Sources and human health risks associated with potentially toxic elements (PTEs) in urban dust: A global perspective.

Long-term exposure to urban dust containing potentially toxic elements (PTEs) poses detrimental impacts on human health. However, studies estimating human health risks in urban dusts from a global perspective are scarce. We evaluated data for twelve PTEs in urban dusts across 59 countries from 463 published articles, including their concentrations, input sources, and probabilistic risks to human health. We found that 34.1 and 60.3% of those investigated urban dusts have been heavily contaminated with As and Cd, respectively. The input of PTEs was significantly correlated with economic structure due to emissions of industrial activities and traffic emissions being the major sources. Based on the Monte Carlo simulation, we found that the mean hazard index below the safe threshold (1.0) could still cause non-negligible risks to human health. Arsenic and Cr were the major PTEs threatening human health, and relatively high risk levels were observed in cities in China, Korea, Chile, Malaysia, and Australia. Importantly, our analysis suggested that PTEs threaten the health of approximately 92 million adults and 280 million children worldwide. Overall, our study provides important foundational understanding and guidance for policy decision-making to reduce the potential risks associated with PTE exposure and to promote sustainable development of urban economies.

A meta-analysis on crop growth and heavy metals accumulation with PGPB inoculation in contaminated soils.

Plant growth-promoting bacteria (PGPB) offer a promising solution for mitigating heavy metals (HMs) stress in crops, yet the mechanisms underlying the way they operate in the soil-plant system are not fully understood. We therefore conducted a meta-analysis with 2037 observations to quantitatively evaluate the effects and determinants of PGPB inoculation on crop growth and HMs accumulation in contaminated soils. We found that inoculation increased shoot and root biomass of all five crops (rice, maize, wheat, soybean, and sorghum) and decreased metal accumulation in rice and wheat shoots together with wheat roots. Key factors driving inoculation efficiency included soil organic matter (SOM) and the addition of exogenous fertilizers (N, P, and K). The phylum Proteobacteria was identified as the keystone taxa in effectively alleviating HMs stress in crops. More antioxidant enzyme activity, photosynthetic pigment, and nutrient absorption were induced by it. Overall, using PGPB inoculation improved the growth performance of all five crops, significantly increasing crop biomass in shoots, roots, and grains by 33 %, 35 %, and 20 %, respectively, while concurrently significantly decreasing heavy metal accumulation by 16 %, 9 %, and 37 %, respectively. These results are vital to grasping the benefits of PGPB and its future application in enhancing crop resistance to HMs.

Sulfur enhances iron plaque formation and stress resistance to reduce the transfer of Cd and As in the soil-rice system.

Sulfur plays an essential role in agricultural production, but few studies have been reported on how sulfur simultaneously impacts the transformation of cadmium (Cd) and arsenic (As) in the soil-rice system. This research selected two soils co-contaminated with both Cd and As, varying in acidity and alkalinity levels, to study the impacts of elemental sulfur (S) and calcium sulfate (CaSO4) on the migration and accumulation of Cd and As by rice. Results indicated that two types of sulfur had a substantial (P < 0.05) impact on decreasing the contents of Cd (28.3-50.4 %) and As (20.1-38.6 %) in brown rice in acidic and alkaline soils. They also increased rice biomass (29.3-112.8 %) and reduced Cd transport coefficient (27.2-45.6 %) significantly (P < 0.05). Notably, sulfur augmented the generation of iron plaque on rice root surfaces, which increased the fixation of Cd (17.6-61.0 %) and As (14.0-45.9 %). SEM-EDS results also indicated that the rice root surface exhibited significant enrichment of Fe, Cd, and As. The mechanism of simultaneous Cd and As immobilization by sulfur application was mainly ascribed to the contribution of iron plaque. Additionally, sulfur reduced the contents of Cd and As in soil porewater and promoted the transformation of As(III) to As(V) to reduce the toxicity of As. The K-edge XAFS of As in iron plaque also confirmed that sulfur application significantly promoted As(III) oxidation. Sulfur also promoted the activities of antioxidant enzymes and the contents of NPT, GSH, and PCs in rice plants. In general, this study establishes a foundation for sulfur to lower As and Cd bioavailability in paddy soils, enhance iron plaque and rice resistance, and reduce heavy metal accumulation.

Meta-analysis of impacts of microplastics on plant heavy metal(loid) accumulation.

The co-occurrence of microplastics (MPs) and heavy metal(loid)s (HMs) has attracted growing scientific interest because of their wide distribution and environmental toxicity. Nevertheless, the interactions between MPs and HMs in soil-plant systems remain unclear. We conducted a meta-analysis with 3226 observations from 87 independent studies to quantify the impact of MPs addition on the plant biomass and HMS accumulation. Co-occurrence of MPs and HMs (except for As) induced synergistic toxicity to plant growth. MPs promoted their uptake in the shoot by 11.0% for Cd, 30.0% for Pb, and 47.1% for Cu, respectively. In contrast, MPs caused a significant decrease (22.6%, 17.9-26.9%) in the shoot As accumulation. The type and dose of MPs were correlated with the accumulation of HMs. MPs increased available concentrations of Cd, Pb, and Cu, but decreased available As concentration in soils. Meanwhile, MPs addition significantly lowered soil pH. These findings may provide explanations for MPs-mediated effects on influencing the accumulation of HMs in plants. Using a machine learning approach, we revealed that soil pH and total HMs concentration are the major contributors affecting their accumulation in shoot. Overall, our study indicated that MPs may increase the environmental risks of HMs in agroecosystems, especially metal cations.

Examining diverse remediation mechanisms of biochar in soil contaminated with polycyclic aromatic hydrocarbon (PAH) of various ring structures: A global meta-analysis.

Biochar is a widely recognized solution for addressing polycyclic aromatic hydrocarbon (PAH) contamination. However, it is unclear how the different physicochemical properties of PAHs affect remediation mechanisms, thereby affecting the remediation efficiency of biochar. In this study, a meta-analysis of 56 studies with 2236 observations was conducted to quantify the contributions of the preparation parameters and physicochemical properties of biochar, soil properties, and application measures to remediation efficiency. The research results revealed that PAH contents in the contaminated soil were significantly reduced by an average of 24.99 % after the application of biochar. Random Forest analysis identified feedstock, biochar SSA, soil pH, and rate of biochar application to be the most critical factors among the four categories. The reduction of PAH contents in the contaminated soil first decreased from 37.61 % to 17.44 % and then increased to 40.29 % with an increase in the number of aromatic rings from 2 to 6. Our study results suggest that biochar prepared from bio-waste at low temperatures could be favorable for reducing the content of NAP in soil. Wood-derived biochar pyrolyzed at a relatively high temperature is recommended for remediation of soil contaminated with PAHs with 5 or 6 aromatic rings. Our study provides a new perspective and an optimized strategy for the remediation of PAH-contaminated soils with biochar.

Silicon reduces toxicity and accumulation of arsenic and cadmium in cereal crops: A meta-analysis, mechanism, and perspective study.

Arsenic (As) and cadmium (Cd) are two toxic metal(loid)s that pose significant risks to food security and human health. Silicon (Si) has attracted substantial attention because of its positive effects on alleviating the toxicity and accumulation of As and Cd in crops. However, our current knowledge of the comprehensive effects and detailed mechanisms of Si amendment is limited. In this study, a global meta-analysis of 248 original articles with over 7000 paired observations was conducted to evaluate Si-mediated effects on growth and As and Cd accumulation in rice (Oryza sativa L.), wheat (Triticum aestivum L.), and maize (Zea mays L.). Si application increases the biomass of these crops under As and/or Cd contamination. Si amendment also decreased shoot As and Cd accumulation by 24.1 % (20.6 to 27.5 %) and 31.9 % (29.0 to 31.9 %), respectively. Furthermore, the Si amendment reduced the human health risks posed by As (2.6 %) and Cd (12.9 %) in crop grains. Si-induced inhibition of Cd accumulation is associated with decreased Cd bioavailability and the downregulation of gene expression. The regulation of gene expression by Si addition was the driving factor limiting shoot As accumulation. Overall, our analysis demonstrated that Si amendment has great potential to reduce the toxicity and accumulation of As and/or Cd in crops, providing a scientific basis for promoting food safety globally.

Interaction of microplastics with heavy metals in soil: Mechanisms, influencing factors and biological effects.

Microplastics (MPs) and heavy metals (HMs) in soil contamination are considered an emerging global problem that poses environmental and health risks. However, their interaction and potential biological effects remain unclear. Here, we reviewed the interaction of MPs with HMs in soil, including its mechanisms, influencing factors and biological effects. Specifically, the interactions between HMs and MPs mainly involve sorption and desorption. The type, aging, concentration, size of MPs, and the physicochemical properties of HMs and soil have significant impacts on the interaction. In particular, MP aging affects specific surface areas and functional groups. Due to the small size and resistance to decomposition characteristics of MPs, they are easily transported through the food chain and exhibit combined biological effects with HMs on soil organisms, thus accumulating in the human body. To comprehensively understand the effect of MPs and HMs in soil, we propose combining traditional experiments with emerging technologies and encouraging more coordinated efforts.

New perspectives on microbiome and nutrient sequestration in soil aggregates during long-term grazing exclusion.

Grazing exclusion alters grassland soil aggregation, microbiome composition, and biogeochemical processes. However, the long-term effects of grazing exclusion on the microbial communities and nutrient dynamics within soil aggregates remain unclear. We conducted a 36-year exclusion experiment to investigate how grazing exclusion affects the soil microbial community and the associated soil functions within soil aggregates in a semiarid grassland. Long-term (36 years) grazing exclusion induced a shift in microbial communities, especially in the <2 mm aggregates, from high to low diversity compared to the grazing control. The reduced microbial diversity was accompanied by instability of fungal communities, extended distribution of fungal pathogens to >2 mm aggregates, and reduced carbon (C) sequestration potential thus revealing a negative impact of long-term GE. In contrast, 11-26 years of grazing exclusion greatly increased C sequestration and promoted nutrient cycling in soil aggregates and associated microbial functional genes. Moreover, the environmental characteristics of microhabitats (e.g., soil pH) altered the soil microbiome and strongly contributed to C sequestration. Our findings reveal new evidence from soil microbiology for optimizing grazing exclusion duration to maintain multiple belowground ecosystem functions, providing promising suggestions for climate-smart and resource-efficient grasslands.

Biological roles of soil microbial consortium on promoting safe crop production in heavy metal(loid) contaminated soil: A systematic review.

Heavy metal(loid) (HM) pollution of agricultural soils is a growing global environmental concern that affects planetary health. Numerous studies have shown that soil microbial consortia can inhibit the accumulation of HMs in crops. However, our current understanding of the effects and mechanisms of inhibition is fragmented. In this review, we summarise extant studies and knowledge to provide a comprehensive view of HM toxicity on crop growth and development at the biological, cellular and the molecular levels. In a meta-analysis, we find that microbial consortia can improve crop resistance and reduce HM uptake, which in turn promotes healthy crop growth, demonstrating that microbial consortia are more effective than single microorganisms. We then review three main mechanisms by which microbial consortia reduce the toxicity of HMs to crops and inhibit HMs accumulation in crops: 1) reducing the bioavailability of HMs in soil (e.g. biosorption, bioaccumulation and biotransformation); 2) improving crop resistance to HMs (e.g. facilitating the absorption of nutrients); and 3) synergistic effects between microorganisms. Finally, we discuss the prospects of microbial consortium applications in simultaneous crop safety production and soil remediation, indicating that they play a key role in sustainable agricultural development, and conclude by identifying research challenges and future directions for the microbial consortium to promote safe crop production.

Deciphering microbiomes dozens of meters under our feet and their edaphoclimatic and spatial drivers.

Microbes inhabiting deep soil layers are known to be different from their counterpart in topsoil yet remain under investigation in terms of their structure, function, and how their diversity is shaped. The microbiome of deep soils (>1 m) is expected to be relatively stable and highly independent from climatic conditions. Much less is known, however, on how these microbial communities vary along climate gradients. Here, we used amplicon sequencing to investigate bacteria, archaea, and fungi along fifteen 18-m depth profiles at 20-50-cm intervals across contrasting aridity conditions in semi-arid forest ecosystems of China's Loess Plateau. Our results showed that bacterial and fungal α diversity and bacterial and archaeal community similarity declined dramatically in topsoil and remained relatively stable in deep soil. Nevertheless, deep soil microbiome still showed the functional potential of N cycling, plant-derived organic matter degradation, resource exchange, and water coordination. The deep soil microbiome had closer taxa-taxa and bacteria-fungi associations and more influence of dispersal limitation than topsoil microbiome. Geographic distance was more influential in deep soil bacteria and archaea than in topsoil. We further showed that aridity was negatively correlated with deep-soil archaeal and fungal richness, archaeal community similarity, relative abundance of plant saprotroph, and bacteria-fungi associations, but increased the relative abundance of aerobic ammonia oxidation, manganese oxidation, and arbuscular mycorrhizal in the deep soils. Root depth, complexity, soil volumetric moisture, and clay play bridging roles in the indirect effects of aridity on microbes in deep soils. Our work indicates that, even microbial communities and nutrient cycling in deep soil are susceptible to changes in water availability, with consequences for understanding the sustainability of dryland ecosystems and the whole-soil in response to aridification. Moreover, we propose that neglecting soil depth may underestimate the role of soil moisture in dryland ecosystems under future climate scenarios.

Microbial communities in terrestrial surface soils are not widely limited by carbon.

Microbial communities in soils are generally considered to be limited by carbon (C), which could be a crucial control for basic soil functions and responses of microbial heterotrophic metabolism to climate change. However, global soil microbial C limitation (MCL) has rarely been estimated and is poorly understood. Here, we predicted MCL, defined as limited availability of substrate C relative to nitrogen and/or phosphorus to meet microbial metabolic requirements, based on the thresholds of extracellular enzyme activity across 847 sites (2476 observations) representing global natural ecosystems. Results showed that only about 22% of global sites in terrestrial surface soils show relative C limitation in microbial community. This finding challenges the conventional hypothesis of ubiquitous C limitation for soil microbial metabolism. The limited geographic extent of C limitation in our study was mainly attributed to plant litter, rather than soil organic matter that has been processed by microbes, serving as the dominant C source for microbial acquisition. We also identified a significant latitudinal pattern of predicted MCL with larger C limitation at mid- to high latitudes, whereas this limitation was generally absent in the tropics. Moreover, MCL significantly constrained the rates of soil heterotrophic respiration, suggesting a potentially larger relative increase in respiration at mid- to high latitudes than low latitudes, if climate change increases primary productivity that alleviates MCL at higher latitudes. Our study provides the first global estimates of MCL, advancing our understanding of terrestrial C cycling and microbial metabolic feedback under global climate change.

Microplastics may increase the environmental risks of Cd via promoting Cd uptake by plants: A meta-analysis.

Microplastics (MPs) and cadmium (Cd) are widely distributed in soil ecosystems, posing a potential threat to agricultural production and human health. However, the coupled effects of MPs and Cd in soil-plant systems remain largely unknown, especially on a large scale. In this study, a meta-analysis was conducted to evaluate the influence of MPs on plant growth and Cd accumulation under the Cd contamination conditions. Our results showed that MPs had significantly negative effects on shoot biomass (a decrease of 11.8 %) and root biomass (a decrease of 8.79 %). MPs also significantly increased Cd accumulation in the shoots and roots by 14.6 % and 13.5 %, respectively, revealing that MPs promote plant Cd uptake. Notably, polyethylene displayed a stronger promoting effect (an increase of 29.4 %) on Cd accumulation among these MP types. MPs induced a significantly increase (9.75 %) in concentration of soil available Cd and a slight decrease in soil pH, which may be the main driver promoting plant Cd uptake. MP addition posed physiological toxicity risks to plants by inhibiting photosynthesis and enhancing oxidative damage, directly demonstrating that MPs in combination with Cd can pose synergetic toxicity risks to plants. We further noted that MPs altered microbial diversity, likely influencing Cd bioavailability in soil-plant systems. Overall, our study has important implications for the combined impacts of Cd and MPs on plants and provides new insights into developing guidelines for the sustainable use of MPs in agriculture.

Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus.

To enhance the nitrogen removal capacity, scrap iron filings and Si-Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH4+-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70-130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe2+, TOC, TP, TN, NH4+-N and NO3--N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Crop residue return sustains global soil ecological stoichiometry balance.

Although soil ecological stoichiometry is constrained in natural ecosystems, its responses to anthropogenic perturbations are largely unknown. Inputs of inorganic fertilizer and crop residue are key cropland anthropogenic managements, with potential to alter their soil ecological stoichiometry. We conducted a global synthesis of 682 data pairs to quantify the responses of soil carbon (C), nitrogen (N), and phosphorus (P) and grain yields to combined inputs of crop residue plus inorganic fertilizer compared with only inorganic fertilizer application. Crop residue inputs enhance soil C (10.5%-12%), N (7.63%-9.2%), and P (2.62%-5.13%) contents, with an increase in C:N (2.51%-3.42%) and C:P (7.27%-8.00%) ratios, and grain yields (6.12%-8.64%), indicating that crop residue alleviated soil C limitation caused by inorganic fertilizer inputs alone and was able to sustain balanced stoichiometry. Moreover, the increase in soil C and C:N(P) ratio reached saturation in ~13-16 years after crop residue return, while grain yield increase trend discontinued. Furthermore, we identified that the increased C, N, and P contents and C:N(P) ratios were regulated by the initial pH and C content, and the increase in grain yield was not only related to soil properties, but also negatively related to the amount of inorganic N fertilizer input to a greater extent. Given that crop residual improvement varies with soil properties and N input levels, we propose a predictive model to preliminary evaluate the potential for crop residual improvement. Particularly, we suggest that part of the global budget should be used to subsidize crop residue input management strategies, achieving to a win-win situation for agricultural production, ecological protection, and climate change mitigation.

Generic CD-MUSIC-eSGC model parameters to predict the surface reactivity of iron (hydr)oxides.

The surface reactivity of iron (hydr)oxides plays a crucial role in controlling their interfacial reactions, for which various surface complexation models have been developed. The diversity of mineralogical properties of iron (hydr)oxides has resulted in a redundancy of model parameters, which hampers the modeling of iron (hydr)oxides in soils and sediments, where goethite, hematite and ferrihydrite dominate the iron (hydr)oxide mass fraction. To capture their combined surface reactivity, optimized generic protonation parameters of the Charge Distribution-Multisite Complexation (CD-MUSIC) extended-Stern-Gouy-Chapman (eSGC) model were derived by reanalyzing literature datasets and tested with some newly synthesized iron (hydr)oxides. It was observed that the proton and monovalent ion affinity constants of the different iron (hydr)oxides were located in a narrow range. For the singly- and triply-coordinated hydroxyl sites the obtained generic log(affinity constants) were 8.3 and 11.7 for the protonation reaction and -0.5 for the reaction with the monovalent background ions. Their combination with fixed site densities of singly-/triply-coordinated hydroxyl sites of 3.45/2.70, 5.00/2.50, and 5.80/1.40 sites/nm2 for goethite, hematite, and ferrihydrite, respectively, provided good results. The Stern layer capacitances of the inner and outer Stern layers were set equal and could be acquired by an empirical correlation with the sample specific surface area (SSA). The CD-MUSIC-eSGC model with the generic model parameters enables good quality predictions of the proton reactivity of iron (hydr)oxides in 1:1 electrolyte solutions regardless of the sample heterogeneity. The advantages of the generic CD-MUSIC-eSGC model are twofold: (1) protonation of iron (hydr)oxides can be described without making use of spectroscopic measurements and proton titrations, and (2) the model calculations are greatly simplified.

Synergistic interplay between Azospirillum brasilense and exogenous signaling molecule H2S promotes Cd stress resistance and growth in pak choi (Brassica chinensis L.).

Inoculation with growth-promoting rhizobacteria inoculation and the addition of exogenous signaling molecules are two distinct strategies for improving heavy metal resistance and promoting growth in crops through several mechanisms. However, whether rhizobacteria and phyllosphere signaling molecules can act synergistically alleviate heavy metal stress and promote growth and the mechanisms underlying these effects remain unclear. Here, a novel strategy involving the co-application of growth-promoting rhizobacteria and an exogenous signaling molecule was developed to reduce cadmium (Cd) phytotoxicity and promote pak choi growth in Cd-contaminated soil. We found that the co-application of Azospirillum brasilense and hydrogen sulfide (H2S) resulted in significant improvements in shoot biomass and antioxidant enzyme content and a decline in the levels of Cd translocation factors. In addition, this co-application significantly improved pak choi Cd resistance. Furthermore, we observed a significant negative correlation between abscisic acid concentration and Cd content of pak choi and a positive correlation between H2S concentration and biomass. These findings revealed that the co-application of rhizobacteria and exogenous signaling molecules synergistically promoted the growth of vegetable crops subjected to heavy metal stress. Our results may serve as a guide for improving the food safety of crops grown in soil contaminated with heavy metals.

Microplastics addition reduced the toxicity and uptake of cadmium to Brassica chinensis L.

The coexistence of microplastics (MPs) and toxic metal contaminants in soils is becoming increasingly common, thereby posing serious threat to soil-plant systems. Cadmium (Cd) is the most common metal contaminant in soil and can easily combine with MPs, thereby altering its bioavailability. However, few studies have focused on the co-pollution of MPs and Cd, particularly the complex phytotoxicity caused by their interaction and the effect of co-exposure on Cd uptake in plants. We conducted pot experiments to compare the effects of exposure to polystyrene (PS) and Cd, as well as the effects of co-exposure (PS + Cd), on the physiological characteristics of Brassica chinensis L. and explored the regulatory factors of MPs on Cd uptake in plant tissues. The results showed that plant biomass, photosynthetic parameters, and chlorophyll content significantly decreased (p < 0.05) with increasing PS doses under treatment with MPs alone. Although the negative effects of PS and Cd co-exposure on plants were higher than those of PS alone, however, the addition of MPs reduced the toxicity effects of Cd on plants and decreased the uptake and accumulation of Cd by plants compared with the Cd treatment alone. Furthermore, plants can resist the increased malondialdehyde content and oxidative stress induced by PS and Cd exposure by increasing the activities of superoxide dismutase and peroxidase. Under the PS + Cd treatment, linear models showed that soil organic carbon and sucrase activity were the key variables affecting Cd uptake by plant shoots and roots, respectively. The results of the partial least squares path modeling further showed that PS indirectly affected Cd uptake by B. chinensis by significantly affecting the physicochemical properties of soil, Cd concentration, and enzyme activity. Our results provide a new perspective and an important reference for further understanding the effects of MPs on the bioavailability and fate of heavy metals.

Microbial metabolic limitation and carbon use feedback in lead contaminated agricultural soils.

Potentially toxic elements (PTEs) pollution causes a great threat to microbial metabolism, which plays a vital role in studying soil nutrient cycling and predicting carbon (C) storage in agroecosystems. However, the responses of microbial metabolism characteristic to heavy metal contamination and the mechanisms through which microbial metabolism mediate nutrient cycling and C dynamics in contaminated soil remain elusive. Here, we performed an incubation experiment over 80 days to investigate the variations in microbial metabolic limitation under various Pb levels (0, 100, 500, 800, 1500, 2000, and 3000 mg Pb kg-1 dry soil) in cropland soil using extracellular enzymatic stoichiometry, and to reveal the impact of Pb stress on soil C storage by associating with microbial metabolic quotients (qCO2) and C use efficiency (CUE). The results showed microbial relative C limitation and phosphorus (P) limitation were observed in Pb-contaminated soils. Pb addition enhanced the microbial relative C limitation by approximately 7.3%, while decreasing the P limitation by approximately 12.3%. Furthermore, Pb addition led to higher qCO2 (from 8.75 to 108 µg C kg-1 MBC-1 d-1) duo to the increase of microbial relative C limitation, suggesting that the more CO2 was released of per unit of microbial biomass C. The increase of microbial relative C limitation reduced CUE (from 0.35 to 0.10) because of the change in microbial metabolism from growth to respiration maintenance under Pb stress. Consequently, the CUE and qCO2 together determined the loss of soil C. Our study reveals that microbial relative C limitation is the dominant driver of soil C loss and provides important knowledge of microbial metabolic limitation regulating soil C turnover in PTEs-contaminated agricultural soils.

Sequestration of heavy metals in soil aggregates induced by glomalin-related soil protein: A five-year phytoremediation field study.

Glomalin-related soil protein (GRSP) is an essential bioactive component that may respond to heavy metal stress; however, its exact influence on metal bioavailability and the associated mechanism remains poorly understood. This study investigated the speciation and distribution of heavy metals in soil aggregates associated with GRSP through macroscopic and microscopic approaches. A field study showed that the metal ions were distributed to the macro-aggregate fraction by partitioning the particle size classes during phytoremediation. Partial least squares path modeling (PLS-PM) demonstrated that the heavy metal bioavailability was negatively affected by aggregate stability (61.5%) and GRSP content (52.8%), suggesting that the soil aggregate properties regarding GRSP were vital drivers in mitigating environmental risk closely associated with toxic metal migration in soil-plant systems. The nonideal competitive adsorption (NICA)-Donnan model fitting suggested that GRSP were rich in acid site density, and the complexation with deprotonated groups dominated the speciation of heavy metals in soil. Further, the microfocus X-ray absorption/fluorescence spectroscopy analysis indicated that GRSP might promote the formation of stable metal species by binding with sulfur-containing sites. This study highlights the role of GRSP in heavy metal sequestration in contaminated soils, providing new guidance on the GRSP intervention for phytoremediation strategies.

Dissolved Organic Matter Promotes the Aging Process of Polystyrene Microplastics under Dark and Ultraviolet Light Conditions: The Crucial Role of Reactive Oxygen Species.

Microplastics (MPs) interact frequently with dissolved organic matter (DOM) commonly found in the environment, but information on the aging behavior of MPs under the participation of DOM is still lacking. Thus, the polystyrene microplastic (PSMP) aging process with DOM participation was systematically studied by electron paramagnetic resonance spectroscopy, high-performance liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy analyses under dark and ultraviolet (UV) light conditions. DOM was found to promote electron transfer to generate reactive oxygen species (ROS) under dark conditions and the aging of PSMPs, while the process of DOM generating ROS under UV light was more susceptible to photoelectrons and accelerated the aging process of PSMPs. However, among the four DOM types, fulvic acid (FA) has a more significant promoting effect on the aging process of PSMPs than humic acid, which can be attributed to the stronger conversion ability of FA to semiquinone radicals. Density functional theory calculations are used to describe the difference in the aging process of different structures of plastics with the participation of DOM. This study provides a necessary theoretical basis for the study of the migration of MPs in groundwater and deep surface water.