01), indicating that hydrophobicity closely related with aromatic structure, and aromatic groups could be broadly hydrophobic. Fluorescence indices were from 1.62 to 2.21 and biological index values ranged from 0.54 to 1.14, where the DOM was mainly sourced from mixed terrestrial and autogenous inputs in most sites. Four universal fluorescence components were identified and characterized by fluorescence EEM-PARAFAC, including two humic-like (components 1 and 2), one tyrosine-like (components 3) and one fulvic-like (components 4) component. Both components 3 and 4 showed fluorescence quenching with increasing Cd concentrations, while components 1 and 2 had no evident change in fluorescence intensity. The logK₃ and logK₄ values ranged from 4.41 to 5.29 and 4.71 to 5.54, respectively, with most logK values of component 3 for Cd binding being smaller than that of component 4, thus, indicating that the fulvic acid substances exhibited stronger and more stable interactions with Cd than protein-like components.

Dynamic changes of rhizosphere soil bacterial community and nutrients in cadmium polluted soils with soybean-corn intercropping

BACKGROUND

Soybean-corn intercropping is widely practised by farmers in Southwest China. Although rhizosphere microorganisms are important in nutrient cycling processes, the differences in rhizosphere microbial communities between intercropped soybean and corn and their monoculture are poorly known. Additionally, the effects of cadmium (Cd) pollution on these differences have not been examined. Therefore, a field experiment was conducted in Cd-polluted soil to determine the effects of monocultures and soybean-corn intercropping systems on Cd concentrations in plants, on rhizosphere bacterial communities, soil nutrients and Cd availability. Plants and soils were examined five times in the growing season, and Illumina sequencing of 16S rRNA genes was used to analyze the rhizosphere bacterial communities.

RESULTS

Intercropping did not alter Cd concentrations in corn and soybean, but changed soil available Cd (ACd) concentrations and caused different effects in the rhizosphere soils of the two crop species. However, there was little difference in bacterial community diversity for the same crop species under the two planting modes. Proteobacteria, Chloroflexi, Acidobacteria, Actinobacteria and Firmicutes were the dominant phyla in the soybean and corn rhizospheres. In ecological networks of bacterial communities, intercropping soybean (IS) had more module hubs and connectors, whereas intercropped corn (IC) had fewer module hubs and connectors than those of corresponding monoculture crops. Soil organic matter (SOM) was the key factor affecting soybean rhizosphere bacterial communities, whereas available nutrients (N, P, K) were the key factors affecting those in corn rhizosphere. During the cropping season, the concentration of soil available phosphorus (AP) in the intercropped soybean-corn was significantly higher than that in corresponding monocultures. In addition, the soil available potassium (AK) concentration was higher in intercropped soybean than that in monocropped soybean.

CONCLUSIONS

Intercropped soybean-corn lead to an increase in the AP concentration during the growing season, and although crop absorption of Cd was not affected in the Cd-contaminated soil, soil ACd concentration was affected. Intercropped soybean-corn also affected the soil physicochemical properties and rhizosphere bacterial community structure. Thus, intercropped soybean-corn was a key factor in determining changes in microbial community composition and networks. These results provide a basic ecological framework for soil microbial function in Cd-contaminated soil.

Effects of environmental factors on soil bacterial community structure and diversity in different contaminated districts of Southwest China mine tailings

A mass of tailings left by mineral exploitation have caused serious environmental pollution. Although many studies have shown that soil microorganisms have the potential to remediate environmental pollution, the interaction mechanism between microorganisms and the surrounding environment of tailings is still unclear. In this study, 15 samples around pyrite mine tailing were collected to explore the ecological effects of environmental factors on bacterial community. The results showed that most of the samples were acidic and contaminated by multiple metals. Cadmium (Cd), copper (Cu), nickel (Ni) migrated and accumulated to into downstream farmlands while chromium (Cr) was the opposite. Proteobacteria, Chloroflex and Actinobacteria were the dominant phyla. Soil pH, total phosphorus (TP), total nitrogen (TN), available potassium (AK), available phosphorus (AP), the bacteria abundance and diversity all gradually increased with the increase of the distance from the tailing. Invertase, acid phosphatase, total organic carbon (TOC), pH, TP and Cr were the main influencing factors to cause the variation of bacterial community. This work could help us to further understand the changes in soil microbial communities around pollution sources.

Accumulation of phthalates under high versus low nitrogen addition in a soil-plant system with sludge organic fertilizers instead of chemical fertilizers

Nitrogen is the main nutrient in soil. The long-term addition of N leads to changes in the soil dissolved organic matter (DOM) and other quality indicators, which affects the adsorption and accumulation of organic pollutants. The use of organic fertilizer is important for the development of green agriculture. However, organic fertilizers (especially sludge organic fertilizers (SOFs) contain phthalates (PAEs) that may accumulate in the soil and result in environmental contamination. How this accumulation response varies with the magnitude of long-term N addition, especially in different soil layer profiles, remains unclear. Here, changes in the content of PAEs in the soil-plant system without and after SOFs application were studied through field experiments in soils with different N addition backgrounds (CK, N1, N3 (0, 100, 300 kg N ha^-1 yr^-1 respectively)). Our results showed that the application of SOFs increase the accumulation of PAEs in soil profiles and plant systems, increasing human health risks. The content of Σ₅PAEs in the topsoil increased from 0.96 ± 0.10 to 1.86 ± 0.09 mg kg^-1. Moreover, under a high N addition background and SOFs application, the characteristics of soil DOM change, and the accumulation of PAEs in soil was nearly 30% higher compared with the low N group. Some suggestions such as removing PAEs from SOFs during preparation, conducting soil surveys before applying PAEs, and using soil amendments, which are provided for optimizing the trialability and environmental safety of SOFs application.