The Fate of Zn in Agricultural Soils: A Stable Isotope Approach to Anthropogenic Impact, Soil Formation, and Soil-Plant Cycling

Global soil pollution by toxic metals threatens agriculture and human health

Toxic metal pollution is ubiquitous in soils, yet its worldwide distribution is unknown. We analyzed a global database of soil pollution by arsenic, cadmium, cobalt, chromium, copper, nickel, and lead at 796,084 sampling points from 1493 regional studies and used machine learning techniques to map areas with exceedance of agricultural and human health thresholds. We reveal a previously unrecognized high-risk, metal-enriched zone in low-latitude Eurasia, which is attributed to influential climatic, topographic, and anthropogenic conditions. This feature can be regarded as a signpost for the Anthropocene era. We show that 14 to 17% of cropland is affected by toxic metal pollution globally and estimate that between 0.9 and 1.4 billion people live in regions of heightened public health and ecological risks.

Stable isotopes analysis combined with X-ray absorption spectroscopy reveal the fate of organic waste-borne copper and zinc in amended soils

Copper (Cu) and zinc (Zn), two potentially toxic trace elements, are commonly abundant in organic wastes (OWs) recycled in soils as fertilizer. Yet current knowledge on the long-term behavior and fate of Cu and Zn in soil following OW spreading is scant. We addressed this issue by studying the fate of OW-borne Cu and Zn in amended soils from four different long-term field experiments. By combining the stable isotope analysis and X-ray absorption spectroscopy, we identified changes in Cu and Zn concentrations, speciation and isotopic compositions in the amended soils only when OW had been applied at high rates over long periods. Under these conditions, we highlighted that: (i) all OW-derived Cu and Zn had accumulated in the topsoil layer regardless of the soil and OW type; (ii) the amended soil isotopic signatures were the result of the mixing of OW-borne and natural Cu and Zn; and (iii) Cu and Zn exhibited distinct speciation patterns in amended soils. Indeed, the unprecedented persistence of OW-borne crystallized Cu(I)-sulfide in the amended soils contrasted with the complete transformation of pig slurry-borne nanosized Zn-sulfide or household compost-derived amorphous Zn phosphate and Zn complexed by organic matter.

Isotopic and trace element record of changing metal source contributions to tropical freshwater Lake Naivasha (Kenya)

Lake Naivasha, Kenya's second-largest freshwater body is a wetland of international ecological importance and currently subjected to unprecedented anthropogenic influence. The study aims to chronologically reconstruct the main human activities and background weathering reactions that govern metal mobilizations into the lake and their potentially adverse effects on its ecological status. We combine extensive geochemical analyses (major, trace elements, Zn-Pb isotope ratios) in a dated lake sediment record and catchment rocks with remote sensing techniques. Downcore geochemical variations reflect natural ecosystem destabilizations occurring as early as the first half of the 20th century. These coincide with changes towards less radiogenic Pb-isotope values which persist towards the top of the core (206Pb/207Pb = 1.243 at core base ∼1843, to 206Pb/207Pb = 1.225 at ∼1978). We interpret the land-clearance for agricultural purposes on the Aberdare Range and documented early aviation activities as possible vectors of this early Pb-isotope excursion. The overlapping Pb-isotope signatures between sediment sources and anthropogenic contributions challenges a straightforward deconvolution of the two. Our conservative model calculations suggest, nevertheless, that an addition of up to ∼1.8 % Pb-gasoline influx to the total Pb flux, peaking in the 1980s is able to explain the Pb distribution trend. Homogeneous Zn-isotope compositions in sediments deposited until ∼1970s (δ66/64Zn = 0.216-0.225 ‰) do not follow major hydro-climatic events or anthropogenic forcing but likely reflect lake-specific natural cycling. Subsequent higher variations to both heavier (up to δ66/64Zn = 0.242 ± 0.005 ‰) and lighter (down to δ66/64Zn = 0.184 ± 0.003 ‰) Zn-isotope values are contemporaneous with intensification of large-scale horticultural industry in the catchment. Together with supporting indicators, the lighter Zn-isotope compositions in youngest analysed sediments (21st century) are attributable to increased biological productivity (algal blooms) and ongoing lake eutrophication. Our study demonstrates the applicability of the heavy metal isotope tool to reconstruct human influences on lake environments with complex geological settings such as the East African Rift System.

Study on the variation mechanism of Zn isotope in polluted farmland soil

Zn isotope can help to clarify the migration, transformation and source contribution of Zn in farmland soil. However, the research on Zn isotope value of different end members in farmland soil is incomprehensive, and the variation of Zn isotope in farmland soil caused by different factors in different polluted areas is unclear, which hinders the usage of Zn isotope tracing method in farmland soil. Thus, a Pb-Zn mine polluted farmland in southwest China was selected as the research object and the end elements and farmland soil samples with different Zn contamination were systematically collected to analyse Zn content, fraction and isotopic composition. The effects of different end members and processes of eluviation, organic adsorption and inorganic adsorption on Zn isotopic composition in soil were analysed, and the relationship between these three processes and environmental variables was analysed to clarify the change mechanism. The results can enrich the fractionation mechanism of Zn isotopes, expand the application of Zn isotope in tracing the sources, and provide geochemical evidence for remediation of Zn pollution in farmland soil.

A comprehensive study of source apportionment, spatial distribution, and health risks assessment of heavy metal(loid)s in the surface soils of a semi-arid mining region in Matehuala, Mexico

BACKGROUND

This study investigates the contamination level, spatial distribution, pollution sources, potential ecological risks, and human health risks associated with heavy metal(loid)s (i.e., arsenic (As), copper (Cu), iron (Fe), manganese (Mn), lead (Pb), and zinc (Zn)) in surface soils within the mining region of Matehuala, located in central Mexico.

OBJECTIVES

The primary objectives are to estimate the contamination level of heavy metal(loid)s, identify pollution sources, assess potential ecological risks, and evaluate human health risks associated with heavy metal(loid) contamination.

METHODS

Soil samples from the study area were analysed using various indices including Igeo, Cf, PLI, mCd, EF, and PERI to evaluate contamination levels. Source apportionment of heavy metal(loid)s was conducted using the APCS-MLR and PMF receptor models. Spatial distribution patterns were determined using the most efficient interpolation technique among five different approaches. The total carcinogenic risk index (TCR) and total non-carcinogenic index (THI) were used in this study to assess the potential carcinogenic and non-carcinogenic hazards posed by heavy metal(loid)s in surface soil to human health.

RESULTS

The study reveals a high contamination level of heavy metal(loid)s in the surface soil, posing considerable ecological risks. As was identified as a priority metal for regulatory control measures. Mining and smelting activities were identified as the primary factors influencing heavy metal(loid) distributions. Based on spatial distribution mapping, concentrations were higher in the northern, western, and central regions of the study area. As and Fe were found to pose considerable and moderate ecological risks, respectively. Health risk evaluation indicated significant levels of carcinogenic risks for both adults and children, with higher risks for children.

CONCLUSION

This study highlights the urgent need for monitoring heavy metal(loid) contamination in Matehuala's soils, particularly in regions experiencing strong economic growth, to mitigate potential human health and ecological risks associated with heavy metal(loid) pollution.

The fate of Cd in Soils with Various Particle Sizes: Characteristics, Speciation Distribution and Influencing Factors

This study investigated the distribution of Cd in soil water-stable aggregate particles of varying sizes, revealing that smaller particles have higher total Cd content as well as different forms of Cd content, with the clay particle showing a greater tendency to accumulate Cd. However, the proportion of high activity Cd is lower in clay particles, posing a lower environmental risk of Cd transformation compared to silt particles. Adsorption experiments indicated that the clay particle exhibits the strongest adsorption capacity and highest adsorption rate. Additionally, correlation and principal component analyses identified Fe-Mn oxides and organic matter as the primary influencing factors on Cd distribution characteristics, with pH playing a secondary role. These findings provide valuable insights for the remediation of heavy metal-contaminated soil.

Towards Sustainable Productivity of Greenhouse Vegetable Soils: Limiting Factors and Mitigation Strategies

Greenhouse vegetable production has become increasingly important in meeting the increasing global food demand. Yet, it faces severe challenges in terms of how to maintain soil productivity from a long-term perspective. This review discusses the main soil productivity limiting factors for vegetables grown in greenhouses and identifies strategies that attempt to overcome these limitations. The main processes leading to soil degradation include physical (e.g., compaction), chemical (e.g., salinization, acidification, and nutrient imbalances), and biological factors (e.g., biodiversity reduction and pathogen buildup). These processes are often favored by intensive greenhouse cultivation. Mitigation strategies involve managing soil organic matter and mineral nutrients and adopting crop rotation. Future research should focus on precisely balancing soil nutrient supply with vegetable crop demands throughout their life cycle and using targeted organic amendments to manage specific soil properties. To ensure the successful adoption of recommended strategies, socioeconomic considerations are also necessary. Future empirical research is required to adapt socioeconomic frameworks, such as Science and Technology Backyard 2.0, from cereal production systems to greenhouse vegetable production systems. Addressing these issues will enable the productivity of greenhouse vegetable soils that meet growing vegetable demand to be sustained using limited soil resources.

Brownfield Topsoil Vertical Heterogeneity: Implications for Germination and Soil Microbial Functioning

Soil vertical heterogeneity refers to the variation in soil properties and composition with depth. In uncontaminated soils, properties including the organic matter content and nutrient concentrations typically change gradually with depth due to natural processes such as weathering, leaching, and organic matter decomposition. In contaminated soils, heavy metals and organic contaminants can migrate vertically through leaching or root uptake and translocation by plants and macrobiota, if present, leading to vertical heterogeneity in contaminant concentrations at different depths. Contaminants can alter soil properties, and we investigated the implications of soil vertical heterogeneity for germination and microbial functioning. We collected soil from an urban brownfield and created two conditions: structured soil samples collected with the soil core intact and mixed (unstructured) samples. When planted, the germination rate was significantly lower in the structured conditions (3.1 ± 1.7%) compared to mixed soils (17 ± 4.6%), suggesting that the vertical heterogeneity of contaminated soil influenced plant germination. To map the vertical distribution of contaminants and nutrient cycling rates in the structured soil samples, we collected 10 cm-deep soil cores from the barren site and a neighboring vegetated reference site and measured heavy metal concentrations, soil enzyme activities, and organic matter content in five 2 cm vertical layers. In the barren soil cores, metals were found concentrated in the top 2 cm layer, while in the vegetated soil cores, metals were uniformly distributed. No significant differences were observed for the organic matter content or moisture along depth. Published studies on vertical distribution of enzyme activities and metal concentrations have treated the top 10-20 cm as a single layer and thus would have not revealed the thin (<2 cm thick) metal cap on the surface of the barren soil core. Despite the metal cap, enzyme activities in the top layer were similar to those in the lower layers of the barren soil core, suggesting that high metal concentrations do not limit soil enzyme activity under all circumstances. Investigating vertical heterogeneity in postindustrial soils can inform efforts to convert industrial barrens to vegetated environments.

Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Cadmium, zinc, and copper leaching rates determined in large monolith lysimeters

Soil mass balances are used to assess the risk of trace metals that are inadvertently applied with fertilizers into agroecosystems. The accuracy of such balances is limited by leaching rates, as they are difficult to measure. Here, we used monolith lysimeters to precisely determine Cd, Cu, and Zn leaching rates in 2021 and 2022. The large lysimeters (n = 12, 1 m diameter, 1.35 m depth) included one soil type (cambisol, weakly acidic) and distinct cropping systems with three experimental replicates. Stable isotope tracers were applied to determine the direct transfer of these trace metals from the soil surface into the seepage water. The annual leaching rates ranged from 0.04 to 0.30 for Cd, 2.65 to 11.7 for Cu, and 7.27 to 39.0 g (ha a)-1 for Zn. These leaching rates were up to four times higher in the year with several heavy rain periods compared to the dry year. Monthly resolved data revealed that distinct climatic conditions in combination with crop development have a strong impact on trace metal leaching rates. In contrast, fertilization strategy (e.g., conventional vs. organic) had a minor effect on leaching rates. Trace metal leaching rates were up to 10 times smaller than fertilizer inputs and had therefore a minor impact on soil mass balances. This was further confirmed with isotope source tracing that showed that only small fractions of Cd, Cu, and Zn were directly transferred from the soil surface to the leached seepage water within two years (< 0.07 %). A comparison with models that predict Cd leaching rates in the EU suggests that the models overestimate the Cd soil output with seepage water. Hence, monolith lysimeters can help to refine leaching models and thereby also soil mass balances that are used to assess the risk of trace metals inputs with fertilizers.

Do contaminants compromise the use of recycled nutrients in organic agriculture? A review and synthesis of current knowledge on contaminant concentrations, fate in the environment and risk assessment

Use of nutrients recycled from societal waste streams in agriculture is part of the circular economy, and in line with organic farming principles. Nevertheless, diverse contaminants in waste streams create doubts among organic farmers about potential risks for soil health. Here, we gather the current knowledge on contaminant levels in waste streams and recycled nutrient sources, and discuss associated risks. For potentially toxic elements (PTEs), the input of zinc (Zn) and copper (Cu) from mineral feed supplements remains of concern, while concentrations of PTEs in many waste streams have decreased substantially in Europe. The same applies to organic contaminants, although new chemical groups such as flame retardants are of emerging concern and globally contamination levels differ strongly. Compared to inorganic fertilizers, application of organic fertilizers derived from human or animal feces is associated with an increased risk for environmental dissemination of antibiotic resistance. The risk depends on the quality of the organic fertilizers, which varies between geographical regions, but farmland application of sewage sludge appears to be a safe practice as shown by some studies (e.g. from Sweden). Microplastic concentrations in agricultural soils show a wide spread and our understanding of its toxicity is limited, hampering a sound risk assessment. Methods for assessing public health risks for organic contaminants must include emerging contaminants and potential interactions of multiple compounds. Evidence from long-term field experiments suggests that soils may be more resilient and capable to degrade or stabilize pollutants than often assumed. In view of the need to source nutrients for expanding areas under organic farming, we discuss inputs originating from conventional farms vs. non-agricultural (i.e. societal) inputs. Closing nutrient cycles between agriculture and society is feasible in many cases, without being compromised by contaminants, and should be enhanced, aided by improved source control, waste treatment and sound risk assessments.

Combining Cd and Pb isotope analyses for heavy metal source apportionment in facility agricultural soils around typical urban and industrial areas

Facility agriculture enhances food production capabilities. However, concerns persist regarding heavy metal accumulation resulting from extensive operation of this type of farming. This study integrated the total content, five fractions, and isotope composition of Cd and Pb in intensively farmed soils in regions characterized by industrialization (Shaoguan, SG) and urbanization (Guangzhou, GZ), to assess the sources and mechanisms causing metals accumulation. We found significantly more severe Cd/Pb accumulation and potential mobility in SG than GZ. Cd displayed higher accumulation levels and potential mobility than Pb. The distinct isotopic signals in SG (-0.54 to 0.47‰ for δ114/110Cd and 1.1755 to 1.1867 for 206Pb/207Pb) and GZ (-0.86 to 0.12‰ for δ114/110Cd and 1.1914 to 1.2012 for 206Pb/207Pb) indicated significant differences in Cd/Pb sources. The Bayesian model revealed that industrial activities and related transportation accounted for over 40% and approximately 30%, respectively, of the average contributions of Cd/Pb in SG. While urban-related (26.6%) and agricultural-related (26.3%) activities primarily contributed to Cd in GZ. The integration of δ114/110Cd and 208Pb/206Pb has further enhanced the regional contrast in sources. The present study established a comprehensive tracing system for Cd-Pb, providing crucial insights into the accumulation and distribution of these metals in facility agricultural soils.

Anthropogenic processes drive heterogeneous distributions of toxic elements in shallow groundwater around a smelting site

Smelting activities have a far-reaching influence on the quality of soil and groundwater, while most studies have neglected the information on the pollution characteristics of groundwater. The hydrochemical parameters of shallow groundwater and the spatial distributions of toxic elements were investigated in this study. Correlations analysis and groundwater evolution revealed that the major ions were primarily determined by silicate weathering and calcite dissolution process, and anthropogenic processes had a significant effect on groundwater hydrochemistry. Almost 79%, 71%, 57%, 89%, 100%, and 78.6% of samples exceeded the standards of Cd, Zn, Pb, As, SO₄^2-, and NO₃^-, and their distribution is closely related to the production process. Analysis of soil geochemistry indicated that the relatively mobile forms of toxic elements strongly influence the origin and concentration in shallow groundwater. Besides, rainfall with high magnitude would lead to a decrease of toxic elements in shallow groundwater, whereas the area once stacked waste residue was the opposite. It is recommended to strengthen risk management of the limited mobility fraction while devising a plan for waste residue treatment in accordance with the local pollution conditions. The research on controlling the mechanism of toxic elements in shallow groundwater, along with sustainable development in the study area and other smelting zones may benefit from this study.

Advances in the application of metallic isotopes to the identification of contaminant sources in environmental geochemistry

The development of the economy and society makes heavy metals (HMs) pollution more and more serious. And, pollution source identification is the primary work of environmental pollution control and land planning. Notably, stable isotope technology has a high ability to distinguish pollution sources, and can better reflect the migration behavior and contribution of HMs from diverse sources, which has become a hot research tool for pollution source identification of HMs. Currently, the rapid development of isotope analysis technology provides a relatively reliable reference for pollution tracking. Based on this background, the fractionation mechanism of stable isotopes and the influence of environmental processes on isotope fractionation are reviewed. Furthermore, the processes and requirements for the measurement of metal stable isotope ratios are summarized, and the calibration methods and detection accuracy of sample measurement are evaluated. Besides, the current commonly used binary model and multi-mixed models in the identification of contaminant sources are also concluded. Moreover, the isotopic changes of different metallic elements under natural and anthropogenic conditions are discussed in detail, and the application prospects of multi-isotope coupling in the traceability of environmental geochemistry are evaluated. This work has some guidance for the application of stable isotopes in the source identification of environmental pollution.

Land-use conversion controls on the mobility of Zn in paddy soils revealed by stable Zn isotopes

Understanding Zn biogeochemical cycling is necessary for monitoring Zn supply for plants and life during land use conversion, which is critical for environmentally sustainable development. But little is known about how the conversion of paddy soil to abandoned land affects the Zn isotope signature. A comparative field observation was conducted in northeast Thailand to investigate the Zn isotope footprint of paddy soils and abandoned paddy soils (PL and NPL). Our results show that Zn (τ_Zn^int : 0.04) slightly retains on PL, but is lost from NPL (τ_Zn^int from - 0.81 to - 0.24) to the river during weathering. Compared to PL (Δ⁶⁶Zn_parent-soil: -0.29 ‰), more ⁶⁶Zn isotopes might enter the river when rice cultivation ceases in NPL (Δ⁶⁶Zn_parent-soil from -0.26 ‰ to -0.47 ‰). Rice harvest and then root decay might result in heavy ⁶⁶Zn isotopes accumulating at the topmost soil in PL (δ⁶⁶Zn: 0.14 ‰) and short-term abandonment (1-2 years) in paddy soils (NPL1 δ⁶⁶Zn: 0.18 ‰). The release of assimilated Zn, and then the high adsorption of Zn in the Fe-SOM-metal(loid)s ternary system positively contribute to the high [Zn] in PL, while this was not observed in NPL. Our findings provide a comprehensive insight into the Zn isotope signature in response to the conversion of land-use types, which is beneficial for understanding the terrestrial Zn geochemical cycle.

Determination of contamination sources and geochemical behaviors of metals in soil of a mine area using Cu, Pb, Zn, and S isotopes and positive matrix factorization

The use of multiple isotopic ratios and statistical methods can substantially increase the reliability and precision of determining contamination sources and pathways. In this study, contamination sources were differentiated in three subareas in one mine area and geochemical processes were investigated using Cu, Pb, Zn, and S isotopes and positive matrix factorization (PMF). Soil samples downstream of the adit seepages exhibited distinctly higher δ⁶⁵Cu values than those from other areas. δ⁶⁵Cu in adit seepages increased substantially from ore sulfides owing to large isotopic fractionation during oxidative dissolution. Although δ⁶⁵Cu decreased during sulfide precipitation in seepage-contaminated soil, the discrimination of δ⁶⁵Cu was still valid. Therefore, δ⁶⁵Cu is particularly useful for differentiating between contamination by sulfides (tailings) and water (adit seepages). Moreover, sulfide precipitation following sulfate reduction was verified by the decreased δ⁶⁶Zn and δ³⁴S in the soil. In addition, the plot of ²⁰⁸Pb/²⁰⁶Pb versus Pb^-1 distinguished contamination sources. Furthermore, PMF analysis confirmed the determination of sources and differentiated between contamination by As- and Cu-enriched tailings. The effect of Cu-enriched tailings further downstream suggested that the lower specific gravity of chalcopyrite compared to that of arsenopyrite affected the distribution of soil contamination.

Cellulose Acetate Microbeads for Controlled Delivery of Essential Micronutrients

The controlled delivery of micronutrients to soil and plants is essential to increase agricultural yields. However, this is today achieved using fossil fuel-derived plastic carriers, posing environmental risks and contributing to global carbon emissions. In this work, a novel and efficient way to prepare biodegradable zinc-impregnated cellulose acetate beads for use as controlled release fertilizers is presented. Cellulose acetate solutions in DMSO were dropped into aqueous antisolvent solutions of different zinc salts. The droplets underwent phase inversion, forming solid cellulose acetate beads containing zinc, as a function of zinc salt type and concentration. Even higher values of zinc uptake (up to 15.5%) were obtained when zinc acetate was added to the cellulose acetate-DMSO solution, prior to dropping in aqueous zinc salt antisolvent solutions. The release profile in water of the beads prepared using the different solvents was linked to the properties of the counter-ions via the Hofmeister series. Studies in soil showed the potential for longer release times, up to 130 days for zinc sulfate beads. These results, together with the efficient bead production method, demonstrate the potential of zinc-impregnated cellulose acetate beads to replace the plastic-based controlled delivery products used today, contributing to the reduction of carbon emissions and potential environmental impacts due to the uptake of plastic in plants and animals.

Possible Sources of Trace Metals in Obese Females Living in Informal Settlements near Industrial Sites around Gauteng, South Africa

Trace metals have been reported in the literature to be associated with obesity. Exposure to some trace metals such as Mn, Cr, Ni, Cd, and Pb may pose a serious health risk to individuals living around a polluted environment. The present study assessed the levels of trace metals in the blood of obese females living around industrial areas in Gauteng, South Africa. The study was carried out using a mixed method approach. Only females with a BMI ≥ 30.0 were considered. A total of 120 obese females participated in the study (site 1: 40-industrial area, site 2: 40-industrial area, and site 3: 40-residential area), aged 18-45 and not in menopause. Blood samples were analysed for trace metals content using inductively coupled plasma mass spectrometry (ICP-MS). The mean concentrations of trace metals were in the order Pb > Mn > Cr > Co > As > Cd (site 1), Pb > Mn > Co > As > Cd (site 2), and Mn > Cr > Co > As > Pb > Cd (site 3). The blood Mn from site 1 ranged from 6.79 µg/L-33.99 µg/L, and the mean differences obtained from the participants from different sites were significant (p < 0.01). The blood levels of Mn, Pb, Cr, Co, As, and Cd were above the recommended limits set by the WHO in some of the participants. The present study noted, among others, closeness to industrial areas, lifestyle decisions such as the use of tobacco products by their partners indoors, and the method used for cooking as factors that might have accounted for the blood levels of Mn, Pb, Cd and Co. The study showed that there is a need for constant monitoring of the levels of trace metals in the blood of those living in these areas.

Mobilisation of Cd, Mn, and Zn in floodplains by action of plants and its consequences for spreading historical contamination and fluvial geochemistry

Cadmium, Mn, and Zn are mobilised by plants commonly growing in floodplains, most notably willows (Salix) and alder (Alnus). These plants accumulate unwanted elements (Cd) or excessive element concentrations (Mn, Zn) in their foliage, thus introducing them into the food web and enriching them in floodplain surface by litterfall. In floodplain of the Litavka River in Czechia, contaminated by historical mining activities, up to 100 mg kg^-1 Cd and up to several thousand mg kg^-1 Mn and Zn are present in willow leaves in autumn, probably close maxima for sustainable plant growth. Willows and alders show seasonal growth of their foliar Mn and Zn. The willow leaves showed Cd/Zn larger than contaminated fluvisol of the Litavka River. Senesced willow leaves thus contribute to spread of risk elements from historically contaminated floodplains back to river water even without the bank erosion. Alders and willows alter geochemical cycles of Cd, Mn, and Zn in fluvial systems and increase Cd/Zn and Mn/Fe concentration ratios and Cd and Mn concentrations in fluvially transported particles relative to global geochemical averages as well as relative to floodplain sediments. Willows, in particular Salix fragilis L., S. aurita L, and S. cinerea L are particularly important "plant pumps". Other common floodplain plants, such as bird cherry (Prunus padus L.) and herbaceous plants (common nettle, Urtica dioica L. and grasses, Poaceae) do not contribute to those phenomena.

Heavy metal levels in the soil near typical coal-fired power plants: partition source apportionment and associated health risks based on PMF and HHRA

In this study, five priority metals recommended by the Ministry of Ecology and Environment of China (MEEC) were investigated. In the Bijie region of Guizhou Province, three typical coal-fired power plants were chosen as the research locations. A combination of 24 soil samples was obtained at various distances and depths from the point source of contamination. The authors found that the average contents of As, Cd, Cr, Ni, and Pb were 14.15, 1.52, 16.80, 40.71, and 53.00 mg kg^-1, respectively, with Cd and Pb pollution prominent. In another, soil heavy metal (SHM) content tends to increase or decrease dependently with the increase of sampling distance and depth, with total concentrations ranging from 77.14 to 157.33 mg kg^-1. Combining PCA and PMF models, the number of source factors was determined more clearly and accurately using PCA, and the Q-value of PMF was used for validation. The PCA-PMF indicated that the primary anthropogenic sources were transportation-related activities and emissions from coal combustion. The health risks of SHMs under three different exposure routes were then assessed using the HHRA. The findings showed the five HMs in order of non-carcinogenic risk were As > Pb > Cr > Ni > Cd. The comprehensive non-carcinogenic risk for children under the oral intake route around plant B and C was greater than 1, pointing to a potential health risk to children from the soils. The carcinogenic risk of HM was less than 1.00E-04 for both single-factor and multifactor under all three exposure routes, which is below the tolerable limit.

Too Much of a Good Thing: Rethinking Feed Formulation and Feeding Practices for Zinc in Swine Diets to Achieve One Health and Environmental Sustainability

The objectives of this review were to summarize current knowledge of Zn in swine nutrition, environmental concerns, potential contribution to antimicrobial resistance, and explore the use of alternative feeding strategies to reduce Zn excretion in manure while capturing improvements in productivity. Zinc is a required nutrient for pigs but is commonly supplemented at concentrations that greatly exceed estimated requirements. Feeding pharmacological concentrations of Zn from ZnO to pigs for 1 to 2 weeks post-weaning reduces post-weaning diarrhea and improves growth performance. Feeding elevated dietary levels of Zn to sows during the last 30 days of gestation can reduce the incidence of low-birth-weight pigs and pre-weaning mortality. Most of the dietary Zn consumed by pigs is not retained in the body and is subsequently excreted in manure, which led several countries to impose regulations restricting dietary Zn concentrations to reduce environmental impacts. Although restricting Zn supplementation in swine diets is a reasonable approach for reducing environmental pollution, it does not allow capturing health and productivity benefits from strategic use of elevated dietary Zn concentrations. Therefore, we propose feeding strategies that allow strategic use of high dietary concentrations of Zn while also reducing Zn excretion in manure compared with current feeding practices.

Zn isotope fractionation in laterites from Yunnan province, southwest China: Implications for the Zn cycles and its environmental impacts in (sub-) tropics

The weathering and development of laterites can influence trace element cycling in (sub-) tropics. Zinc (Zn) is a ubiquitous trace metal that involves both abiotic and biotic processes in soils. To explore Zn behavior in laterites, Zn cycling in (sub-) tropics, and the environmental impacts, Zn isotope systematics were presented for two laterite profiles from Yunnan province, southwest China. The laterite samples exhibit the δ⁶⁶Zn of 0.02 ‰-0.56 ‰, indicating a light shift of Zn isotope ratios (Δ⁶⁶Zn_{laterite-parent rock} = -0.47 ‰-0.07 ‰) relative to bulk parent granite. This observation is attributed to the preferential preservation of light Zn isotopes on the surface of secondary Fe oxides. As a result, laterites are likely to control the instantaneous riverine δ⁶⁶Zn in (sub-) tropical regions heavier than unweathered rocks. The isotopic signature of different vegetation covered soils show that shrub-covered soils are stronger leached (average τ_Zn = -0.61) and have a smaller Δ⁶⁶Zn_{laterite-parent rock} (=-0.15 ‰), relative to forest-covered soils (=-0.20 ‰). Due to the strong loss of Zn (average τ_Zn = -0.61 to -0.12) and large amounts of low-bioavailable Zn preserved in oxides, the micronutrient supplies for plant growth are difficult to maintain and need more fertilization. This study is helpful for a better understanding of global Zn cycling and the management of micronutrients in (sub-) tropical soil-plant systems.

Machine learning can identify the sources of heavy metals in agricultural soil: A case study in northern Guangdong Province, China

Source tracing of heavy metals in agricultural soils is of critical importance for effective pollution control and targeting policies. It is a great challenge to identify and apportion the complex sources of soil heavy metal pollution. In this study, a traditional analysis method, positive matrix fraction (PMF), and three machine learning methodologies, including self-organizing map (SOM), conditional inference tree (CIT) and random forest (RF), were used to identify and apportion the sources of heavy metals in agricultural soils from Lianzhou, Guangdong Province, China. Based on PMF, the contribution of the total loadings of heavy metals in soil were 19.3% for atmospheric deposition, 65.5% for anthropogenic and geogenic sources, and 15.2% for soil parent materials. Based on SOM model, As, Cd, Hg, Pb and Zn were attributed to mining and geogenic sources; Cr, Cu and Ni were derived from geogenic sources. Based on CIT results, the influence of altitude on soil Cr, Cu, Hg, Ni and Zn, as well as soil pH on Cd indicated their primary origin from natural processes. Whereas As and Pb were related to agricultural practices and traffic emissions, respectively. RF model further quantified the importance of variables and identified potential control factors (altitude, soil pH, soil organic carbon) in heavy metal accumulation in soil. This study provides an integrated approach for heavy metals source apportionment with a clear potential for future application in other similar regions, as well as to provide the theoretical basis for undertaking management and assessment of soil heavy metal pollution.

Organo-mercury species in a polluted agricultural flood plain: Combining speciation methods and polymerase chain reaction to investigate pathways of contamination

The analysis of organic mercury (Hg) species in polluted soils is a necessary tool to assess the environmental risk(s) of mercury in contaminated legacy sites. The artificial formation of monomethylmercury (MeHg) during soil extraction and/or analysis is a well-known limitation and is especially relevant in highly polluted areas where MeHg/Hg ratios are notoriously low. Although this has been known for almost 30 years, the thorough characterisation of artificial formation rates is rarely a part of the method development in scientific literature. Here we present the application of two separate procedures (inorganic Hg (iHg) spiking and double-spike isotope dilution analyses (DSIDA)) to determine and correct for artificial Hg methylation in MeHg-selective acid-leaching/organic solvent extraction procedure. Subsequently, we combined corrected MeHg and ethylmercury (EtHg) measurements with PCR amplification of hgcA genes to distinguish between naturally formed MeHg from primary deposited MeHg in soils from a legacy site in a Swiss mountain valley. We found the DSIDA procedure incompatible with the organomercury selective extraction method due to the quantitative removal of iHg. Methylation factors from iHg spiking were in the range of (0.0075 ± 0.0001%) and were consistent across soils and sediment matrices. Further, we suggest that MeHg was deposited and not formed in-situ in two out of three studied locations. Our line of evidence consists of 1) the concomitant detection of EtHg, 2) the elevated MeHg concentrations (up to 4.84 μg kg^-1), and 3) the absence of hgcA genes at these locations. The combination of Hg speciation and methylation gene (hgcA) abundance analyses are tools suited to assess Hg pollution pathways at Hg legacy sites.

Antagonistic Cd and Zn isotope behavior in the extracted soil fractions from industrial areas

The remobilization of metals accumulated in contaminated soils poses a threat to humans and ecosystems in general. Tracing metal fractionation provides valuable information for understanding the remobilization processes in smelting areas. Based on the difference between the isotopic system of Cd and Zn, this work aimed to couple isotope data and their leachability to identify possible remobilization processes in several soil types and land uses. For soil samples, the δ^66/64Zn values ranged from 0.12 ± 0.05‰ to 0.28 ± 0.05‰ in Avilés (Spain) and from - 0.09 ± 0.05‰ to - 0.21 ± 0.05‰ in Příbram (Czech Republic), and the δ^114/110Cd ranged from - 0.13 ± 0.05‰ to 0.01 ± 0.04‰ in Avilés and from - 0.86 ± 0.27‰ to - 0.24 ± 0.05‰ in Příbram. The metal fractions extracted using chemical extractions were always enriched in heavier Cd isotopes whilst Zn isotope systematics exhibited light or heavy enrichment according to the soil type and land uses. Coupling Zn and Cd systematics provided a tool for deciphering the mechanisms behind the remobilization processes: leaching of the anthropogenic materials and/or metal redistribution within the soil components prior to remobilization.

Effect of Fiber Fermentation and Protein Digestion Kinetics on Mineral Digestion in Pigs

Nutrient kinetic data and the timing of nutrient release along the gastrointestinal tract (GIT), are not yet widely used in current feed formulations for pigs and poultry. The present review focuses on interactions between fermentable substrates (e.g., starch, fiber, and protein) and selected minerals on nutrient digestion and absorption to determine nutritional solutions to maximize animal performance, principally in the grower-finisher phase, with the aim of minimizing environmental pollution. For phosphorus (P), myo-inositol 1,2,3,4,5,6-hexakis (dihydrogen phosphate) (InsP6), copper (Cu), and zinc (Zn), no standardized methodologies to assess in vitro mineral digestion exist. The stepwise degradation of InsP6 to lower inositol phosphate (InsP) forms in the GIT is rare, and inositol phosphate4 (InsP4) might be the limiting isomer of InsP degradation in diets with exogenous phytase. Furthermore, dietary coefficients of standardized total tract digestibility (CSTTD) of P might be underestimated in diets with fermentable ingredients because of increased diet-specific endogenous P losses (EPL), and further clarification is required to better calculate the coefficients of true total tract digestibility (CTTTD) of P. The quantification of fiber type, composition of fiber fractions, their influence on digestion kinetics, effects on digesta pH, and nutrient solubility related to fermentation should be considered for formulating diets. In conclusion, applications of nutrient kinetic data should be considered to help enhance nutrient digestion and absorption in the GIT, thereby reducing nutrient excretion.

Zinc in soil reflecting the intensive coal mining activities: Evidence from stable zinc isotopes analysis

In the mining area affected by coal mining activities for a long time, heavy metal Zn pollution poses a serious threat to soil quality and human health, and direct evidence showing the relationship between Zn accumulation mechanism in soils and mining activities is lacking. In this study, the Zn content and isotopes composition (δ⁶⁶Zn) from soil and environmental samples around mining area were determined and analyzed to clarify the Zn characteristics in soil. Moreover, the distribution and source of Zn content in soil of mining area were analyzed by mathematical statistics, correlation analysis and isotope mass mixing model. The results showed that: (1) the Zn content in soil ranged from 95 to 327 mg·kg^-1 (mean: 233 mg·kg^-1), exceeding the control point and the soil background value of Anhui Province; (2) the results of Zn isotope analysis showed that Zn in soil mainly derived from the wind dispersion input of fine particles in gangue and fly ash, followed by the natural weathering of parent material; (3) isotopic mass mixing model can be used to distinguish the contribution of anthropogenic and natural Zn sources. Mining input was the main contribution source of Zn in soil (mean: 67%), followed by natural background (mean: 33%). The employment of Zn isotopes can effectively evaluate the impact of anthropogenic and natural long-term processes on Zn in the soil of the mining area, and provide important information for the formulation of soil metal pollution control measures.

Zinc Essentiality, Toxicity, and Its Bacterial Bioremediation: A Comprehensive Insight

Zinc (Zn) is one of the most abundantly found heavy metals in the Earth's crust and is reported to be an essential trace metal required for the growth of living beings, with it being a cofactor of major proteins, and mediating the regulation of several immunomodulatory functions. However, its essentiality also runs parallel to its toxicity, which is induced through various anthropogenic sources, constant exposure to polluted sites, and other natural phenomena. The bioavailability of Zn is attributable to various vegetables, beef, and dairy products, which are a good source of Zn for safe consumption by humans. However, conditions of Zn toxicity can also occur through the overdosage of Zn supplements, which is increasing at an alarming rate attributing to lack of awareness. Though Zn toxicity in humans is a treatable and non-life-threatening condition, several symptoms cause distress to human activities and lifestyle, including fever, breathing difficulty, nausea, chest pain, and cough. In the environment, Zn is generally found in soil and water bodies, where it is introduced through the action of weathering, and release of industrial effluents, respectively. Excessive levels of Zn in these sources can alter soil and aquatic microbial diversity, and can thus affect the bioavailability and absorption of other metals as well. Several Gram-positive and -negative species, such as Bacillus sp., Staphylococcus sp., Streptococcus sp., and Escherichia coli, Pseudomonas sp., Klebsiella sp., and Enterobacter sp., respectively, have been reported to be promising agents of Zn bioremediation. This review intends to present an overview of Zn and its properties, uses, bioavailability, toxicity, as well as the major mechanisms involved in its bioremediation from polluted soil and wastewaters.

Zinc isotopic signature in tropical soils: A review

The micronutrient cycling in tropical latitudes is an issue of great concern because tropical soils are not only suffering micronutrient deficiency, but also influencing the global cycling of trace metals. With the development of stable isotope techniques, Zn isotopic composition (δ⁶⁶Zn) has been an powerful tool to interpret the Zn behaviour, signature, and cycling in soils. This review compiles δ⁶⁶Zn ratios of ten types of soils from both tropical and non-tropical latitudes, to (i) discuss the Zn isotopic signature in tropical soils and at the interfaces of soil-plant-river-ocean, (ii) disclose the Zn mass balance in tropical latitudes, and (iii) provide an implication for the eco-environmental effects of Zn cycling in tropical latitudes. Zinc isotopic signature is constrained by soil constituents. Our review summarized that the precipitation of secondary Fe oxides and organic complexation in the aqueous phases are likely to result in the preferential preservation of light Zn isotopes in tropical soils. The extreme weathering and material leaching of tropical soils can remove large amounts of Zn and thus result in Zn deficiency in tropical latitudes and pose risks to plant growth. The removed Zn is likely to influence the instantaneous riverine δ⁶⁶Zn heavier than that of the crustal average. However, the modern oceanic δ⁶⁶Zn will ultimately approach those of the parent materials by mass balance, at large geological timescales. Future direction should be concerned with the isotopic studies on Zn speciation in tropical soils and the association of isotopic ratios with the flux of Zn to quantitatively estimate of the Zn mass balance in tropical regions. The prospect of this review is to help solve the issue of plant micronutrition, as well as riverine and marine bio-availablity.

Biodegradation and effects of EDDS and NTA on Zn in soil solutions during phytoextraction by alfalfa in soils with three Zn levels

In chelator-enhanced Zn phytoremediation studies, it is crucial to understand how the degradable chelators and the competition from other ions influence the concentration of Zn in soil solutions. This study investigated the biodegradability of two chelators (EDDS: Ethylenediamine-N,N'-disuccinic acid, and NTA: Nitrilotriacetic acid) and their effects on the Zn concentration in the soil solution during the growth of alfalfa (Medicago Sativa L.). The chelators were added at four doses (0, 0.5, 2 and 5 mmol kg^-1) in soils with varying Zn levels (189, 265 and 1496 mg kg^-1). The results showed that the lag phase before EDDS and NTA biodegradation varied from 0 to 7 days in the three soils. EDDS and NTA were completely decomposed within the assessed 57 days regardless of the applied dosage, with a half-life of 1.3-3.0 days in highly Zn-contaminated soil and 4.2-10.8 days in the two other soils. In soil solutions, the change in solubilized Zn was in line with EDDS and NTA degradation kinetics. Cu, Al, Fe and Mn were the main metal ions that competed against Zn for chelation. Besides, Ni competed with Zn in the whole process. Ca did not compete effectively in the three soils, while Mg was a competitor only at the initial stage. Our results show the importance of considering both the biodegradation rate and the competition between the target cation and other elements present in the soil when using chelators to enhance phytoremediation. A 30-day explorative incubation experiment is recommended to evaluate the appropriate application time of chelators and the target Zn exposure time for plants during phytoremediation.

Effects of Agriculture and Animal Husbandry on Heavy Metal Contamination in the Aquatic Environment and Human Health in Huangshui River Basin

Huangshui River (HSR) is the mother river of Qinghai province. Croplands and grasslands cover more than 76% of the total area, and highland agriculture and animal husbandry are the dominant industries. The use of pesticides, fertilizers, and feed additives increases the risk of heavy metal (HM) contamination. In this study, the concentration of HMs in the main stream and tributaries of HSR were investigated. The Positive Matrix Factorization model was used for source apportionment, and Health Risk Assessment method was used to assess the human health risks. To further analyze the effect of agriculture and animal husbandry on aquatic environment and human health, we considered agriculture and animal husbandry as two factors in the source apportionment process, defined the effect of the factors, established the calculation formula, and quantified the effects. The results show that the overall situation of aquatic environment in HSR is good; natural processes, traffic tail gas and atmospheric deposition, agricultural planting, industrial wastewater discharge, and animal husbandry are the main sources of HMs in the water. These HMs present noncarcinogenic and carcinogenic risks for infants. A total effect of agricultural and animal husbandry on HMs or HI in HSRB is approximately 20%, while on TCR is 40%. However, the effects of agriculture on the hazard quotient of arsenic, carcinogenic risk of nickel and lead, and that of animal husbandry on carcinogenic risk of cadmium were significant. This study can provide a theoretical basis for local managers of agriculture and animal husbandry to perform their work effectively.

The Use of Stable Zinc Isotope Soil Labeling to Assess the Contribution of Complex Organic Fertilizers to the Zinc Nutrition of Ryegrass

Manure and sewage sludge are known to add significant amounts of zinc (Zn) and other metals to soils. However, there is a paucity of information on the fate of Zn that derives from complex organic fertilizers in soil-plant systems and the contribution of these fertilizers to the Zn nutrition of crops. To answer these questions, we grew Italian ryegrass in the presence of ZnSO₄, sewage sludge, and cattle and poultry manure in an acidic soil from Heitenried, Switzerland, and an alkaline soil from Strickhof, Switzerland, where the isotopically exchangeable Zn had been labeled with ⁶⁷Zn. This allowed us to calculate the fraction of Zn in the shoots that was derived from fertilizer, soil, and seed over 4 successive cuts. In addition, we measured the ⁶⁷Zn:⁶⁶Zn isotope ratio with the diffusive gradients in thin films technique (DGT) on soils labeled with ⁶⁷Zn and incubated with the same fertilizers. After 48 days of growth, the largest fraction of Zn in the ryegrass shoots was derived from the soil (79-88%), followed by the Zn-containing fertilizer (11-20%); the least (<2.3%) came from the seed. Only a minor fraction of the Zn applied with the fertilizer was transferred to the shoots (4.7-12%), which indicates that most of the freshly added Zn remained in the soil after one crop cycle and may thereby contribute to a residual Zn pool in the soil. The ⁶⁷Zn:⁶⁶Zn isotope ratios in the DGT extracts and the shoots measured at cut 4 were identical, suggesting that the DGT and plant took up Zn from the same pool. The proportion of Zn derived from the fertilizers in the DGT extracts was also identical to that measured in ryegrass shoots at cut 4. In conclusion, this work shows that stable Zn isotope labeling of the soil available Zn can be used to precisely quantify the impact of complex organic fertilizers on the Zn nutrition of crops. It also demonstrates that DGT extractions on labeled soils could be used to estimate the contribution of Zn fertilizers to plant nutrition.

Spatial distribution and human health risk assessment of soil heavy metals based on sequential Gaussian simulation and positive matrix factorization model: A case study in irrigation area of the Yellow River

The content of Cd, Cu, Pb, Zn, Cr, Ni and As from 250 soil samples was measured in agricultural soil of Ningxia section of the Yellow River. Positive matrix factorization (PMF) was to identify the main sources of these heavy metals; Sequential Gaussian Simulation (SGS) was to identify their spatial distribution and high-risk areas; and Human Health risk (HHR) model was to measure the health risk. Results showed that the average content of Cd and As exceeds the risk screening value of "Soil Environmental Quality-Agricultural Land Soil Pollution Risk Control Standard" (GB 15618-2018), which belongs to slight-level pollution. Although the content of other types of HMs (Cu, Pb, Zn, Cr, Ni) is below the risk screening value, they are still included heavily in the soil (except Cr). PMF indicated that mixed sources of agriculture and industry accounted for 27.06%, natural sources accounted for 14.12%, industrial sources accounted for 23.04%, traffic sources accounted for 21.50%, and Yellow River sedimentary sources accounted for 14.28%. PMF-HHR showed that the mixed sources of agriculture and industry are the most important factor causing non-carcinogenic risk (HI) to children (accounting for 55.75%). Industrial sources and traffic sources were the two main factors that cause HI to adults (industrial sources accounted for 25.16%, and traffic sources accounted for 28.78%). Mixed sources of agriculture and industry and natural sources were the two main factors that cause carcinogenic risk (CR) (mixed sources of agriculture and industry account for 35.34%, and natural sources account for 33.23%). SGS indicated that 0.64% and 9.32% of the total areas were posing as higher HI areas to children and adults respectively; in particular, 0.68% and 1.12% of the areas were identified as higher HI of As and Cr areas at a critical probability of 0.9.

Uptake of Cd, Pb, U, and Zn by plants in floodplain pollution hotspots contributes to secondary contamination

Willows, woody plants of genus Salix common in floodplains of temperate regions, act as plant pumps and translocate the Cd and Zn in the soil profiles of uncontaminated and weakly contaminated floodplains from the sediment bulk to the top strata. We suggest this process occurs because the Cd and Zn concentrations in willow leaves exceed those in the sediments. Senescing foliage of plant species common in floodplains can increase the Cd and Zn ratios as compared to other elements (Pb and common 'lithogenic elements' such as Al) in the top strata of all floodplains, including those that have been severely contaminated. The top enrichment is caused by the root uptake of specific elements by growing plants, which is followed by foliage deposition. Neither the shallow groundwater nor the plant foliage shows that Cd, Zn, and Pb concentrations are related to those in the sediments, but they clearly reflect the shallow groundwater pH, with the risk element mobilised by the acidity that is typical for the subsurface sediments in floodplains. The effect that plants have on the Pb in floodplains is significantly lower than that observed for Cd and Zn, while U can be considered even less mobile than Pb. Groundwater and plant leaves can contribute to secondary contamination with Cd and Zn from floodplain pollution hotspots, meaning that plants can accumulate these elements on the floodplain surface or even return them back to the fluvial transport, even if bank erosion would not occur. For Pb and U at the sites studied, these risks were negligible.

Quantitative source apportionment and ecological risk assessment of heavy metals in soil of a grain base in Henan Province, China, using PCA, PMF modeling, and geostatistical techniques

Heavy metals' pollution of agricultural soil is an environmental problem of widespread concern, especially in China, but, how to accurately identify their sources is still a great challenge. Here, we implemented a high-density sampling strategy (2194 samples collected) and combined correlation analysis (CA), principal component analysis (PCA), positive matrix factorization (PMF) modeling, and geostatistical analyses to identify and quantify the source contributions of heavy metals in agricultural soil in a key commodity grain base of China. The results showed that the excess risk rate of Cd was the highest (4.3%), and contributing to 42.7% of the total potential ecological risk in the region. Hence, Cd is the most important ecological risk factor. Heavy metals in the region mainly originated from oil extraction and smelting (7.5%), parent materials and traffic emissions (59.2%), coal-related industrial activities (11.6%), and agricultural sources (21.7%). Cd was mainly derived from oil extraction and smelting (90.1%). As (73.6%), Cr (90.3%), and Pb (67.1%) were closely associated with parent materials and traffic emissions. Hg (87.7%) was mainly originated from coal-related industrial sources. In addition, As (25.6%) and Pb (24.3%) were also from agricultural sources (such as fertilizer, pesticides, and manure). This study confirms that the combination of these methods can accurately identify the sources of heavy metals in agricultural soil. These findings can assist governmental agencies in implementing targeted control strategies to prevent the spread of heavy metals.

Mass balance of nine trace elements in two karst catchments in southwest China

High geological background levels of trace elements (TEs) and high population density in the karst areas of southwest China have imposed environmental pressure on the fragile ecosystems in this region. Understanding the mass budget of TEs, especially the toxic ones, is of great importance to sustain future developments. This study investigates the mass balance and fate of nine TEs (cadmium, arsenic, lead, chromium, copper, nickel, zinc, thallium, and antimony) in two karst catchments (Huilong and Chenqi) in southwest China through estimation of their mass budgets in throughfall, open field precipitation, total suspended particulate matter (TSP), litterfall, fertilization, harvested crops, surface runoff, and underground runoff. The estimated net fluxes are positive, indicating a source region, for four elements (Cu, Cr, Ni, and Tl) and negative, indicating a sink region, for five elements (As, Cd, Pb, Sb, and Zn) in both catchments. The net fluxes for the nine elements in Chenqi catchment are within a relatively small range (2.6, 2.0, 1.6, 0.6, -0.05, -0.5, -0.5, -2.9, and -3.3 mg m^-2 yr^-1 for Cu, Ni, Cr, Tl, Cd, Zn, Sb, Pb, and As, respectively), but in Huilong catchment in quite a large range (15.5, 6.0, 1.0, 0.8, -0.3, -0.9, -4.5, -7.5, and -8.7 mg m^-2 yr^-1 for Tl, Cr, Ni, Cu, Cd, Sb, As, Pb, and Zn, respectively). Rainfall (12.3%-66.2%) and litterfall (18.4%-81.3%) are the major input flux pathways, while crops harvest (16%-99%) is the major output flux pathway for the TEs in both catchments, indicating that the fate of TEs is shaped by both natural factors such as precipitation and litterfall and human activities such as fertilization and crop harvesting in these forestland-farmland compound karst catchments. Results from this study suggest that restoring forests from low-yield sloping farmlands will be useful for controlling TEs pollution in these fragile karst regions with high geological background TEs.

Catchment Runoff in Industrial Areas Exports Legacy Pollutant Zinc from the Topsoil Rather than Geogenic Zn

In highly industrialized, densely populated parts of Central Europe, mobilization of legacy Zn pollution from forest ecosystems may negatively affect the quality of water resources. To test this hypothesis, we determined the ⁶⁶Zn/⁶⁴Zn isotope ratios of 15 Zn reservoirs and fluxes in an acidified, spruce die-back affected mountain-slope catchment in northern Czech Republic. The δ⁶⁶Zn values of precipitation, organic horizon, and runoff were statistically indistinguishable. In contrast, δ⁶⁶Zn values of bedrock orthogneiss and mineral soil were significantly different from δ⁶⁶Zn values of runoff. The magnitude of within-site Zn isotope fractionations appeared to be relatively small. Despite the large potential source of Zn in bedrock, runoff exported mostly young pollutant Zn that had been temporarily stored in the organic horizon. This conclusion was corroborated by comparing Zn input-output mass balances in the polluted northern catchment and in a relatively unpolluted catchment situated 250 km to the south. Seven-times higher Zn export via runoff at the northern site was controlled by a combination of 10-times higher atmospheric Zn input and five-times higher DOC leaching, compared to the southern site. In industrial areas, atmospherically deposited Zn is leached from headwater catchments in a direct analogy to leaching of highly toxic pollutant Pb.

Possible application of stable isotope compositions for the identification of metal sources in soil

Metals in soil are potentially harmful to humans and ecosystems. Stable isotope measurement may provide "fingerprint" information on the sources of metals. In light of the rapid progress in this emerging field, we present a state-of-the-art overview of how useful stable isotopes are in soil metal source identification. Distinct isotope signals in different sources are the key prerequisites for source apportionment. In this context, Zn and Cd isotopes are particularly helpful for the identification of combustion-related industrial sources, since high-temperature evaporation-condensation would largely fractionate the isotopes of both elements. The mass-independent fractionation of Hg isotopes during photochemical reactions allows for the identification of atmospheric sources. However, compared with traditionally used Sr and Pb isotopes for source tracking whose variations are due to the radiogenic processes, the biogeochemical low-temperature fractionation of Cr, Cu, Zn, Cd, Hg and Tl isotopes renders much uncertainty, since large intra-source variations may overlap the distinct signatures of inter-source variations (i.e., blur the source signals). Stable isotope signatures of non-metallic elements can also aid in source identification in an indirect way. In fact, the soils are often contaminated with different elements. In this case, a combination of stable isotope analysis with mineralogical or statistical approaches would provide more accurate results. Furthermore, isotope-based source identification will also be helpful for comprehending the temporal changes of metal accumulation in soil systems.

Zn phytoextraction and recycling of alfalfa biomass as potential Zn-biofortified feed crop

Zn is an essential micronutrient for living organisms and, in that capacity, it is added to animal feed in intensive livestock production to promote growth and eliminate diseases. Alfalfa (Medicago sativa L.) may have the potential to compensate and substitute the need for chemical Zn additives in feeds as a Zn-biofortified feed crop when grown on Zn-enriched soils. Thus, this possibility was investigated with a greenhouse experiment using three soils with Zn concentrations (mg kg^-1) of 189 (soil A), 265 (soil B) and 1496 (soil C). Ethylenediamine-N,N'-disuccinate acid (EDDS) and Nitrilotriacetic acid (NTA) at different rates (0 as control, 0.5, 2 and 5 mmol kg^-1) were applied as soil additives to enhance the phytoextraction efficiency of alfalfa. The results showed that Zn was highly transferable in alfalfa tissues in the three soils even without additives. EDDS was more effective than NTA in enhancing Zn phytoextraction by alfalfa. The maximum Zn accumulation in the third cutting shoots was obtained with the EDDS concentration of 5 mmol kg^-1 in soil A and of 2 mmol kg^-1 in soil B, with a 462% and 162% increase compared with controls, respectively. However, the higher EDDS concentration resulted in a significant reduction in biomass production. In soil C, all EDDS concentrations resulted in similar Zn accumulations in the third shoot. To improve the phytoextraction efficacy of Zn while minimizing its phytotoxicity on alfalfa, the rate of 2 mmol kg^-1 EDDS proved to be optimal for soil B, and 0.5 mmol kg^-1 EDDS for soils A and C. Findings suggest that phytoextraction of Zn-enriched soil can be combined with Zn biofortification, thus allowing to recycle Zn into biomass that can, to an extent, substitute Zn feed additives. This study provided a primary data set for the combination of Zn-biofortification and Zn-phytoextraction.

Dairy manure as a potential source of crop nutrients and environmental contaminants

Although animal manure is applied to agricultural fields for its nutrient value, it may also contain potential contaminants. To determine the variability in such contaminants as well as in valuable nutrients, nine uncomposted manure samples from Idaho dairies collected during 2.5 years were analyzed for macro- and micro-nutrients, hormones, phytoestrogens, antibiotics, veterinary drugs, antibiotic resistance genes, and genetic elements involved in the spread of antibiotic resistance. Total N ranged from 6.8 to 30.7 (C:N of 10 to 21), P from 2.4 to 9.0, and K from 10.2 to 47.7 g/kg manure. Zn (103 - 348 mg/kg) was more abundant than Cu (56 - 127 mg/kg) in all samples. Phytoestrogens were the most prevalent contaminants detected, with concentrations fluctuating over time, reflecting animal diets. This is the first study to document the presence of flunixin, a non-steroidal anti-inflammatory drug, in solid stacked manure from regular dairy operations. Monensin was the most frequently detected antibiotic. Progesterones and sulfonamides were regularly detected. We also investigated the relative abundance of several types of plasmids involved in the spread of antibiotic resistance in clinical settings. Plasmids belonging to the IncI, IncP, and IncQ1 incompatibility groups were found in almost all manure samples. IncQ1 plasmids, class 1 integrons, and sulfonamide resistance genes were the most widespread and abundant genetic element surveyed, emphasizing their potential role in the spread of antibiotic resistance. The benefits associated with amending agricultural soils with dairy manure must be carefully weighed against the potential negative consequences of any manure contaminants.

Using Zn isotopes to trace Zn sources and migration pathways in paddy soils around mining area

Paddy soils around mining areas suffer from the great threat of heavy metal pollution. The traditional source-tracing methods based on metal concentrations limit our ability to quantify the sources of heavy metals and trace their transport processes to paddy soils. In this study, Zn isotope compositions of paddy soils in Dabaoshan mine area, a typical sulfide deposit in southern China, have been systematically studied. According to a plot between 1/Zn (i.e. inverse concentration) and δ⁶⁶Zn value, all the polluted paddy soils fall on the mixing line between acid mine drainage precipitate (AMD-precipitate) and fertilizer while the unpolluted paddy soil falls on the mixing line between fertilizer and bedrock. This indicates the mixing of Zn sources at least three end-members: the mining end-member (i.e. AMD-precipitate), the agricultural end-member (i.e. fertilizer), and bedrock whose geochemical signature is often overprinted by the former two sources around the mining area. The quantitative calculations to apportion the end-member's contributions show that the mining activity contributes most Zn in the paddy soils with an average of ∼66.2%. The contribution of mining activities has significant spatial variations. Specifically, the mining activities have relatively low impacts on the lower reach and the deep soil. Additionally, the apparent Zn isotope fractionation between AMD and AMD-precipitate (Δ⁶⁶Zn_{AMD-precipitate} - _AMD of -0.35 to -0.08‰) in the tailings dam suggests that Zn cations in AMD coprecipitated with the secondary Fe-bearing minerals (e.g. jarosite and goethite). After being discharged from the tailings dam, Zn is mainly carried by the Fe-oxide minerals and migrated during surface runoff. Our study highlights the contribution of human activities to the Zn pollution in the paddy soils and the key role of Fe-bearing minerals in the migration of Zn. These findings provide a scientific base for the development of policy for pollution control in mining-affected region.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Zinc toxicity in seedlings of three trees from the Fabaceae associated with arbuscular mycorrhizal fungi

Due to diverse human activities zinc (Zn) may reach phytotoxic levels in the soil. Here, we evaluated the differential sensibility of three Brazilian tree species from the Fabaceae to increasing soil Zn concentrations and its physiological response to cope with excess Zn. A greenhouse experiment was conducted with the species: Mimosa caesalpiniaefolia, Erythrina speciosa and Schizolobium parahyba, and the addition of 0, 200, 400 and 600 mg Zn kg^-1 to the soil. Plants were harvested after three months of cultivation, and growth, root symbiosis, biochemical markers and elemental composition were analyzed. Soil Zn addition reduced seedling growth, irrespective of the species, with a strong reduction in M. caesalpiniaefolia. Regarding root symbiosis, in N₂-fixing species, nitrogenase activity was reduced by the highest Zn concentrations. Zn addition caused plants nutritional imbalances, mainly in roots. The content of photosynthetic pigments in leaves decreased up to 40%, suggesting that high Zn contents interfered with its biosynthesis, and altered the content of foliar polyamines and free amino acids, depending on the species and the soil Zn concentration. Zn toxicity in M. caesalpiniaefolia plants was observed at available soil Zn concentrations greater than 100 mg kg^-1 (DTPA-extractable), being the most sensitive species and E. speciosa was moderately sensitive. S. parahyba was a moderately tolerant species, which seems to be related to polyamines accumulation and to mycorrhizal association. This last species has the potential for revegetation of areas with moderately high soil Zn concentration and for phytostabilization purposes. Future research evaluating the tolerance to multiple metal stress under field conditions should confirm S. parayba suitability in Zn contaminated areas of tropical regions.

A partition computing-based positive matrix factorization (PC-PMF) approach for the source apportionment of agricultural soil heavy metal contents and associated health risks

Apportion soil heavy metal sources across large-scale regions is a challenging task. The present study developed a modified receptor model to estimate the contributions of various sources to soil heavy metals and the associated health risks at a large scale. A positive matrix factorization model based on a partition computing approach was employed; the entire study area was divided into several zones for the source apportionment and then calculated together, termed partition computing-PMF (PC-PMF). The agricultural soil in Tianjin, China, was chosen for the case study. The PC-PMF results showed that irrigation, atmospheric deposition and sludge application were the main anthropogenic sources, with contributions of 26.60 %, 19.56 % and 2.86 %, respectively. We subsequently combined PC-PMF with a human health risk assessment model (HHRA) to obtain the human health risk of every source category. The natural background was regarded as a major factor influencing human health in the study area, with contributions of 38.03 % for the noncarcinogenic risk and 28.68 % for the carcinogenic risk. The results indicated that PC-PMF performed better at the source apportionment of soil heavy metals than PMF. This study provides a good example of how the spatial variability can be utilized to reduce the uncertainty in source apportionment.

The Use of Q-ICPMS to Apply Enriched Zinc Stable Isotope Source Tracing for Organic Fertilizers

Organic fertilizer applications can contribute to Zinc (Zn) biofortification of crops. An enriched stable isotope source tracing approach is a central tool to further determine the potential of this biofortification measure. Here, we assessed the use of the widely available quadrupole single-collector ICPMS (Q-ICPMS, analytical error = 1% relative standard deviation) and the less accessible but more precise multicollector ICPMS as reference instrument (MC-ICPMS, analytical error = 0.01% relative standard deviation) to measure enriched Zn stable isotope ratios in soil-fertilizer-plant systems. The isotope label was either applied to the fertilizer (direct method) or to the soil available Zn pool that was determined by isotope ratios measurements of the shoots that grew on labeled soils without fertilizer addition (indirect method). The latter approach is used to trace Zn that was added to soils with complex insoluble organic fertilizers that are difficult to label homogeneously. To reduce isobaric interferences during Zn isotope measurements, ion exchange chromatography was used to separate the Zn from the sample matrix. The ⁶⁷Zn:⁶⁶Zn isotope ratios altered from 0.148 at natural abundance to 1.561 in the fertilizer of the direct method and 0.218 to 0.305 in soil available Zn of the indirect method. Analysis of the difference (Bland-Altman) between the two analytical instruments revealed that the variation between ⁶⁷Zn:⁶⁶Zn isotope ratios measured with Q-ICPMS and MC-ICPMS were on average 0.08% [95% confidence interval (CI) = 0.68%]. The fractions of Zn derived from the fertilizer in the plant were on average 0.16% higher (CI = 0.49%) when analyzed with Q- compared to MC-ICPMS. The sample matrix had a larger impact on isotope measurements than the choice of analytical instrument, as non-purified samples resulted on average 5.79% (CI = 9.47%) higher isotope ratios than purified samples. Furthermore, the gain in analytical precision using MC-ICPMS instead of Q-ICPMS was small compared to the experimental precision. Thus, Zn isotope measurements of purified samples measured with Q-ICPMS is a valid method to trace Zn sources in soil-fertilizer-plant systems. For the indirect source tracing approach, we outlined strategies to sufficiently enrich the soil with Zn isotopes without significantly altering the soil available Zn pool.