Temporal changes of metal bioavailability and extracellular enzyme activities in relation to afforestation of highly contaminated calcareous soil

Recalcification stabilizes cadmium but magnifies phosphorus limitation in wastewater-irrigated calcareous soil

Long-term wastewater irrigation leads to the loss of calcium carbonate (CaCO3) in the tillage layer of calcareous land, which irreversibly damages the soil's ability to retain cadmium (Cd). In this study, we selected calcareous agricultural soil irrigated with wastewater for over 50 years to examine the recalcification effects of sugar beet factory lime (SBFL) at doses of 0%, 2.5%, 5%, and 10%. We found that SBFL promoted Cd transformation in the soil from active exchangeable species to more stable carbonate-bonded and residual species, which the X-ray diffraction patterns also confirmed results that CdSO4 reduced while CdS and CaCdCO3 increased. Correspondingly, the soil bioavailable Cd concentration was significantly reduced by 65.6-84.7%. The Cd concentrations in maize roots and shoots were significantly reduced by 11.7-50.6% and 13.0-70.0%, respectively, thereby promoting maize growth. Nevertheless, SBFL also increased the proportion of plant-unavailable phosphorus (P) in Ca8-P and Ca10-P by 4.3-13.0% and 10.7-25.9%, respectively, reducing the plant-available P (Olsen P) content by 5.2-22.1%. Consequently, soil P-acquiring associated enzyme (alkaline phosphatase) activity and microbial (Proteobacteria, Bacteroidota, and Actinobacteria) community abundance significantly increased. Our findings showed that adding SBFL to wastewater-irrigated calcareous soil stabilized Cd, but exacerbated P limitation. Therefore, it is necessary to alleviate P limitations in the practice of recalcifying degraded calcareous land.

Tradeoffs between pH, dissolved organic carbon, and mineral ions regulate cadmium uptake by Solanum hyperaccumulators in calcareous soil

Soil solution pH and dissolved organic carbon (DOC) influence cadmium (Cd) uptake by hyperaccumulators but their tradeoff in calcareous soils is unclear. This study investigated the mechanisms of Solanum nigrum L. and Solanum alatum Moench in calcareous soil using a combination of concentration gradient experiments (0.6-100 mg Cd kg-1) and soil solution composition analysis. The results showed that the soil solution pH of S. nigrum remained stable despite Cd stress. On average, the soil solution pH of S. alatum was 0.23 units higher than that of S. nigrum, although pH decreased significantly under high Cd stress. In addition, the concentrations of potassium (K) and calcium (Ca) in the soil solution of S. nigrum increased and decreased under low and high levels of Cd stress, respectively. In S. alatum, the K and Ca concentrations in the soil solution generally increased with increasing Cd stress levels. Moreover, the level of DOC in the soil solution of both plants was higher under Cd stress compared to the control, and a gradually increasing trend with Cd stress level was observed in S. alatum. Consequently, the bioconcentration factors of the roots (2.62-19.35) and shoots (1.20-9.59) of both plants were >1, while the translocation factors were <1, showing an obstacle of Solanum hyperaccumulators in transferring Cd into their aboveground parts. Redundancy analysis revealed that the Cd concentration in S. nigrum roots was significantly negatively correlated with the soil solutions of K and Ca. In contrast, Cd concentrations in S. alatum roots and shoots were significantly positively correlated with soil solution DOC, K, and Ca but negatively correlated with pH. Our results suggest that calcareous soil neutralizes the acidity of released protons but does not affect cation exchange, inhibiting DOC in assisting the translocation of Cd within plants.

Intercropping Sedum alfredii Hance and Cicer arietinum L. does not present a suitable land use pattern for multi-metal-polluted soil

Intercropping of specific accumulators with industrial crops is used in moderately metal contaminated agricultural land. The distribution characteristics and environmental risks of non-accumulated ions in intercropping fields have rarely been reported. This study analyzed dissolved organic matter (DOM) fractionation and metal chemical forms to investigate the bioavailability, transformation, and uptake of non-hyperaccumulated metals in different cultivation patterns of a Cd hyperaccumulator (Sedum alfredii Hance) and a commercial crop (Cicer arietinum L.). The study focused on the distribution and transformation of heavy metals, with a particular emphasis on the role of DOM in intercropping. The contents of DOM in the rhizosphere soils of the Cd hyperaccumulator monoculture and the intercropping field were obviously greater than the DOM concentration in the commercial crop monoculture. The content of soluble Cd was significantly lower in the former two planting patterns than in the latter. In contrast, soluble Pb and Cu exhibited opposite content characteristics. In addition, the metal extraction ability of DOM extracted from the C. arietinum monoculture was lower than those from the Cd hyperaccumulator monoculture and the intercropping field. The concentrations of Cd in both below-ground and aerial parts of C. arietinum intercropping were significantly lower than those in its monoculture, since S. alfredii depleted soil Cd. Contrastingly, the contents of Cu and Pb in C. arietinum harvested from intercropping were significantly greater than those in its monoculture because the intercropped Cd hyperaccumulator activated Cu and Pb by changing soil DOM content and fractionations without absorbing them. The findings provide valuable insights into the use of intercropping to remediate moderately metal-contaminated agricultural land and highlight the potential risks associated with intercropping in multi-metal-contaminated fields.

Changes in soil Cd contents and microbial communities following Cd-containing straw return

Contamination of soil with cadmium (Cd) threatens food safety and human health. In general, crop straws from contaminated soils could accumulate considerable amounts of Cd. The addition of Cd-containing rice straw can have negative effects on soil environment. In this study, straws varying in Cd concentration were added to soil at a rate of 5% (w/w) to investigate the effects of Cd-containing straw on soil Cd dynamics and soil microbial communities. Results showed that large amounts of Cd, especially bioavailable Cd, were released into soil during the decomposition of Cd-containing straws. The addition of straws with 10, 20 and 40 mg kg^-1 Cd increased total Cd in soils from 0.31 mg kg^-1 to 0.89, 1.39 and 2.09 mg kg^-1, respectively, exceeding the screening value of total Cd < 0.4 mg kg^-1 for paddy soils of pH 5.5-6.5 according to Chinese Soil Environmental Quality Standards. Moreover, the addition of Cd-containing straw decreased alpha-diversity of bacterial and fungal communities compared to the clean straw. Indeed, changes in soil factors including pH, Eh, dissolved organic C and Cd level jointly reconstructed soil microbial communities. The addition of Cd-containing straw increased the relative abundance of bacterial species Acidobacteria and Proteobacteria but decreased that of Firmicutes. Meanwhile, it increased the relative abundance of fungal species Basidiomycota and Fusarium which were considered Cd-tolerant. This study revealed the potential environmental risk and the variation of microbial communities caused by increasing soil Cd bioavailability after direct application of Cd-containing rice straw to the field.

Exploring the safe utilization strategy of calcareous agricultural land irrigated with wastewater for over 50 years

The trace element (TE) contamination of farmland caused by wastewater irrigation threatens food security and food safety. We selected a typical calcareous soil area in western China that has been irrigated with wastewater for >50 years to explore safe use strategies for flax farmland contaminated by cadmium (Cd) and arsenic (As). We found that Cd and As were mainly accumulated in flax roots rather than seeds. However, regardless of the type of TE and acceptor, direct ingestion of the flaxseed would seriously endanger human health (hazard quotient >1). According to the results of redundancy analysis and Pearson correlation analysis, the concentration of Cd and As in flaxseed depended on the concentration of soil total TE, Olsen phosphorus, dissolved organic carbon, soil organic matter, and active calcium carbonate (CaCO₃). This was largely because the pH and total CaCO₃ content in topsoil of flax farmland decreased by 1.05 units and 37 %, respectively, compared with their background levels before wastewater irrigation. Interestingly, after pressing, Cd and As in flaxseed transferred to flaxseed oil were 3.87-10.55 % and 17.21-30.48 %, respectively, which led to an acceptable risk of adults and children (hazard quotient <1) consuming flaxseed oil. Our results suggest that with the production of flaxseed oil as the goal, the long-term wastewater-irrigated calcareous land can be safely utilized while obtaining income.

Effects of magnetically treated Sedum alfredii seeds on the dissolved organic matter characteristics of Cd-contaminated soil during phytoextraction

The effects of magnetic field treatments on the two determining factors of phytoremediation, growth status and element uptake capacity, of Sedum alfredii Hance. have been thoroughly studied; however, minimal studies have been performed to determine the influence of the Cd hyperaccumulator S. alfredii, grown from magnetically treated seeds, on the dissolved organic matter (DOM) characteristics in its rhizosphere. A series of pot experiments were conducted to evaluate the variations in the DOM concentration and fractionations in the rhizosphere of S. alfredii treated with external magnetic fields. Compared with the untreated seeds, S. alfredii grown from magnetically treated seeds excreted more DOM in its rhizosphere. Additionally, the hydrophilic DOM fractionation proportion, which presented a greater capacity to mobilize Cd in the soil, increased from 42.7 % in the control sample to 47.2 % in the 150 mT magnetically treated S. alfredii sample. The water-soluble and exchangeable forms of Cd in the rhizosphere of the magnetically treated S. alfredii were significantly lower than those of the control sample. Furthermore, the Cd extraction capacity of DOM from the rhizosphere of the magnetically treated S. alfredii was greater than that of the control sample, thereby increasing the Cd uptake ability of the magnetically treated species. This study proves that a suitable magnetic field treatment can enhance the phytoremediation effect of S. alfredii, and reveals the mechanism of the phenomenon from the perspective of changes in soil DOM.

Phosphorus supply level is more important than wheat variety in safe utilization of cadmium-contaminated calcareous soil

Screening and cultivating crop varieties with low Cd accumulation is an effective way to safely utilize the Cd slightly contaminated soil. The characteristics and mechanism of Cd uptake by 13 wheat varieties in two calcareous soils with similar Cd contamination level but different P supply level were studied. The grain Cd concentration of almost all varieties in low-P soil was significantly higher than that in high-P soil and exceeded the maximum level of 0.2 mg kg^-1 recommended by the Codex Alimentarius Commission. The pH value of low-P soil was significantly lower than that of high-P soil by 0.27 units, while leaf [Mn] (proxy for rhizosphere carboxylates) and the activities of soil alkaline phosphatase and phytase were significantly higher than those of high-P soil by 35%, 55%, and 286%, respectively. The exchangeable Cd concentration in low-P soil was 2.93 times higher than that in high-P soil, while the Cd concentration of oxides and organic species was significantly lower than that in high-P soil by 21% and 64%, respectively, collectively increasing soluble Cd concentration in low-P soil by 38%. In low-P calcareous soil, P mobilization induced the change of root-zone microenvironment, resulting in the mobilization of Cd.

Effect of light combination on the characteristics of dissolved organic matter and chemical forms of Cd in the rhizosphere of Arabidopsis thaliana involved in phytoremediation

Light, one of the most important natural resources for plant species, significantly influences the biomass yield and nutrient uptake capacity in plants. Light sources with different spectra combinations can impact the bioavailability, toxicity, and solubility of heavy metals in soils by altering the concentrations and fractionations of soil dissolved organic matter (DOM). A series of light irradiation treatments were performed to evaluate the influence of red, yellow, and blue lights on the characteristics of DOM in the rhizosphere soils of Arabidopsis thaliana. The results showed that monochromatic red light significantly raised the levels of DOM and proportions of hydrophilic fractionations in the rhizosphere of A. thaliana relative to the control, while monochromatic blue light had the opposite effect. Moreover, the proportions of hydrophobic acid, which can mobilize Cd effectively, also raised with increasing doses of red light, which stimulated Cd mobilization. The application of yellow light not only increased the levels of hydrophobic acid in monochromatic red light treatment but also decreased the proportion of hydrophobic fractions in monochromatic blue light treatment, partially weakening the negative impacts of pure blue light on soil Cd activation. Moreover, DOM from the combined red, yellow, and blue lights resulted in a significantly stronger Cd extraction efficiency than the other light irradiation treatments, consequently enhancing the Cd phytoextraction efficiency of A. thaliana. The findings of this study demonstrated that a suitable light combination could enhance the phytoremediation effect of A. thaliana by activating soil Cd, and this method can be extrapolated to the real field, where light irradiation can be easily applied and modulated.

Redistribution of calcium and sodium in calcareous soil profile and their effects on copper and lead uptake: A poplar-based phytomanagement

Poplar serves as a phytostabilizator in phytomanagement of the trace metals (TMs) copper (Cu) and lead (Pb) contaminated land. In the process of long-term phytomanagement, it is not clear how the cycling of the mineral nutrients calcium (Ca) and sodium (Na) in calcareous soil will affect poplar remediation mechanisms. We selected a site contaminated by Cu and Pb and phytomanaged by Populus simonii Carr. stands of different ages (7, 14, and 28 years) to study the influencing mechanisms. The results showed that after afforestation, the Ca in the subsoil returned to the topsoil through fallen leaves, whereas the Na in the topsoil migrated downward to the subsoil by leaching, resulting in the redistribution of mineral nutrients in the soil profile. In addition, the Ca content in soil solution of the root-zone was significantly lower relative to that of the bulk soil, whereas the Na content in soil solution was significantly higher in all stands. As a result, because of the competitive adsorption of mineral nutrient and TM cations on the soil surface, the pool of bioavailable TM in root-zone soils did not significantly decrease with stand age. On the contrary, the TM content in poplar leaves (Cu: 31-37 mg kg^-1; Pb: 62-84 mg kg^-1) and litter (Cu: 230-790 mg kg^-1; Pb: 394-1366 mg kg^-1) increased significantly with stand age. Nevertheless, the TM content in poplar wood (Cu < 3 mg kg^-1; Pb < 12 mg kg^-1) remained at an extremely low level in all stands. Our results highlighted that strengthening leaf collection is necessary to eliminate ecological risks and ensure the safe production of poplar wood in the long-term phytomanagement of TM-contaminated land.

Temporal changes of calcareous soil properties and their effects on cadmium uptake by wheat under wastewater irrigation for over 50 years

Calcareous soil has a strong buffering capacity for neutralizing acid and stabilizing cadmium (Cd) because of the high calcium carbonate (CaCO₃) content. However, it is not clear whether the buffering capacity of calcareous soil can be maintained after long-term wastewater irrigation. We selected a typical area in western China that has been irrigated with wastewater for over 50 years to study the temporal changes of soil properties and their effects on Cd uptake by wheat. The results showed that compared with the background level before the 1960s, the soil pH and CaCO₃ content in 2018 were lower by 0.80 units and 35%, respectively, while the soil organic matter (SOM) content, Olsen phosphorus (P) content, and soil total Cd content in 2018 increased by 1.54, 13.05, and 164 times, respectively. Due to the significant decrease in the soil pH and CaCO₃, the high load of soil total Cd and electrical conductivity, the low soil clay content, and the coupling of SOM with soil nitrogen and P, the input Cd was activated. Furthermore, the activated Cd was effectively taken up by wheat roots and transported to grains with the assistance of dissolved organic carbon. Our results highlight that long-term wastewater irrigation caused irreversible damage to soil buffering capacity, resulting in the Cd activation and the enhancement of Cd uptake by wheat.

Differences in bacterial N, P, and COD removal in pilot-scale constructed wetlands with varying flow types

The mechanisms of bacterial nitrogen (N), phosphorus (P), and chemical oxygen demand (COD) removal in pilot-scale constructed wetlands (CWs) were investigated in the present work. Three types of CWs were assessed: vertical flow (VF), horizontal flow (HF), and surface flow (SF), each with three planting conditions, with either Thalia, Canna or without plants. The results show that construction types affected microbes more than planting conditions. VF CWs promoted the aerobic processing of total N, total P, COD, and NH₃-N, increasing the respective removal efficiencies by 4-19%, 13-32%, 19-29%, and 75-80%, respectively, compared with SF CWs. The relative abundance of nitrifying, denitrifying, methanotrophic and dephosphorized bacteria, and functional genes such as nxrA, nirK, nosZ, mmoX, and phoD were higher in VF CWs. Positive and simple gene networks in VF CWs can effectively reduce the redundancy in functional genes, enhance bacterial function and gene interactions, thus promoting nutrient removal.

Nitrogen induced DOC and heavy metals leaching: Effects of nitrogen forms, deposition loads and liming

Atmospheric nitrogen (N) deposition is believed to accelerate dissolved organic carbon (DOC) production and could lead to increased heavy metal mobility into water resources. We sampled intact soil cores from the Isle of Skye with low background N deposition history and having Serpentine rock known for its higher heavy metal concentrations including zinc (Zn), copper (Cu), nickel (Ni) and lead (Pb). The effects of 16 (16kgN) and 32 kg N ha^-1 year^-1 (32kgN), and liming with 32kgN (32kgN+Lime) on soil solution chemistry and heavy metal mobilization were investigated over the 15-month study. Nitrogen in deposition load was added at five ammonium (NH₄⁺) to nitrate (NO₃^-) ratios of 9:1, 5:1, 1:1, 1:5 and 1:9 along NO₃^-dominance. We found significant effects of load on Cu and NH₄⁺/NO₃^- ratio on pH, DOC and Zn in soil solution. However, under lime and ratio experimental factors, liming significantly influenced pH, DOC, Cu and Pb, and NH₄⁺/NO₃^- ratio pH, DOC, Ni and Zn whereas interactions between lime and ratio was significant for Ni and Cu. pH and DOC increased with N load, liming and NO₃^- dominance, and both correlated significantly positively. Liming under NH₄⁺ dominance enhanced DOC production due to supply of base cations in lime. Mobilization of Cu, Ni and Pb was driven by DOC concentrations and, therefore, increased with load, liming and NO₃^- dominance in deposition. However, in contrast, low pH and high NH₄⁺ dominance was associated with Zn mobilization in soil solution. On the contrary, despite of some patterns, heavy metals in soil HNO₃ extracts were devoid of any load, lime and NH₄⁺/NO₃^- ratio effects. Our study suggests that the effects of N load and forms in deposition on sites with high accumulated loads of metals need to be better quantified through soil solution partitioning models.

Responses of soil specific enzyme activities to short-term land use conversions in a salt-affected region, northeastern China

Soil enzyme activity is a sensitive indicator of soil quality changes. The response of soil enzyme activity to different land uses is important in addressing the issues of agricultural sustainability. The objectives of this study were to investigate the effects of short-term land use conversions on soil specific enzyme activity (per unit microbial biomass carbon) of sodic soils and compare the responses of soil absolute (per unit soil mass) and specific enzyme activities in northeastern China. Four specific enzyme activities, including catalase, invertase, urease and alkaline phosphatase were assayed at 0 to 20 cm depth under five land uses, including cropland (CL), alfalfa perennial forage (AF), monoculture grassland (AG), monoculture grassland for hay once a year (AG + M) and successional regrowth grassland (RG). The specific activities of catalase, urease and alkaline phosphatase at 10 to 20 cm depth were 117.3%, 40.0% and 35.6% higher than that in 0 to 10 cm depth, irrespective to the land uses. Conversion of cropland to re-vegetation land increased the specific activities of catalase (2.8%), invertase (99.0%), urease (14.3%) and alkaline phosphatase (14.0%). Under land uses of AF, AG + M, AG and RG, the geometric mean (0.2%, 32.8%, 65.7% and 24.3%, respectively) and sum (2.6%, 38.0%, 82.8% and 29.6%, respectively) of specific enzyme activities at 0 to 20 cm depth were higher than that under CL treatment. The soil specific enzyme activities showed the better discrimination to different land uses than the soil absolute enzyme activities. In conclusion, re-vegetation has a positive effect on the improvement of soil enzyme activity in northeastern China, and the responses of soil specific enzyme activities to short-term land-use conversions are more obvious than the absolute enzyme activities, which could be used as s suitable and sensitive indicator of land use change in semiarid agroecosystems.

Impact of poplar-based phytomanagement on metal bioavailability in low-phosphorus calcareous soil with multi-metal contamination

Bioavailability of trace metals (TMs) is the key component in the management of TM-contaminated soils; however, its impact mechanism is unclear in low-phosphorus (P) calcareous soils afforested by fast-growing tree species for a long duration (>10 years). We selected a site contaminated with multiple TMs and phytomanaged by poplar (Populus hopeiensis Hu & Chow) to study the impact mechanism of plant-soil interactions on TM bioavailability along a long-term chronosequence (i.e., 10, 15, 20, and 25 years). We found that phytomanagement significantly decreased soil organic carbon (SOC) content, soil total nitrogen (N) content, and soil C/P and N/P ratios with stand age, but did not significantly change soil total P content. In contrast, soil available P content significantly changed in rhizospheric soils compared with the bulk soil, suggesting the tight coupling between the amplification of P turnover and N availability. Soil pH in rhizospheric soils significantly decreased by 0.22 to 0.32 units, while calcium carbonate (CaCO₃) content decreased by 14% to 39%, as compared with the bulk soil. Bioavailable concentrations of cadmium, lead, and zinc were positively correlated with soil available P, whereas bioavailable nickel concentration was negatively correlated with soil pH. Furthermore, TM bioavailability in rhizospheric soils significantly increased with stand age, regardless of the metal type. Our results suggest that P mobilization associated with SOC depletion induced soil acidification followed by CaCO₃ dissolution, collectively leading to metal mobilization with stand age.

Effect of roxarsone metabolites in chicken manure on soil biological property

Roxarsone (ROX), an organoarsenic feed additive, occurs as itself and its metabolites including As(V), As(III), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in animal manure. Animal manure improves soil biological property, whereas As compounds impact microorganisms. The integral influence of animal manure bearing ROX metabolites on soil biological quality is not clear yet. Herein, the effect of four chicken manures excreted by chickens fed with four diets containing 0, 40, 80 and 120 mg ROX kg^-1, on soil biological attributes. ROX addition in chicken diets increased total As and ROX metabolites in manures, but decreased manure total N, ammonium and nitrate. The elevated ROX metabolites in manures increased soil total As, As species and total N, and increased first and then decreased soil nitrate and nitrite, but did not affect soil ammonium in manure-applied soils. The promoting role of both soil As(III) and ammonium on soil microbial biomass carbon and nitrogen, respiration and saccharase activity, were exceeded or balanced by the inhibiting effect of soil nitrate. The suppression of soil catalase activity by soil As(V) was surpassed by the enhancement caused by soil nitrate and nitrite. Soil urease, acid phosphatase and polyphenol oxidase activities were not suitable bioindicators in the four manure-amended soils. Soil DMA did not affect soil biological properties, and MMA was not detectable in all manure-amended soils. The above highlights the complexity of joint influence of soil As and N on biological attributes. Totally, when ROX is used at allowable dose in chicken diet, soil biological quality would be suppressed in manure-amended soil.

Effects of tree-herb co-planting on the bacterial community composition and the relationship between specific microorganisms and enzymatic activities in metal(loid)-contaminated soil

Tree-herb co-planting is regarded as an ecologically sustainable approach for the remediation of metal(loid)-contaminated soil. In this study, two herb species, Pteris vittata L. and Arundo donax L., and two woody species, Morus alba L. and Broussonetia papyrifera L., were selected for the tree-herb co-planting, and their impacts on the changing of microbial community structure in metal(loid)-contaminated soil were studied by high-throughput sequencing. The results showed that the microbial diversity was stably maintained by the tree-herb interactions, while the composition of the microbial community was clearly affected in metal(loid)-contaminated soil. According to the Venn and flower diagrams, heat map and principal coordinate analysis, both plant monocultures and co-planting had specific microbial community structures, which suggested that the composition and abundance of bacterial communities varied between plant monoculture and tree-herb co-planting treatments. In particular, A. donax L. played a vital role in increasing the abundances of Cyanobacteria (>1%) in metal(loid)-contaminated soil when co-planted with woody plants. Furthermore, some specific microorganisms combined with plants played a key role in improving enzyme activity in the contaminated soil. Correspondingly, sucrase and acid phosphatase activities in monoculture and co-planting treatments significantly (p < 0.05) increased by 1.05-3.37 and 7.24-20.3 times. These results indicated that the rhizospheric interactions in the tree-herb co-planting system positively affected the soil microbes and had stronger impacts on the composition of soil microorganisms, which was closely related to the improvement of the biological quality in the metal(loid)-contaminated soil.

Accumulation, fractionation, and risk assessment of mercury and arsenic in the soil-wheat system from the wastewater-irrigated soil in Baiyin, northwest China

Wastewater irrigation can increase metal concentrations in soil and wheat, thereby posing metal-associated health risk via food ingestion. We investigated levels of mercury (Hg) and arsenic (As) in roots, husks, stems, leaves, and grains of wheat and their fractionations in farmland soil from Baiyin City, an industrial and mining city, northwest China. Results show that the mean concentrations of Hg in soils from Dongdagou and Xidagou stream in Baiyin were 8.5 times and three times higher than local soil background values, respectively. Those of As were 4.5 times and 1.6 times higher, respectively. Most Hg and As were mainly accumulated in wheat leaves. The spatial distributions of As in soils and grains exhibit a very similar pattern, which suggest that As pollution in soils might be predicted by its level in wheat grains. Residual fractions for Hg (RES-Hg) and As (RES-As) are the highest compared to other fractions, indicating weak mobility of Hg and As in soil. The crop oral intake hazard quotients of both Hg and As for children were approximately two times higher than that for adults, indicating that children have higher exposure risks to Hg- and As-contaminated wheat. The crop oral intake was the main route of exposure causing non-carcinogenic and carcinogenic risk for local residents.