Modeling of Trace Metal Migration and Accumulation Processes in a Soil-Wheat System in Lihe Watershed, China

Bayesian risk prediction model: An accessible strategy to predict cadmium contamination risk in wheat grain grown in alkaline soils

Excessive cadmium (Cd) concentration in wheat grain is becoming a widespread concern in China. Considering the complexity of Cd transfer in the soil-wheat system, how the Cd risk in wheat grain be accurately predicted from the limited details available is of great significance for the risk management of Cd. Bayes' theory could leverage existing data by combining prior information and observational data, providing a promising strategy with which to calculate a more robust posterior probability of a grain sample exceeding the food safety standard (FSS) for Cd (0.1 mg kg-1). In the current study, a risk prediction model, based on Bayes' theory, was established to achieve a more accurate prediction of the wheat grain Cd risk from a limited number of soil parameters. The risk prediction model could predict the risk probability of wheat grain with a Cd concentration exceeding the FSS under a given soil concentration of either total Cd or diethylenetriaminepentaacetic acid (DTPA)-extractable Cd. Soil total Cd concentration proved to be a better variable for the model with greater predictive accuracy. The model predicted that fewer than 5% of the wheat grain would have a Cd concentration exceeding the FSS when grown in soil with a total Cd concentration of less than 0.299 mg kg-1. The risk probability rose significantly to 50% when the soil total Cd reached 0.778 mg kg-1. The accuracy of the model was greater than the widely applied multiple linear regression model, whereas previously published data from similar soil conditions also confirmed that the Bayesian model could predict wheat Cd risk with minimal error. The proposed model provides an accurate, accessible and cost-effective methodology for predicting Cd risk in wheat grown in alkaline soils before harvest. The wider application to other soil conditions, crops or contaminants using the Bayesian model is also promising for risk management authorities.

Mechanisms of lead uptake and accumulation in wheat grains based on atmospheric deposition-soil sources

Lead (Pb) accumulation in wheat grains depends on two aspects: i) Pb uptake by the roots and shoots, and ii) the translocation of organ Pb into the grain. However, the underlying mechanism of the uptake and transport of Pb in wheat remains unclear. This study explored this mechanism by establishing field leaf-cutting comparison treatments. Interestingly, as the organ with the highest Pb concentration, only 20.40 % of the root's relative contribution to grain Pb. The relative contributions of the spike, flag leaf, second leaf, and third leaf to grain Pb were 33.13 %, 23.57 %, 13.21 %, and 9.69 %, respectively, which was opposite to their Pb concentration distribution trends. According to Pb isotope analysis, it was found leaf-cutting treatments reduced the proportion of atmospheric Pb in grain, and grain Pb predominantly comes from atmospheric deposition (79.60 %). Furthermore, from the bottom to the top, the concentration of Pb in internodes decreased gradually, and the proportions of Pb originating from soil in the nodes also decreased, revealing that wheat nodes hindered the translocation of Pb from roots and leaves to the grain. Therefore, the hindering effect of nodes on the migration of soil Pb in wheat resulted in atmospheric Pb having a more convenient pathway to the grain than soil Pb, and further leading grain Pb accumulation primarily depended on the contribution of the flag leaf and spike.

Relative contribution of environmental medium and internal organs to lead accumulation of wheat grain

Lead (Pb) pollution in wheat has received considerable research attention globally due to its persistence and ease of accumulation, posing severe health risks to humans. This study explored the relative contribution of the environmental medium (atmospheric deposition and soil) and wheat internal organs to Pb accumulation in wheat grains, using field experiments by contrasting treatments. The concentration and bioavailability of Pb in the soil were significantly lower than those of atmospherically deposited Pb (P < 0.05). Pb accumulation rate in wheat grains was consistent with the grain filling rate, which first increased and then decreased, reaching the highest level at the middle filling stage. Pb isotope analysis showed that atmospheric deposition was the main source of Pb in the shoots of wheat plants, contributing more than 80.0% of Pb in grains. Although the roots had the highest Pb concentration, the spikes had the greatest relative contribution (58.4%) to Pb accumulation in the wheat grains, followed by that of the leaves (24.5%), whereas the contribution of roots was the lowest (17.1%) among all plant organs. In addition, among all leaves, the contribution of flag leaves to Pb accumulation in the grain was higher than the cumulative contribution of all other leaves, where flag leaves and other leaves contributed 13.8% and 10.7%, respectively. Collectively, the absorption of atmospherically deposited Pb by wheat spikes is the leading cause of Pb pollution in wheat grains. These results may aid in formulating strategies to reduce Pb concentration in grains and ensure food quality and safety.

Contribution of the flag leaf to lead absorption in wheat grain at the grain-filling stage

Wheat flag leaf (FL) is one of the primary sources of carbohydrates in grains; however, its role in grain lead (Pb) absorption remains unclear. A field experiment was conducted to assess the relative contribution of the FL to Pb accumulation in wheat grain by two contrasting treatments: without (CK) and with FL removal (FLR) at the grain-filling stage. The Pb concentration in leaves was closely related to leaf strata and decreased from FL to the third leaf. FLR treatment significantly reduced the yield and grain Pb concentration by 2.79% and 11.47%, respectively. The contribution of FL to grain Pb accumulation decreased gradually with the filling process, from 35.08% (at early stage) to 13.94% (at maturity stage). After FLR, the contribution proportion of atmospheric fallout to grain Pb decreased from 69.01% (CK) to 62.43% (FLR). Combined isotope analysis with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) revealed that the main contribution of FLs to grain Pb originated from Pb fallout in fine atmospheric particles. Therefore, taking measures to reduce the influence of fine atmospheric particles on wheat may be an effective way to control wheat grain Pb contamination.

Risks Assessment Associated with Different Sources of Metals in Abandoned Soil of Zhuxianzhuang Coal Mine, Huaibei Coalfield (Anhui, China)

In this paper, the 36 topsoil (0-10 cm) samples were collected and the contents of Cr, Mn, Co, Ni, Cu, Zn, As, Cd and Pb were analyzed. The results indicated that the contents of Cu and As in all samples exceeded the soil background values of Anhui province, while the Co and Pb contents were lower than the background values. Geo-statistics and positive matrix factorization were applied to identify the sources of soil heavy metals, which were nature factor (15.7%), industrial activities (21.2%), coal mine (50.9%) and traffic emission (12.2%), respectively. The calculation results of health risk model based on positive matrix factorization model showed that coal mine activities accounted for the largest proportion of total source contribution, followed by industrial activities. In addition, compared with adults, the trend of health risk of children from four sources in three lands were same as adults, but their health risk was higher than adults.

Improving spatial prediction of health risk assessment for Hg, As, Cu, and Pb in soil based on land-use regression

Heavy-metal pollution is a significant health and environmental concern in areas of rapid industrialization in China. The accuracy of spatial mapping of pollutant is the main constraint on spatial prediction of health risks. Our study addressed the possibility of improving spatial prediction accuracy of risk assessment. We developed land-use regression (LUR) models for Hg, As, Cu, and Pb based on surface soil sampling, land-use data, and soil properties. The regression results suggested that LUR was more accurate than ordinary kriging method. Spatial prediction accuracy of Hg, As, Cu, and Pb were improved by 15%, 59%, 36%, and 20%, respectively. Then, spatial distribution of health risk was assessed by using distributions of heavy metal and exposure parameters. Chronic risk of children was controlled by distribution of Pb and carcinogenic controlled by As. The result indicated that Pb and As were the main sources of health risk for children in Kunshan. Chronic and carcinogenic risk maps could clearly show where we should pay attention to and control the risk. This study provided a simple approach to draw spatially explicit maps of health risk which were useful for pollution control and public health risk management.

Direct evidence of lead contamination in wheat tissues from atmospheric deposition based on atmospheric deposition exposure contrast tests

A field experiment was conducted to assess the atmospheric deposition effects on lead (Pb) contamination in wheat by two contrasting treatments: wheat exposed or not to atmospheric deposition. Plants were housed in a shed during wheat greening for the non-exposed treatment. The Pb contents of wheat during different growth stages, of soil and of atmospheric deposits were analysed and combined with Pb stable isotope data to quantify the contribution of atmospheric deposition and soil to Pb in wheat tissue. The Pb content in atmospheric deposits was significantly higher than those in soil and wheat tissue, and the Pb content in wheat tissue exposed to atmospheric deposition was significantly higher than the Pb content in non-exposed tissue (p < 0.05). The ²⁰⁶Pb/²⁰⁷Pb of soil was significantly higher than the ²⁰⁶Pb/²⁰⁷Pb of atmospheric deposits (p < 0.05), and soil and atmospheric deposition were the two sources of Pb in wheat tissue. Atmospheric deposition was the main source of wheat tissue Pb in the exposed treatment, and most of the wheat tissue Pb, except for that in the stem, also came from atmospheric deposition in the maturing stage. The proportion of Pb from atmospheric deposition in roots, stems and leaves evidently decreased after the shed was erected, and the contribution of Pb from atmospheric deposition to wheat tissue was significantly higher in the exposed treatment than in the non-exposed treatment (p < 0.05). This contrast test directly confirmed that atmospheric deposition was the main source of Pb in the wheat tissues. Therefore, taking measures to reduce the absorption of Pb by wheat from atmospheric deposition can effectively ensure food safety.

Quantitative analysis of lead sources in wheat tissue and grain under different lead atmospheric deposition areas

Due to rapid growth of industrialization and human activities, such as mining and smelting, lead (Pb) has become a major environmental contaminant. As Pb can pose risks to human health, preventing Pb pollution in wheat is important for food safety, requiring accurate verification of pollution sources. Pb concentrations and isotope ratio levels in soil, in the atmosphere, and wheat tissue (root, stem, leaf, grain) in an area of high-Pb deposition (in the vicinity of a Pb smelter in Jiyuan city) and an area of low deposition (the northwest suburb of Zhengzhou city) were examined. The Pb isotope ratio and the binary mixed model were used to quantify the contribution of soil and atmospheric deposition to Pb content in wheat tissues. Results show that Pb content in soil, atmospheric deposition, and wheat in the high deposition area were significantly higher than those in the low deposition area. Pb content in soil, atmospheric deposition, wheat roots, stems, leaves, and grains in the high-deposition area were 355.32 ± 14.78, 5477.90 ± 187.85, 158.72 ± 9.56, 21.36 ± 1.72, 26.49 ± 1.96, and 0.94 ± 0.02 mg kg^-1, respectively. Pb content in the low-deposition area were 6.10 ± 0.75, 78.50 ± 4.35, 2.47 ± 0.23, 1.03 ± 0.07, 2.11 ± 0.13, and 0.08 ± 0.01 mg kg^-1, respectively. The Pb isotope ratio recorded obvious differences between soil and atmospheric deposition in the two areas. Combined with the significant correlation between Pb isotopes in various tissues of wheat and environmental media, and analysis of the isotopic composition characteristics of wheat and environmental media, in the high-deposition area, the contribution rate of atmospheric deposition in wheat roots, stems, leaves, and grains was 14%, 66%, 84%, and 77%, respectively. And the soil contribution rate was 86%, 34%, 16%, 23%, respectively. In the low-deposition area, the contribution rate of atmospheric deposition in wheat roots, stems, leaves, and grains was 49%, 73%, 93%, and 83%, respectively. And the soil contribution rates were 51%, 27%, 7%, and 17%, respectively. In the low-Pb deposition area, the contribution rate of atmospheric deposition in wheat was higher than that in the high-deposition area. Atmospheric deposition was the main source of Pb in grains, leaves, and stems of wheat in different depositional areas. Pb in wheat roots mainly derives from soil, and the Pb contribution rate of soil to wheat roots in the high-deposition area was significantly higher than that in the low-deposition area.

Effects of Atmospheric Fallout on Lead Contamination of Wheat Tissues Based on Stable Isotope Ratios

In order to trace the source of Pb pollution in wheat, the contribution ratio of soil and atmospheric fallout source was quantified based on stable isotope ratios. Results showed that the average Pb content of soil was significantly lower than that of fallout, and Pb in the fallout had a higher weak acid fraction than soil. Pb in wheat had a distinct distribution in its tissues and the content of Pb in wheat roots was significantly higher than it in shoots. The ²⁰⁶Pb/²⁰⁷Pb ratio of soil was significantly higher than that in atmospheric fallout (p < 0.05). According to a binary mixing model, the ²⁰⁶Pb/²⁰⁷Pb ratio in wheat roots, leaves, and grains reflect 67%, 65%, and 90% of Pb content contributions from fallout, respectively. This results suggest that fallout Pb was absorbed by wheat leaves and transferred to other organs, and it is important to develop effective strategies to control fallout Pb risks.