Research progress on the environmental risk assessment and remediation technologies of heavy metal pollution in agricultural soil

Synergistic effects of allantoin and Achyranthes japonica-biochar profoundly alleviate lead toxicity during barley growth

Lead (Pb), a toxic metal, causes severe health hazards to both humans and plants due to environmental pollution. Biochar addition has been efficiently utilized to enhance growth of plants as well as yield in the presence of Pb-induced stress. The present research introduces a novel use of biochar obtained from the weed Achyranthes japonica to enhance the growth of plants in Pb-contaminated soil. An experiment was performed with 7 treatments: Control, Pb2+ (10 mg kg-1) only, biochar (4 %) only, allantoin (4 g kg-1) only, biochar combined with Pb2+, allantoin combined with biochar, as well as a combination of allantoin and biochar with Pb2+. Lead toxicity alone markedly diminished plant growth metrics, including root and shoot length, biomass (wet and dry), chlorophyll concentration, and grain production. The application of biochar, allantoin, or their joint administration markedly enhanced the length of shoots (by 50.3 %, 29 %, and 70 %), length of roots (by 69 %, 50 %, and 69 %), and fresh biomass of shoots (by 5 %, 29 %, and 5 %), respectively. This enhancement is ascribed to improved soil characteristics and more efficient absorption of nutrients. The application of biochar, allantoin and their combination improved the tolerance against Pb2+ by increasing the total chlorophyll level by 12 %, 16 %, and 17 %, respectively, vs. the control. Likewise, these amendments significantly (p < 0.05) improved the activity of antioxidant enzymes, including SOD, POD, and CAT by 49 %, 29 %, and 49 %, respectively. The resistance towards Pb2+ was enhanced by biochar, allantoin, and their combined application, with lower Pb2+ concentrations in shoots (59.9 %, 40.1 %, and 49.8 %), roots (48.2 %, 24.1 %, and 58.3 %), and grains (60.2 %, 29.7 %, and 40.1 %) compared to solely Pb-stress, respectively. In summary, converting the weed Achyranthes japonica into biochar and integrating it with allantoin provides an eco-friendly approach to control its proliferation while efficiently alleviating Pb-induced toxicity in plants.

Field-scale screening of pumpkin cultivars for cost-effectiveness of "repairing while producing" in cadmium-arsenic co-contaminated agricultural land

Soil contamination with heavy metals poses a significant health risk as these metals can be transferred to humans through agricultural products. This study aimed to identify pumpkin varieties with low cadmium and arsenic accumulation. To this end, we evaluated 25 pumpkin varieties. Results indicated that the accumulated contents of heavy metals in each organ followed the order: root (0.148 mg·kg-1) > leaf (0.100 mg·kg-1) > stem (0.076 mg·kg-1).The transfer coefficients of these 25 pumpkin varieties were greater for leaves than for stems, yet none exceeded 1. Rhizosphere pH is a critical factor influencing the uptake of Cd by pumpkins, with a less significant impact on As uptake. This study provides a foundation for identifying pumpkin varieties with low Cd and As accumulation potential.Additionally, it contributes to the exploration of pumpkin's potential as a phytoremediation resource and its safe production and utilization in heavy metal-contaminated soils.

Sequential distribution, potential sources, and health risk assessment of persistent toxic substances in sewage sludge used as organic fertilizer in Indo-Gangetic region

This study evaluates the environmental and human health impact of sewage sludge generated in the Indo-Gangetic region (Uttarakhand and Uttar Pradesh) used as organic fertilizer and landfill disposal. The research conducts a comprehensive risk assessment, including physicochemical and heavy metals analysis, on triplicate sludge samples obtained from 30 sewage treatment plants. The study provides both qualitative and quantitative insights into potential hazards associated with sewage sludge. The results indicate varying concentrations (mg/kg) of heavy metals in sewage sludge (expressed in mean ± SD) as determined by inductively coupled plasma mass spectrometry (ICP-MS) with the following order, zinc (966.15 mg/kg ± 279) > chromium (851.23 mg/kg ± 3079) > cadmium (150.07 mg/kg ± 307) > copper (186.09 mg/kg ± 56.25) > arsenic (5.24 mg/kg ± 3.54) > nickel (21.97 mg/kg ± 2.13) > mercury (1.05 mg/kg ± 0.12). The metal pollution indices underscore greater non-compliance in samples from STPs of Uttar Pradesh compared to those from Uttarakhand, with 40% of samples falling into poor to particularly poor categories. Multivariate analysis of samples reveals potential pollution sources, implicating industrial effluents and agricultural runoff, with identified controlling factors being Cu (0.948, p < 0.05); Zn (0.941 p < 0.05); Pb (0.921 p < 0.05); Ni (0.806 p < 0.05); Cd (0.717, p < 0.05); and electrical conductivity (0.620, p < 0.05). Monte Carlo-based uncertainty analysis emphasizes sludge-based chromium (Cd) as the highest risk at 62.86% (p < 0.001), trailed by chromium (59.29%, p < 0.001) for target cancer risk. The study also suggests potential management options, including the application of AI-based sensors for heavy metal monitoring, exploration of improved trapping or diluting technologies, and raising public awareness about stringent rules regarding sewage sludge remediation for effective risk mitigation strategies.

Effects of Distiller's Grains Biochar and Lactobacillus plantarum on the Remediation of Cd-Pb-Zn-Contaminated Soil and Growth of Sorghum-Sudangrass

Soil contamination with heavy metals is a significant environmental issue that adversely affects plant growth and agricultural productivity. Biochar and microbial inoculants have emerged as a promising approach to solving this problem, and previous studies have focused more on the remediation effects of single types of materials on heavy metal soil pollution. This study examined the impact of both standalone and combined applications of distiller's grains biochar, Lactobacillus plantarum thallus, and the bacterial supernatant on the availability of cadmium (Cd), lead (Pb), and zinc (Zn) in soil, its physicochemical features, and its enzyme activities; this study also examined the growth, physiological and biochemical characteristics, and heavy metal accumulation of Sorghum-sudangrass. The findings suggest that the application of distiller's grains biochar, Lactobacillus plantarum thallus, and the bacterial supernatant can improve the soil's physical and chemical properties and enhance soil enzyme activity while reducing the availability of heavy metals in the soil. Furthermore, the addition of these materials promoted plant growth, increased stress resistance, and significantly decreased the accumulation of heavy metals in the plants. A thorough analysis of the results shows that applying 0.025% Lactobacillus plantarum thallus along with 4.4% distiller's grains biochar produced the best results.

Insights on Immobilization of Cd Contamination in Soil: Synergic Impacts of Water Management and Bauxite Residue

To immobilize the activity and bioavailability of soil Cd, the single treatment only flooding (F) and the combined treatments with flooding plus bauxite residue (F-B) or lime (F-L) were designed to investigate the impacts of different treatments on the toxicity and bioavailability of Cd in contaminated soil. Compared with the single treatment (F), the combined treatments (F-B and F-L) improved soil-associated organic functional groups and aggregated stability in soil. The average particle sizes of soil aggregates increased from 126 nm (F-treated soil) to 256 and 270 nm following F-B and F-L treatments, respectively. Relative to F treatment, the combined treatments (F-B and F-L) increased soil pH, soil EC, and residual Cd content in soil and reduced exchangeable Cd and acid-soluble Cd content in soil. The exchangeable Cd contents in soils were decreased to 3.17 and 3.42 mg/kg following F-B and F-L treatments in comparison with F-treated soils (4.31 mg/kg), respectively. For the soils with F-B and F-L treatments, soil residual Cd contents increased from 54% (F treatment) to 57 and 56%, respectively, and soil acid-soluble Cd contents decreased from 46% (F treatment) to 37 and 43%, respectively. A negative correlation was found in soil pH versus soil exchangeable Cd and soil acid-soluble Cd. In addition, the F-B treatment exhibited superiority in suppressing toxicity and bioavailability of soil Cd, owing to that F-B treatment is easy to induce neutralization reaction and immobilization effect in contaminated soil. The findings offer evidences that F-B treatment is a facile approach to suppress toxicity and bioavailability of soil Cd, which shows potential for immobilization of Cd in soil.

A Review on Remediation Technology and the Remediation Evaluation of Heavy Metal-Contaminated Soils

With the rapid development of industry and agriculture, soil contamination has become a significant environmental issue, and the heavy metal contamination of soils is an important part of it. The main methods for the remediation of heavy metal-contaminated soils include physical methods, chemical methods, biological methods, and combined remediation methods have been proposed as research deepens. However, the standards and evaluation methods for the remediation of heavy metal-contaminated soils are still not well-established. This article discusses the sources and contamination status of heavy metals in soils, the advantages and disadvantages of remediation technology for heavy metal-contaminated soils, remediation standards, and post-remediation evaluation methods. It also proposes scientific issues to be addressed in future research and provides an outlook on future development, hoping to assist in subsequent remediation studies of heavy metal-contaminated soils.

Composites Based on Natural Zeolites and Green Materials for the Immobilization of Toxic Elements in Contaminated Soils: A Review

Soil contamination by toxic elements is a global problem, and the remediation of contaminated soils requires complex and time-consuming technology. Conventional methods of soil remediation are often inapplicable, so an intensive search is underway for innovative and environmentally friendly ways to clean up ecosystems. The use of amendments that stabilize the toxic elements in soil by reducing their mobility and bioavailability is one of the simplest and most cost-effective ways to remediate soil. This paper provides a summary of studies related to the use of composites based on natural zeolites and green materials for the immobilization of toxic elements in contaminated soils and highlights positive examples of returning land to agricultural use. The published literature on natural zeolites and their composites has shown that combinations of zeolite with biochar, chitosan and other clay minerals have beneficial synergistic effects on toxic element immobilization and soil quality. The effects of zeolite properties, different combinations, application rates, or incubation periods on toxic elements immobilization were tested in laboratory scale or field experiments, whereas the mobility of toxic elements in soil was evaluated by chemical extractions of toxic elements transferred to the plants. This review highlights the excellent potential of natural zeolites to be used as single or combined sustainable green materials to solve environmental pollution problems related to the presence of toxic elements.

Multidimensional effects of green waste vermicomposting on cadmium contaminated soil ecosystems: From physicochemical properties to microbial communities

Soil heavy metal pollution and green waste accumulation have emerged as two major environmental challenges, necessitating the development of sustainable remediation and management technologies. This study investigated the remediation effects of vermicomposted green waste (JE) on cadmium (Cd)-polluted soil. Batch adsorption tests and soil microcosm experiments were conducted to examine the impact of JE on soil quality, microbial community structure, and Cd biotransformation. Results demonstrated that, compared with untreated green waste, JE significantly increased the Cd2+ adsorption capacity by 55.94 %. This enhancement was attributed primarily to increased surface functional groups and altered crystal structure through vermicomposting. JE treatment effectively improved the soil physicochemical properties, increased the nutrient content and elemental exchangeability, and increased soil enzyme activities. At the microbial level, JE drove the assembly and modification of soil microbial communities, increasing their diversity and abundance, particularly those of beneficial bacterial groups. Environmental matrix analysis revealed complex interactions among soil properties, enzyme activities, and soil microbial communities in terms of Cd biotransformation. Overall, vermicomposted green waste rapidly improved the Cd adsorption efficiency and, upon its soil application, effectively enhanced the Cd-polluted soil quality while optimizing soil microbial community structure and function. This ultimately led to Cd immobilization and inert transformation in the soil. This study provides a solid theoretical and practical foundation for the safe utilization and sustainable remediation of heavy metal-polluted agricultural soils, as well as the resource utilization of green waste.

Mapping of heavy metal pollution density and source distribution of campus soil using geographical information system

In this study, the pollution intensity, spatial distribution, and index-based risk distribution in campuses, which are a small prototype of cities, were mapped and the sources of heavy metals in the soil were investigated. Soil samples were taken from 9 different points from the Aksaray University Central campus, which was determined as the study area. It has been determined that the pH value in the collected soil samples varies between 8.7 and 11.0. This situation created an effect on reducing the accumulation and mobility of heavy metals in the soil. When the study area was evaluated based on the geo-accumulation index, Pb heavy metal was much denser in the places indicated as circulation areas and where students were actively present. Based on the pollution load index, it was concluded that 75% of the study area was moderately/highly polluted, and the rest consisted of unpolluted soils. Pearson correlation analysis and APCS-MLR analyses conducted to determine the source distribution showed that the contributions of natural sources, mixed sources of industrial and traffic activities, agricultural activity-based sources, and other sources were 57.49%, 21.44%, 12.67%, and 8.40%, respectively. Pb is mainly related to the mixed sources of industrial and traffic activities. Therefore, to clear up its long-term impact on the accumulation of heavy metals in the soil, it is important to conduct continuous heavy metal monitoring in the soil throughout the campus.

Stabilization effect of nano-SiO2@iron-phosphorus on ferrallisols, calcareous soil and organic soil heavily polluted by heavy metals: A fast reaction curing stabilization process

The remediation of soil pollution by heavy metals (HMs) presents a significant challenge in environmental restoration. Stabilization remediation technology has proven effective in treating HMs contaminated soil. However, its development is constrained by drawbacks such as slow reaction kinetics and low adsorption capacity. This research synthesized a nano-SiO2@iron‑phosphorus (FPOH) material by SiO32- encapsulating the iron-phosphate precipitate obtained from Fe ion and phosphate. In addition, this research applied this material to ferrallisols, calcareous soils and organic soils with three different levels of high pollution by Cd, Pb, Cu and Zn. The experimental results indicate that all experimental soils stabilized rapidly within 1 day and met the requirements of remediation engineering standards (ChinaMEE HJ 1282-2023). Analysis of the possible mechanisms suggests that the FPOH material effectively fills voids with phosphate mineral formation, preventing the secondary release of HMs. During the stabilization process, FPOH involves the adsorption of free ions and small organic molecules in the soil, which does not affect its high reactivity. The development and utilization of FPOH offer valuable insights for soil stabilization remediation.

Assessing soil remediation effect of Cr and Pb based on bioavailability using DGT, BCR and standardized determination method

In the field of soil remediation, the importance of bioavailability of pollutants has not received adequate attention, leading to the excessive application of remediation measures. Therefore, to ensure the safe use of farmland soil, a scientific method is needed to assess labile contaminants and their translocation in plants. To evaluate soil remediation effect based on bioavailability, the concentrations of these heavy metals in soil were analyzed using by the method for total metal content, the Community Bureau of Reference (BCR) extraction, and the diffusive gradients in thin films (DGT) technique. The results reveal that the correlation coefficients between metal concentrations measured by DGT and those accumulated in rice grains are the highest (Cr-R2 = 0.8966, Pb-R2 = 0.9045). However, the capability of method for total metal content to evaluate the remediation effect of heavy metals is very limited. In contrast, although Cr and Pb measured by BCR show a high correlation with HMs in rice plants, the method still falls short in precisely assessing bioavailability. Significantly, DGT proves to be more effective, successfully distinguishing the remediation effects of different treatments. Generally, DGT offers a more accurate and simpler assessment method, underscoring its practical significance for monitoring soil remediation and environmental management.

Predicting Cd accumulation in crops and identifying nonlinear effects of multiple environmental factors based on machine learning models

The traditional prediction of the Cd content in grains (Cdg) of crops primarily relies on the multiple linear regression models based on soil Cd content (Cds) and pH, neglecting inter-factorial interactions and nonlinear causal links between external environmental factors and Cdg. In this study, a comprehensive index system of multi-type environmental factors including soil properties, geology, climate, and anthropogenic activity was constructed. The machine learning models of the tree-based ensemble, support vector regression, artificial neural network for predicting Cdg of rice and wheat based on the environmental factor indexes significantly improved the accuracy than the traditional models of linear regression based on soil properties. Among them, the tree-based ensemble models of XGboost and random forest exhibited highest accuracies for predicting Cdg of rice and wheat, with R2 in the test dataset of 0.349 and 0.546, respectively. This study found that soil properties, including Cds, pH, and clay, have greater impacts on Cdg of rice and wheat, with combined contribution rates accounting for 65.2 % and 29.7 % respectively. Since wheat sampling areas are located in central and northern China, they are more constrained by precipitation and temperature than rice sampling areas in the south. Geologic and climate factors have a greater impact on Cdg of wheat, with a combined contribution rate of 49.9 %, which is higher than the corresponding rate of 20.9 % in rice. Furthermore, the Cdg of rice and wheat did not exhibit an absolute linear relationship with Cds, and excessively high Cds can reduce the bioconcentration factor of Cd accumulation in crops. Meanwhile, other environmental factors such as temperature, precipitation, elevation have marginal effects on the increase of Cdg of crops. This study provides a novel framework to optimize traditional soil plant transfer models, as well as offer a step towards realizing high precision prediction of Cd content in crops.

Insights into the critical roles of water-soluble organic matter and humic acid within kitchen compost in influencing cadmium bioavailability

Compost has demonstrated potential as a cadmium (Cd) remediation agent, while it still remains unclear about the core components in driving the bioactive transformation of Cd. To address this issue, this study isolated three components-kitchen compost powder (KC), humic acid (HA), and water-soluble organic matter (DOM)-from kitchen compost to regulate soil properties, bacterial community structures and functions, and Cd migration risks. The results revealed that the addition of 20% KC and HA reduced the bioavailability factor of Cd by 47.20% and 16.74%, respectively, with HA contributing 35.47% of the total reduction achieved with KC. Conversely, the application of DOM increased the Cd risk through a reduction in soil pH and an increase in the abundance of Cd-activating bacteria, which adversely affected the stability of Cd complexes. However, the porous structure and organic matter in KC and HA provided adsorption sites for Cd passivation and promoted the growth of Cd-fixing bacteria. This study effectively identifies both the positive and negative effects of key compost components on Cd migration and provides scientific guidance for applying kitchen compost in soil management.

Advancements in nanoparticle-modified zeolites for sustainable water treatment: An interdisciplinary review

The contamination of water sources with heavy metals, dyes, and other pollutants poses significant challenges to environmental sustainability and public health. Traditional water treatment methods often exhibit limitations in effectively addressing these complex contaminants. In response, recent developments in nanotechnology have catalyzed the exploration of novel materials for water remediation, with nanoparticle-doped zeolites emerging as a promising solution. This comprehensive review synthesizes current literature on the integration of nanoparticles into zeolite frameworks for enhanced contaminant removal in water treatment applications. We delve into synthesis methodologies, elucidate mechanistic insights, and evaluate the efficacy of nanoparticle-doped zeolites in targeting specific pollutants, while also assessing considerations of material stability and environmental impact. The review underscores the superior adsorptive and catalytic properties of nanoparticle-doped zeolites, owing to their high surface area, tailored porosity, and enhanced ion-exchange capabilities. Furthermore, we highlight recent advancements in heavy metal and organic pollutant uptake facilitated by these materials. Additionally, we explore the catalytic degradation of contaminants through advanced oxidation processes, demonstrating the multifunctionality of nanoparticle-doped zeolites in water treatment. By providing a comprehensive analysis of existing research, this review aims to guide future developments in the field, promoting the sustainable utilization of nanoparticle-doped zeolites as efficient and versatile materials for water remediation endeavors.

Improved mapping of heavy metals in agricultural soils using machine learning augmented with spatial regionalization indices

The accurate spatial mapping of heavy metal levels in agricultural soils is crucial for environmental management and food security. However, the inherent limitations of traditional interpolation methods and emerging machine-learning techniques restrict their spatial prediction accuracy. This study aimed to refine the spatial prediction of heavy metal distributions in Guangxi, China, by integrating machine learning models and spatial regionalization indices (SRIs). The results demonstrated that random forest (RF) models incorporating SRIs outperformed artificial neural network and support vector regression models, achieving R2 values exceeding 0.96 for eight heavy metals on the test data. Hierarchical clustering for feature selection further improved the model performance. The optimized RF models accurately predicted the heavy metal distributions in agricultural soils across the province, revealing higher levels in the central-western regions and lower levels in the north and south. Notably, the models identified that 25.78 % of agricultural soils constitute hotspots with multiple co-occurring heavy metals, and over 6.41 million people are exposed to excessive soil heavy metal levels. Our findings provide valuable insights for the development of targeted strategies for soil pollution control and agricultural soil management to safeguard food security and public health.

Phytoremediation of molybdenum (Mo)-contaminated soil using plant and humic substance

The severity of soil molybdenum (Mo) pollution is increasing, and effective management of contaminated soil is essential for the sustainable development of soil. To investigate this, a pot experiment was carried out to assess the impact of different rates of humic acid (HA) and fulvic acid (FA) on the mobility of Mo in soil solution and its uptake by alfalfa, wheat and green bristlegrass. The concentration of Mo in Plants and soil was determined using an Atomic Absorption Spectrophotometer. The findings revealed that the application of HA led to an increase in Mo accumulation in the shoot and root of green bristlegrass and wheat, ranging from 10.56 % to 28.73 % and 62.15-115.79 % (shoot), and 17.52-46.53 % and 6.29-81.25 % (root), respectively. Nonetheless, the use of HA resulted in a slight inhibition of plant Mo uptake, leading to reduced Mo accumulation in alfalfa roots compared to the control treatment (from 3284.49 mg/kg to 2140.78-2813.54 mg/kg). On the other hand, the application of FA decreased Mo accumulation in the wheat shoot (from 909.92 mg/kg to 338.54-837.45 mg/kg). Furthermore, the bioavailability of green bristlegrass (with HA) and wheat (with FA) decreased, and the percentage of residual fraction of Mo increased (from 0.39 % to 0.78-0.96 %, from 3.95 % to 3.97∼ 4.34 %). This study aims to elucidate the ternary interaction among Mo, humic substances, and plants (alfalfa, wheat, and green bristlegrass), to enhance both the activation and hyperaccumulation of Mo simultaneously.

Harnessing Lignocellulosic Crops for Phytomanagement of Contaminated Soils: A Multi-Country Study

The dwindling availability of agricultural land, caused by factors such as rapid population growth, urban expansion, and soil contamination, has significantly increased the pressure on food production. To address this challenge, cultivating non-food crops on contaminated land has emerged as a promising solution. This approach not only frees up fertile soil for food production but also mitigates human exposure to contaminants. This work aimed to examine the impact of soil contamination with Cd, Pb, Ni, and Zn on the growth, productivity, metal accumulation, and the tolerance of five lignocellulosic non-food crops: switchgrass (Panicum virgatum L.), biomass sorghum (Sorghum bicolor L. Moench), giant reed (Arundo donax L.), African fodder cane (Saccharum spontaneum L. spp. aegyptiacum Willd. Hackel), and miscanthus (Miscanthus × giganteus Greef et Deu.). A two-year pot experiment was conducted in Greece, Italy, and Portugal, following the same protocols and applying various levels of metals: Cd (0, 4, 8 mg kg-1), Pb and Zn (0, 450, 900 mg kg-1), and Ni (0, 110, 220 mg kg-1). The experimental design was completely randomized, with three replicates for each treatment. The results showed that switchgrass and sorghum generally maintained their height and productivity under Cd and Pb stress but were adversely affected by high Zn and Ni concentrations. Giant reed and African fodder cane showed reduced height and productivity at higher Ni and Zn levels. Miscanthus exhibited resilience in height but experienced productivity reductions only at the highest Zn concentration. Heavy metal uptake varied among crops, with switchgrass and sorghum showing high Cd and Pb uptake, while giant reed accumulated the most Cd and Zn. Miscanthus had the highest Ni accumulation. The tolerance indices indicated that switchgrass and sorghum were more tolerant to Cd and Zn at lower concentrations, whereas miscanthus had lower tolerance to Cd but a higher tolerance to Zn at higher concentrations. Giant reed and African fodder cane demonstrated stable tolerance across most heavy metals. Accumulation indices highlighted the effectiveness of switchgrass and sorghum in Cd and Pb uptake, while miscanthus excelled in Ni and Zn accumulation. The cluster analysis revealed similar responses to heavy metal stress between African fodder cane and giant reed, as well as between sorghum and miscanthus, with switchgrass displaying distinct behavior. Overall, the study highlights the differential tolerance and accumulation capacities of these crops, indicating the potential for phytoremediation applications and biomass production in heavy metal-contaminated soils.

Investigating the environmental capacity of soil heavy metals and its determinants in agro-pastoral regions of the qinghai-tibetan plateau

Environmental capacity (EC) serves as the basis for environmental planning and management, as a key indicator for assessing environmental risk and quality, and as a foundation for achieving sustainable development. Studies on EC typically address agricultural or urban rather than pastoral areas, with few examining agro-pastoral areas. The EC of the Tibetan Plateau is particularly important, considering its importance as an agricultural area and ecological reserve. To address this gap, the Qingshizui area in Menyuan County, a typical agro-pastoral area on the Tibetan Plateau, was selected to quantify soil EC and its spatial distribution. In terms of the dynamic and static annual soil EC for this region, the heavy metals were ranked as follows, in ascending order: Cd, Hg, Co, As, Sb, Ni, Cu, Pb, Cr, and Zn. Most of the areas with high residual EC were in the west. For the 10 heavy metals, residual EC was significantly affected by geological background. For all the heavy metals except Zn and Hg, residual EC was significantly affected by soil type. The heavy metal elements in the agro-pastoral area's soil are mildly enriched, suggesting minimal human impact. The composite EC index of this soil is 0.98, indicating an intermediate EC and low health risk. This study underscores that integrating agriculture and pastoralism can optimize land use and mitigate ecological pressures associated with these practices when done separately. Our research provides valuable insights for resource optimization, environmental conservation, and enhancing the welfare of farmers and herders in the Qinghai-Tibet region.

Identifying interactive effects of spatial drivers in soil heavy metal pollutants using interpretable machine learning models

The accurate identification of spatial drivers is crucial for effectively managing soil heavy metals (SHM). However, understanding the complex and diverse spatial drivers of SHM and their interactive effects remains a significant challenge. In this study, we present a comprehensive analysis framework that integrates Geodetector, CatBoost, and SHapley Additive exPlanations (SHAP) techniques to identify and elucidate the interactive effects of spatial drivers in SHM within the Pearl River Delta (PRD) region of China. Our investigation incorporated fourteen environmental factors and focused on the pollution levels of three prominent heavy metals: Hg, Cd, and Zn. These findings provide several key insights: (1) The distribution of SHM is influenced by the combined effects of various individual factors and interactions within the source-flow-sink process. (2) Compared with the spatial interpretation of individual factors, the interaction between Hg and Cd exhibited enhanced spatial explanatory power. Similarly, interactions involving Zn mainly demonstrated increased spatial explanatory power, but there was one exception in which a weakening was observed. (3) Spatial heterogeneity plays a crucial role in determining the contributions of environmental factors to soil heavy metal concentrations. Although individual factors generally promote metal accumulation, their effects fluctuate when interactions are considered. (4) The SHAP interpretable method effectively addresses the limitations associated with machine-learning models by providing understandable insights into heavy metal pollution. This enables a comparison of the importance of environmental factors and elucidates their directional impacts, thereby aiding in the understanding of interaction mechanisms. The methods and findings presented in this study offer valuable insights into the spatial heterogeneity of heavy metal pollution in soil. By focusing on the effects of interactive factors, we aimed to develop more accurate strategies for managing SHM pollution.

Evaluating the Response of the Soil Bacterial Community and Lettuce Growth in a Fluorine and Cadmium Co-Contaminated Yellow Soil

The impact of cadmium (Cd) and fluorine (F) on plant and human health has provoked significant public concern; however, their combined effects on plant and soil bacterial communities have yet to be determined. Here, a pot experiment was conducted to evaluate the effects of exogenous F, Cd, and their combination (FCd) on lettuce growth and soil bacterial communities. The results revealed that F and Cd concentrations in lettuce ranged from 63.69 to 219.45 mg kg-1 and 1.85 to 33.08 mg kg-1, respectively, presenting lower values in shoots than in the roots. Moreover, low contamination levels had no discernable influence on lettuce growth, but showed a synergistic negative on plant biomass when exogenous F and Cd exceeds 300 and 1.0 mg kg-1, respectively. The results of 16S rRNA gene sequencing indicated that the most abundant bacterial community at the phylum level was Proteobacteria, with the relative abundance ranging from 33.42% to 44.10% across all the treatments. The contaminants had little effect on bacterial richness but impacted the structure of bacterial communities. The PCoA showed that compartment and contaminants were the primary contributors to the largest source of community variation, while the VPA indicated that F and Cd synergistically affected the bacterial communities. In turn, lettuce plants could enhance the resistance to the combined stress by increasing the relative abundance of Oxyphotobacteria, Subgroup 6, Thermoleophilia, and TK10 classes in the rhizosphere.

Effect of humic substances on the fraction of heavy metal and microbial response

Contamination of soils by Molybdenum (Mo) has raised increasing concern worldwide. Both fulvic acid (FA) and humic acid (HA) possess numerous positive properties, such as large specific surface areas and microporous structure that facilitates the immobilization of the heavy metal in soils. Despite these characteristics, there have been few studies on the microbiology effects of FA and HA. Therefore, this study aimed to assess the Mo immobilization effects of FA and HA, as well as the associated changes in microbial community in Mo-contaminated soils (with application rates of 0%, 0.5% and 1.0%). The result of the incubation demonstrated a decrease in soil pH (from 8.23 ~ 8.94 to 8.05 ~ 8.77). Importantly, both FA and HA reduced the exchangeable fraction and reducible fraction of Mo in the soil, thereby transforming Mo into a more stable form. Furthermore, the application of FA and HA led to an increase in the relative abundance of Actinobacteriota and Firmicutes, resulting in alterations to the microbial community structure. However, it is worth noting that due to the differing structures and properties of FA and HA, these outcomes were not entirely consistent. In summary, the aging of FA and HA in soil enhanced their capacity to immobilization Mo as a soil amendment. This suggests that they have the potential to serve as effective amendments for the remediation of Mo-contaminated soils.