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Article

Influencing Factors and Prediction Models of Mercury Phytoavailability and Transference in a Soil–Lettuce System under Chinese Agricultural Soils

by
Subhan Ullah
1,2,
Sajjad Hussain
3,
Yousaf Noor
4,
Tasawar Khanam
5,6,
Xing Xia
3,
Aminu Inuwa Darma
1,
Ya Feng
3 and
Jianjun Yang
1,*
1
State Key Laboratory of Efficient Utilization of Arid and Semi-arid Arable Land in Northern China (Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences), Beijing 100081, China
2
Cereal Crops Research Institute (CCRI), Pirsabak, Nowshera 24050, Pakistan
3
Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China
4
Agriculture Research Institute (ARI), Tarnab, Peshawar 24330, Pakistan
5
Ecohealth and Toxicology Laboratory, Department of Biosciences, COMSATS University Islamabad, Islamabad 44000, Pakistan
6
Department of Zoology, University of Chakwal, Chakwal 48800, Pakistan
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(7), 1394; https://doi.org/10.3390/agronomy14071394
Submission received: 30 April 2024 / Revised: 29 May 2024 / Accepted: 12 June 2024 / Published: 27 June 2024
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

:
Mercury (Hg) is a highly toxic contaminant posing serious ecological and human health risks. This study investigates the Hg transfer characteristics and prediction models in a soil–lettuce system, employing bioconcentration factors (BCF), path analysis (PA), and Freundlich-type functions. A pot experiment was conducted in a greenhouse, where lettuce was grown in a range of Chinese agricultural soils (n = 21) and deliberately spiked with Hg using Hg(NO3)2 solution. The results indicated that lettuce grown in Hg-spiked acidic soils (pH < 6.5) accumulated total Hg (THg) levels up to 14.01 µg kg−1, surpassing the safe consumption limit of 10 µg kg−1. The BCF for lettuce THg was less than 1.0, suggesting a low transfer of Hg from soil to lettuce. Notably, BCF values were significantly higher in acidic soils (0.02) compared to alkaline soils (0.005). Path analysis accounted for 82% of the variation in lettuce THg content, identifying soil THg, pH, and amorphous (Amo) Al and Fe oxides as primary direct factors. Additionally, soil-available Hg (AvHg), exchangeable Hg (ExHg), clay, and organic matter (OM) were significant indirect factors affecting lettuce THg content. To validate the findings of the path analysis, an extended Freundlich-type equation was developed using stepwise multiple linear regression (SMLR). This model exhibited high predictive accuracy (R2 = 0.82, p ≤ 0.001), with soil pH, THg, and amorphous Al and Fe oxides being the key variables for predicting Hg transfer in the soil–lettuce system. The insights from this study can guide the management of safe lettuce production in Hg-contaminated soils, ensuring the mitigation of Hg exposure through agricultural produce.

1. Introduction

Mercury (Hg), one of the most toxic pollutants, endangers ecosystems and human health through accumulation and biomagnification in the food chain [1,2]. Soil Hg contamination has become a serious environmental issue in China due to rapid urbanization and economic growth, making the country one of the largest sources of anthropogenic Hg emissions worldwide [3,4]. These emissions are eventually deposited on the surface soil layers through wet and dry deposition. Approximately 1.6% of agricultural land in China is contaminated with Hg [5], primarily due to sewage irrigation, mining activities, irrational fertilizer use, and the application of Hg-containing chemicals, in addition to atmospheric deposition [6]. Once in soils, Hg undergoes various biogeochemical processes that significantly impact the environment and food security [7]. Among these, the bioavailability of Hg depends on soil properties, Hg concentration, and crop species. Therefore, understanding the soil–crop Hg transfer characteristics and the main controlling soil variables is essential for minimizing Hg exposure risks.
However, despite Hg’s low phytoavailability due to adsorption or binding to solid phases [7], significant translocation and accumulation of Hg from soils to plants can occur during the growth period [8]. Key soil properties influencing Hg mobility and bioavailability include pH, organic matter (OM), clay content, and oxides of Al and Fe [9,10]. Soil pH directly affects Hg speciation in soil solutions, producing more bioavailable Hg under acidic conditions [11,12]. Lower pH enhances metal bioavailability by releasing adsorbed cations and dissolving metal-containing soil minerals [13]. Therefore, soil pH is a critical factor controlling the bioavailability and transference of Hg in soil–plant systems [10,12]. Organic matter forms strong complexes with Hg [1,14,15], although its influence can be complex and sometimes contradictory. High OM content can both stabilize Hg [16] and, in some cases, increase its bioavailability [17]. Similarly, soil clay content and oxides of Al, Fe, and Mn have high adsorption capacities for Hg, influencing its availability to plants [1,18,19,20].
Several modeling approaches have been developed to estimate the risks of heavy metal pollution, including species sensitivity distribution [21], potential ecological risk [22], soil-to-plant transfer models [23,24], positive matrix factorization [25], and human health risks from exposure to contaminated soils [26,27]. In this study, we focused on empirical models of soil–plant metal transfer based on soil and plant variables [12,28]. Freundlich-type models are particularly advantageous for their simplicity and applicability to real field conditions, although their validity can be limited by the specific crop species and soil metal contents used [29]. Previous studies have developed soil–plant Hg transfer models for various crops, identifying key soil variables such as soil Hg content, pH, and metal oxides. Ding and Zhang [10] investigated Hg transfer from soil to carrots and identified soil Hg, pH, and free Al oxides as the most significant variables controlling carrot Hg content. Similarly, wheat Hg uptake was mainly influenced by soil total and available Hg, pH, organic matter, and amorphous Mn [14]. More recently, Hussain and Yang [12] studied Hg transfer in soil–pepper systems across 21 different soil types and found that soil THg, available Hg, pH, and free Fe/Al oxides were closely related to pepper fruit Hg content.
Vegetables produced in Hg-contaminated soil pose a significant risk of Hg exposure [30]. Leafy vegetables, which have become a staple in the Chinese diet [31], are particularly efficient at transferring and accumulating Hg from soil [32]. Lettuce (Lactuca sative L.), a major leafy vegetable, is extensively cultivated in China, which produces more than half of the world’s supply [33]. Lettuce grown in Hg-contaminated soil can accumulate Hg to levels significantly above recommended limits [34]. Given lettuce’s importance and its potential for Hg accumulation [31], it is essential to understand the Hg transfer in soil–lettuce systems.
To the best of our knowledge, no systematic studies have been conducted on Hg transfer in soil–lettuce systems. Therefore, this investigation aims to (1) characterize Hg transfer and identify its main controlling soil variables in a soil–lettuce system and (2) develop a prediction model for Hg transfer in a soil–lettuce system using a range of agricultural soils collected across China. By understanding Hg transfer and its key controlling soil variables in a soil–lettuce system, this research will provide a theoretical basis for developing strategies to ensure safe lettuce production in Hg-contaminated soils.

2. Materials and Methods

2.1. Sample Collection

Composite soil samples were collected from twenty-one different locations across China, representing a wide range of soil physicochemical characteristics in agricultural lands (Table S1). The selected locations were predominantly used for vegetable cultivation. At each uncontaminated sampling site, five sub-samples were taken from the surface soil layer (0–20 cm) using a stainless-steel auger [12]. These sub-samples were then carefully combined into one composite soil sample. Before analyzing the soil’s physicochemical characteristics and applying Hg treatments, the samples were air-dried, crushed, mixed, and sieved through a 2 mm mesh to remove crop residue and bulky pebbles [12].

2.2. Physicochemical Characteristics of Soils

The analyzed soil’s physicochemical characteristics are displayed in (Table S2). Soil EC (1:5) and pH (1:2.5) were determined in soil-distilled water suspensions with a digital pH meter (PHS-3C, Leici, Shanghai, China) [9]. Soil texture was determined by the procedure of Bowman and Hutka [35]. Soil OM was estimated via wet digestion, following the procedure described by Nelson and Sommers [36]. Cation exchange capacity (CEC) was assessed using a 1 M ammonium-acetate leaching procedure (pH 7.0), as outlined by Hussain and Yang [12]. Free Al and Fe oxides were measured using the Na2S2O4-Na3C6H5O7-NaHCO3 method [37]. All analyses were carried out in triplicate.

2.3. Soil Hg Treatments and Experimental Setup

The present study was conducted in a greenhouse at the Institute of Environment and Sustainable Development in Agriculture (IEDA), Chinese Academy of Agricultural Sciences (CAAS), Beijing, China. The experimental soils were contaminated with known quantities of Hg (as Hg(NO3)2 water solution) according to the Soil Environmental Quality Standards of China (GB 15618-1995) [38] for Hg limits in soil. The Hg limits for vegetable cultivation are 0.3, 0.5, and 1.0 mg Hg kg−1 soil for soils with pH < 6.5, 6.5–7.5, and >7.5, respectively. Based on these standards, the following three Hg treatments were designed: control (CK, no Hg added), low-Hg (Hg-I, one times the Hg limit in soil), and high-Hg (Hg-II, two times the Hg limit in soil) (Table S3). The required amounts of Hg(NO3)2 water solution were calculated according to soil pH and continuously sprayed on 15 kg of air-dried soil to achieve the desired Hg contamination levels. The Hg-contaminated soils were thoroughly mixed in pots and left to age for three months in the natural environment to stabilize the added Hg [10,12]. During the aging process, the soils were regularly irrigated every week to maintain 80% water-holding capacity. After the aging period, the soils were air-dried, minced, and sieved through a 2 mm mesh to fill the experimental pots (5 kg soil/pot). The experimental pots were then arranged in a randomized complete block design (RCBD), with three replicates for each control and Hg treatment. Leaf lettuce seeds were directly sown in the experimental pots and thinned to three plants per pot after emergence. The recommended doses of NPK fertilizers (0.15–0.05–0.1 g NPK kg−1 soil) were thoroughly mixed into the pots before sowing [12]. Analytical reagent (AR) grade urea, Ca(H2PO4)2, and K2SO4 were used as sources of N, P, and K, respectively. All cultural and agronomic practices, including irrigation, weeding, and insect pest control, were uniformly carried out throughout the experimental period.

2.4. Soil and Plant Sampling

After the aging process, soil samples were collected, freeze-dried, ground, and passed through a 149 μm (100 mesh) sieve for a total Hg analysis [12]. Once the pot experiment was completed and the lettuce reached maturity, the leaves were harvested and thoroughly washed with Milli-Q water. The leaves were then dried to a stable weight in an oven at 30 °C to minimize Hg volatilization loss [31]. The dried leaf samples were finely ground into powder for total Hg analysis.

2.5. Hg Analysis

Soil total Hg (THg) content was analyzed by digesting approximately 1.0 g of soil sample with 2.0 mL of concentrated HNO3 and 1.0 mL of concentrated HCl in a 50 mL glass volumetric tube. The mixture was heated on a digestion apparatus (Hanon-SH230N, Beijing, China) at 100 ± 2 °C for 2 h. After cooling to room temperature, the volume was brought up to 25 mL with deionized water [12]. A soil-available Hg (AvHg) concentration was estimated by extracting the soil with 1.0 mol L−1 NH4Ac (pH 7.0) using a 2 g/150 mL soil-to-extractant ratio. The suspension was shaken at room temperature (24 ± 1 °C) for 30 min at a continuous rate of 60 rpm on an end-over-end shaker [39]. A sequential extraction scheme was employed to extract soil Hg fractions as follows: (1) water-soluble Hg (deionized water), (2) exchangeable Hg (1 M CaCl2), (3) organic-bound Hg (0.2 M NaOH and 4% (v/v) CH3COOH), and (4) residual Hg (HNO3:H2SO4:HClO4 in a 1:5:1 ratio) [40]. For lettuce THg analysis, about 0.5 g of plant sample was added to 50 mL volumetric tubes containing concentrated HClO4 and HNO3 (2:3 v/v). The tubes were left overnight and then heated on a digestion apparatus at 100 °C for 3 h. After digestion, the tubes were cooled and the volume was adjusted to 25 mL with deionized water [12]. All soil and plant Hg determinations were carried out in triplicate using cold vapor atomic fluorescence spectrophotometry (CVAFS, Beijing Titans Instruments Co., Ltd., Beijing, China), with a detection limit of 0.02 μg kg−1. To ensure the quality of the analytical technique, method blanks, certified reference materials, and replicates were used. Certified reference materials for the soil (GBW07450, 19.0 μg Hg kg−1) and plants (GBW(E)100350, 4.30 μg Hg kg−1) showed recovery ratios ranging from 93 to 107% and 95 to 104%, respectively. All chemicals used for soil and plant Hg analysis were obtained from the Sinopharm Chemical Reagents Corporation Ltd., Beijing, China, and those used for determining soil physicochemical characteristics were of analytical reagent grade.

2.6. Data Analysis

All statistical data analyses were performed using SPSS (version 27), and the graphical data presentations were created with Origin Pro (2022). The effects of Hg treatments and soil types on lettuce total Hg contents and bioconcentration factors (BCF) were analyzed using a one-way analysis of variance (ANOVA). Mean comparisons were conducted using Fisher’s least significant difference (LSD) test at a significance level of 0.05 [41].

2.7. BCF (Bioconcentration Factor)

The bioconcentration factor (BCF), which is the ratio of metal concentration in plants to the metal concentration in soil, estimates the soil–plant metal transfer capability [14,42]. The BCF of lettuce Hg was calculated to describe the soil–lettuce Hg transfer characteristics in the investigated soils as follows.
BCF = Lettuce Hg/Soil Hg
where lettuce Hg and soil Hg are the total Hg concentrations in lettuce and soil, respectively.

2.8. Path Analysis

Path analysis (PA), as previously applied [12,43], was used to assess the association between lettuce Hg content and various soil variables. In the PA model (Figure 1), lettuce THg content (10) was used as the dependent variable, while soil properties, including (1) soil THg, (2) available Hg, (3) exchangeable Hg, (4) soil pH, (5) organic matter, (6) clay content, (7) cation exchange capacity, (8) amorphous Al, and (9) amorphous Fe, were used as independent variables. This setup was employed to establish the direct and indirect effects of soil properties on lettuce THg content using SPSS-AMOS (IBM Corporation, Armonk, New York, USA). In Figure 1, the unidirectional arrow indicates the direct effect of a soil variable on lettuce Hg content, while the bidirectional arrow indicates a simple correlation coefficient between soil variables. The multiplication of a unidirectional and a bidirectional arrow produces the indirect effect of a soil variable on lettuce Hg content. A simple multiple linear regression between lettuce Hg and soil variables and a simple correlation analysis between soil variables were employed to determine the direct and indirect effects of soil variables on lettuce Hg content. Furthermore, the unexplained part of the path model, represented by the uncorrelated residue (U), was estimated as follows:
U = √1 − R2
where (R2) represents the determination coefficient of multiple linear regression between soil variables and lettuce Hg content.

2.9. Prediction Model

The Freundlich-type functions are commonly employed to derive soil–plant metal transfer empirical models [10,12] as follows:
Log (Cplant) = a + b log (Csoil)
Cplant and Csoil are the metal concentrations in plants and soil, respectively, while a and b are constants. Extended Freundlich-type functions can be obtained using simple multiple linear regression between independent and dependent variables. In this study, stepwise multiple linear regression (SMLR) was applied to develop extended Freundlich-type equations capable of predicting soil–lettuce Hg transfer using soil variables. Soil variables that did not achieve statistical significance (p > 0.05) were omitted from the regression model. In developing the Freundlich model for lettuce leaf Hg, we collected comprehensive data on various soil properties from each studied area, including pH, total Hg (THg), amorphous (Amo) Al and Fe oxides, available Hg (AvHg), exchangeable Hg (ExHg), clay content, and organic matter (OM). Using correlation analysis and expert judgment, identified the most relevant soil properties that have significant impacts on Hg phytoavailability and uptake in lettuce. Then, the SMLR technique was employed to develop prediction models for lettuce leaf Hg content. In this process, variables were included step-by-step based on their statistical significance (p-value) and their contribution to the model’s explanatory power (R2). Tested various combinations of the selected soil properties to identify the optimal model. The final model was evaluated based on its goodness-of-fit (R2) and predictive accuracy (p ≤ 0.001). Then, the final prediction model was validated by comparing the predicted Hg contents with the observed values, ensuring that the model accurately reflects the influence of the selected soil properties.

3. Results

3.1. Soil Total Hg (THg) Concentration (µg kg−1)

The soil total Hg (THg) concentrations in the control (CK) and Hg-treated soils are shown in (Table 1 and Figure 2). In the CK soils, THg concentrations ranged from 15.98 to 156.02 µg kg−1, with a mean concentration of 54.10 ± 27.17 µg kg−1. In the Hg-I treated soils, THg concentrations ranged from 281.04 to 1272.93 µg kg−1, with a mean concentration of 606.58 ± 248.38 µg kg−1. For the Hg-II treated soils, THg concentrations ranged from 503.77 to 1991.90 µg kg−1, with a mean concentration of 1024.48 ± 314.18 µg kg−1 (Table 1). Comparing the mean THg concentrations for the different Hg treatments revealed significantly higher (p ≤ 0.05) soil THg levels in the high-Hg treatment (1024.48 µg kg−1) compared to the low-Hg treatment (606.58 µg kg−1) and the control (54.10 µg kg−1) (Table 1). These results showed a clear dose response to the Hg additions, highlighting the importance of monitoring and managing soil Hg levels to mitigate potential environmental and health risks associated with Hg contamination.

3.2. Lettuce Total Hg (THg) Concentrations (µg kg−1)

Lettuce THg concentrations, as influenced by soil types and Hg treatments, are shown in (Table 2, and Figure 3a). In certain soils (soils 15, 19, 20, and 21), the lettuce grown in the high-Hg (Hg-II) treated soils exhibited peak THg concentrations that exceeded the safe Hg limit of 10 µg kg−1 for vegetables, as advised by the National Food Safety Standards. However, in all other cases, including all control, low-Hg (Hg-I), and high-Hg (except soils 15, 19, 20, and 21) treatments, the lettuce THg concentrations were below the safe limit of 10 µg kg−1 for vegetable consumption. Furthermore, comparing the lettuce THg mean concentrations for different Hg treatments (Table 2) revealed that significantly higher (p ≤ 0.05) THg levels (10.32 µg kg−1) were recorded in the high-Hg treatment compared to the low-Hg treatment (7.17 µg kg−1) and the control (1.06 µg kg−1). This suggests that lettuce THg uptake was significantly influenced by Hg additions and higher soil Hg content. Similarly, comparing the lettuce THg mean concentrations across different soil types (Figure 3b) indicated significantly higher (p ≤ 0.05) Hg uptake in soils with acidic pH (9.48 µg kg−1) compared to alkaline soils (6.85 µg kg−1). This suggests that lettuce THg uptake is also affected by soil type, particularly soil pH. These results illustrate the impact of Hg additions, soil types, and lettuce cultivation in Hg-contaminated soils. The elevated lettuce THg concentrations from high-Hg-treated soils revealed the significance of Hg additions in increasing soil Hg levels, which subsequently influences plant uptake. Higher Hg uptake in acidic soils can be attributed to the increased bioavailability of Hg under acidic conditions, whereas lower pH levels facilitate the release of Hg from soil particles, making it more available for plant absorption. Lettuce, being a leafy vegetable, has the capacity for Hg uptake, which further emphasizes the importance of monitoring and managing soil Hg levels to ensure safe vegetable production. This highlights the need for careful assessment of both soil contamination levels and soil pH when evaluating the risks associated with Hg accumulation in crops.

3.3. Uptake and Accumulation of Hg in a Soil–Lettuce System

The bioconcentration factor (BCF) of lettuce THg is shown in (Figure 4a). The BCF values for lettuce THg in all control and Hg-treated soils were less than one, indicating that the uptake and accumulation of Hg in lettuce were much lower than the total Hg concentration in the soil. When comparing the BCF values for different Hg treatments across each soil type, it was observed that for most soils (soils 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13, and 18), BCF values were significantly higher (p ≤ 0.05) in the control compared to Hg-treated soils. This is likely due to the low background soil total Hg concentrations in the control treatment. However, in some soils (soils 15, 16, 19, and 21) with an acidic pH (<6.5), BCF values were significantly higher (p ≤ 0.05) in the Hg-treated soils compared to the control. When comparing the BCF values between low and high Hg treatments, insignificant differences were observed in soils 1, 2, 3, 4, 9, 10, 12, 13, 14, 15, 18, and 19. However, significantly higher BCF values were found for the low-Hg treatment in soils 5, 6, 7, 8, 11, 16, 17, 20, and 21 compared to the high Hg treatment. Furthermore, a comparison of mean BCF values across different Hg treatments showed significantly higher (p ≤ 0.05) BCF values (0.020) for the control compared to the low-Hg (0.012) and high-Hg (0.01) treatments (Table 3). Similarly, when comparing mean BCF values for different soil types (Figure 4b), significantly higher (p ≤ 0.05) BCF values were recorded for acidic soils (0.017) compared to alkaline soils (0.007), indicating that lettuce Hg uptake and accumulation were influenced by soil type. The results demonstrate the role of Hg additions, soil types, and lettuce cultivation on Hg transfer. The lower BCF values in Hg-treated soils, especially in the high-Hg treatment, suggest that while the total soil Hg concentration increases, the proportion of Hg available for plant uptake does not increase linearly, likely due to the binding of Hg to soil particles, making it less bioavailable. The higher BCF values in acidic soils can be attributed to the increased solubility and bioavailability of Hg under low pH conditions, facilitating greater uptake by lettuce. This highlights the importance of considering soil pH and Hg concentrations when assessing the risks of Hg accumulation in crops and the complex interactions between soil chemistry and plant physiology in determining metal uptake and transference.

3.4. Factors Affecting Hg Uptake and Accumulation in a Soil–Lettuce System

Path analysis (PA) was employed to distinguish the direct and indirect effects of various soil properties on total Hg uptake and accumulation in a soil–lettuce system. The uncorrelated residue (U) was 0.42, and the coefficient of determination (R2) was 0.82, indicating that PA elucidated 82% of the variation in lettuce total Hg uptake and accumulation (Table 4). PA split each correlation coefficient (r) between lettuce’s total Hg content and soil properties into one direct and eight indirect effects. The results showed that soil total Hg content had a significant positive direct effect (P110 = 0.79, p < 0.01) on lettuce Hg uptake, while soil pH (P410 = −0.86, p < 0.01), amorphous Fe (P810 = −0.27, p < 0.05), and amorphous Al (P910 = −0.47, p < 0.05) had significant negative direct effects. Soil organic matter (OM) also had a positive direct effect (P510 = 0.17), although it was not statistically significant (p > 0.05). Additionally, soil total Hg content had positive indirect effects through available Hg (r12P110 = 0.16), exchangeable Hg (r13P110 = 0.20), and soil pH (r14P110 = 0.28). Conversely, it had negative indirect effects through clay content (r16P110 = −0.21), amorphous Fe (r18P110 = −0.16), and amorphous Al (r19P110 = −0.17). Soil pH also exerted positive indirect effects through soil-available Hg (r24P410 = 0.31), organic matter (r45P410 = 0.18), clay (r46P410 = 0.19), amorphous Fe (r48P410 = 0.15), and amorphous Al (r49P410 = 0.16) but had a negative indirect effect through soil total Hg (r14P110 = −0.30). However, the simple correlation coefficients between lettuce total Hg content and amorphous Fe or amorphous Al were not significant, indicating that simple correlation alone was insufficient for understanding the association between plant Hg uptake and soil variables. These results highlight the complex interplay between different soil properties and their cumulative impact on Hg uptake by lettuce. The significant positive direct effect of soil total Hg content emphasizes its primary role in determining Hg availability for plant uptake. Meanwhile, the negative direct effects of soil pH, amorphous Fe, and amorphous Al suggest that these factors can reduce Hg bioavailability, possibly by influencing Hg speciation or binding in the soil. The indirect effects further illustrate how interdependent soil properties collectively control Hg dynamics, emphasizing the necessity of comprehensive soil management to mitigate Hg accumulation in crops.

3.5. Hg Transfer in a Soil–Lettuce System (Prediction Model)

Extended Freundlich-type functions are commonly used to derive reliable soil–plant metal transfer models through simple multiple linear regression (SMLR) using soil variables. In this study, the physicochemical characteristics of 21 different soil types and their respective lettuce total Hg contents were analyzed with SMLR to develop extended Freundlich-type equations for Hg transfer in a soil–lettuce system. The prediction models for different Hg inputs (control and pooled data of the two Hg treatments) are presented in Table 5. The SMLR results for control soils identified soil total Hg content as the sole significant parameter influencing lettuce total Hg content (R2 = 0.32, p < 0.001). However, when data from the two Hg treatments were pooled, due to non-significant differences in the bioconcentration factor (BCF), SMLR results corroborated the path analysis findings, showing that soil total Hg content, pH, amorphous Al (Amo Al), and amorphous Fe (Amo Fe) were the most significant soil factors controlling lettuce total Hg content (R2 = 0.82, p < 0.001). Furthermore, when the lettuce total Hg data from all three Hg treatments were pooled, the model’s predictability remained high (R2 = 0.81, p < 0.001), showing a similar trend in which lettuce total Hg content was positively influenced by soil total Hg content and negatively affected by soil pH, Amo Al, and Amo Fe. For control soils, soil pH, Amo Al, and Amo Fe were omitted from the model due to their non-significant effects. Typically, the predictability of soil–plant metal transfer models improves with the inclusion of soil factors compared to models based solely on soil metal content. In this study, the predictability of Hg transfer in the soil–lettuce system significantly improved (R2 = 0.32 vs. R2 = 0.82, p < 0.001) with the inclusion of soil pH, Amo Al, and Amo Fe in the prediction models. Other soil properties, such as EC, OM, and clay content, did not significantly enhance the model’s predictability and were, therefore, omitted. These prediction models highlight the complex interactions between soil properties and Hg availability for plant uptake. Soil total Hg content is a primary determinant of Hg availability. However, soil pH plays a crucial role by influencing Hg speciation and mobility, with lower pH (more acidic conditions) generally increasing Hg solubility and bioavailability. Amorphous Fe and Al oxides can adsorb Hg, reducing its availability for plant uptake. Thus, including these variables in the prediction models significantly enhances their accuracy, reflecting the multifaceted nature of Hg dynamics in soil–plant systems.

4. Discussion

Mercury (Hg) is a highly toxic contaminant that poses serious ecological and human health risks [1]. Recent research has shown that soil Hg can readily accumulate in food crops and vegetables [12,31]. Due to its potential health risks, the soil–plant Hg transfer in agricultural soils has garnered significant research interest [10,12,14,44]. The phytoavailability of Hg depends on variations in soil properties and crop species, making the assessment of ecological risks complex. Therefore, deriving prediction models that account for soil variables and different crop species is essential for an accurate ecological risk assessment of Hg in diverse soil types [45]. Several soil–vegetable metal transfer models have been developed to address this issue [12,14,46,47]. However, the characteristics of soil–lettuce Hg transfer and its prediction models have not been thoroughly explored. Lettuce, a commonly consumed leafy vegetable, has shown potential for Hg accumulation, which raises concerns about food safety [34]. Given the importance of lettuce in the diet and its susceptibility to Hg uptake, it is essential to understand the factors influencing Hg transference in the soil–lettuce system. This study aimed to characterize the transfer of Hg in the soil–lettuce system and develop prediction models using 21 different agricultural soil types collected across China. By analyzing a wide range of soil physicochemical properties, we aimed to identify key factors influencing Hg uptake in lettuce and improve the predictability of soil–plant Hg transfer models.

4.1. Lettuce Hg Concentration (ug kg−1) in the Investigated Soils

The uptake and accumulation of mercury (Hg) in soil–plant systems are influenced by several factors, including soil type, total Hg content, and soil physicochemical characteristics [12,31,44]. Our study found that peak Hg concentrations in lettuce were observed in soils with high total Hg levels, particularly under the high Hg treatment, in which Hg levels exceeded the safe limit of 10 µg kg−1 for vegetable consumption in some cases. These findings align with earlier studies that linked elevated soil Hg levels to increased Hg content in various crops, such as carrots, leafy vegetables, and peppers [12,31,44]. The higher total Hg content in these soils likely contributed to an increased pool of bioavailable Hg, as Hg availability is positively correlated with its total concentration in the soil [39]. This suggests that cultivating lettuce in soils with Hg levels exceeding the current Soil Environmental Quality Standard by twofold could pose significant health risks. Additionally, our results demonstrated that lettuce grown in acidic soils (pH < 6.5) had significantly higher Hg concentrations compared to those grown in alkaline soils (pH > 7.5). This observation is consistent with previous research indicating that plant Hg uptake and accumulation are higher in acidic soil conditions [10,14,31,44]. The behavior of metal contaminants in soils, including processes such as precipitation-dissolution, acid–base reactions, complexation, and sorption, is primarily controlled by soil pH [48,49]. Soil pH influences metal bioavailability by regulating the mobility of cations and the solubility of metals [11]. Specifically, the speciation of Hg in soil solution is directly affected by soil pH, with lower pH levels resulting in more bioavailable free ionic Hg species [11,50]. Lower soil pH enhances the bioavailability of metal cations because H+ ions displace adsorbed cations from exchange sites, and soil minerals containing metal cations dissolve more readily [13]. In alkaline soils, metals tend to form insoluble phosphates and carbonates, which reduce their bioavailability, while more soluble and bioavailable organo-metallic species are prevalent under acidic conditions [51,52]. This stabilization of Hg in alkaline soils leads to lower Hg phytoavailability [9]. Therefore, soil pH is a critical factor that regulates the bioavailability and transfer of metals, particularly Hg, in soil–plant systems [10,12,44]. Given these dynamics, the careful management of lettuce cultivation in acidic soils (pH < 6.5) with high Hg content is essential to mitigate the risks of Hg contamination. Thus, understanding the influence of soil pH and Hg concentration on the bioavailability of Hg in agricultural soils is essential for developing strategies to minimize the health risks associated with Hg contamination.

4.2. Uptake and Accumulation of Hg in Soil–Lettuce System

Soil–plant metal transfer characteristics are often described by the bioconcentration factors (BCF), which express the ratio of metal concentration in the plant to the metal concentration in the soil [31,44]. In our study, the BCF of Hg in lettuce was much lower than 1 (<1), indicating that soil total Hg content may not exclusively regulate Hg transference in soil–vegetable systems [31,53]. This is because the bioavailable Hg fractions in soils are influenced by multiple interrelated soil factors, resulting in a comparatively low degree of soil–plant Hg transferal [54]. Our findings align with previous research, which also reported low BCF values for Hg in plants [10,12,55]. The BCF of the control treatment was significantly higher (p ≤ 0.05) than in the two Hg-treated soils due to the low background soil Hg contents. This suggests that the higher BCF values in the control soils might be due to the uptake of Hg by plants, along with other cationic nutrients from the soil solution. In the treated soils, the total Hg content was much higher than in the control soils, leading to lower BCF values in the Hg-treated soils (Table 3). Similar results have been reported previously with exogenous Hg additions [10,31]. Moreover, higher lettuce Hg BCF values were recorded in acidic soils (pH < 6.5) compared to alkaline soils (pH > 7.5), indicating the increased phytoavailability and transference of Hg in an acidic soil–lettuce system. Previous research has identified several soil variables, including pH, cation exchange capacity (CEC), organic matter (OM) content, clay, and amorphous Al and Fe oxides, as significant factors influencing Hg bioavailability in soil–vegetable systems [10,12,31]. Among these factors, pH is the most prominent, governing metal solid phases, precipitation-dissolution, and acid–base reactions [12,48,56]. The higher BCF values observed in acidic soil conditions can be explained by the fact that low soil pH favors the presence of free metal ions in solution and prevents metal precipitation and complexation with soil minerals [14,57]. This phenomenon has been well-documented in previous studies, which found that acidic soils (pH < 6.5) exhibited significantly higher BCF values than alkaline soils (pH > 7.5) [10,12]. Thus, lettuce cultivation in Hg-contaminated acidic soils (pH < 6.5) should be approached with caution to avoid health risks associated with Hg-contaminated lettuce. To manage Hg-contaminated soils, it is necessary to understand the mechanisms underlying the interaction of soil properties and Hg bioavailability. Additionally, exploring the potential of soil amendments and other remediation strategies to reduce Hg bioavailability in contaminated soils would be valuable.

4.3. Factors Affecting Hg Uptake and Accumulation in a Soil–Lettuce System

Path analysis (PA) is a statistical technique used to distinguish between the direct and indirect influences of independent variables on the dependent variable. In the present study, PA explained 82% of the variation in lettuce Hg content, highlighting soil Hg, pH, amorphous aluminum (Amo Al), and amorphous iron (Amo Fe) as the main direct controlling factors of Hg uptake and accumulation in the soil–lettuce system (Table 4). Previous studies investigating the transfer of Hg in soil–plant systems have established significant relationships between plant Hg content and soil variables such as total Hg, pH, Amo Al, and Amo Fe content [8,10,44]. Soil total Hg had a positive direct effect on lettuce Hg concentration, while pH, Amo Al, and Amo Fe had negative direct effects. Free Al and Fe oxides in the soil have high affinities for sorbing, adsorbing, or co-precipitating Hg from the soil solution due to their large surface areas, leading to decreased Hg phytoavailability [49,58]. This was supported by our findings, which showed that Amo Al and Fe oxides decrease Hg uptake by lettuce. These results align with previous studies that have demonstrated the significant role of these oxides in controlling Hg bioavailability [8,59]. For example, Gabriel and Williamson [19]) demonstrated that oxides and oxyhydroxides of Al, Fe, and Mn have high adsorption affinities for Hg2+, second only to humic substances. Similarly, Fe oxides have been identified as critical geochemical factors controlling heavy metal uptake and accumulation in soil–vegetable systems [55]. One of the advantages of PA is its ability to identify specific variables that cause significant changes through indirect effects. In our study, PA identified soil-available Hg, exchangeable Hg, clay, and organic matter (OM) content as indirect factors affecting lettuce Hg concentration through soil Hg, pH, Amo Al, or Amo Fe (Table 4). Research suggests that evaluating soil-available Hg through chemical extraction provides a more reliable estimate of Hg phytoavailability compared to solely measuring total Hg content [10,12,60]. This is because soil-available and exchangeable Hg fractions are more directly related to the bioavailable pools of Hg in the soil [12]. The fractionation of Hg in soil revealed that acidic soils contain a higher proportion of exchangeable Hg compared to alkaline soils [14]. The recent literature suggests that the most bioavailable pools of Hg in soil are the soil-available and exchangeable Hg contents [14,61]. Organic matter (OM) in soils typically forms strong complexes with Hg, affecting its bioavailability [1,14,15]. There are conflicting findings on soil OM-Hg interactions due to the complex transformations of OM in soils. Some studies, like that of Wang and Qing [16], observed decreased Hg phytoavailability with increased soil humus, indicating stable Hg-OM complexes. However, other studies have found more bioavailable Hg forms in soils with higher OM content [17]. The mineralization of OM, which complexes with Hg in soils, along with a significant decline in soil pH, can affect the availability and mobility of Hg [1,62]. Soil clay content, due to its large surface area and ability to form soil sorption complexes, also influences Hg binding to soil solid phases [1,63,64]. However, the availability of Hg in soils with lower organic matter content is not necessarily limited by this fact [16,64]. Even in soils with a minimum clay content of 15%, clay minerals can significantly bind Hg, demonstrating their importance in controlling Hg availability [16]. The results of this study emphasize the complex interplay of soil properties in determining Hg phytoavailability and uptake by lettuce. These findings highlight the need for the careful management of Hg-contaminated soils, particularly those with acidic pH, to mitigate health risks associated with Hg-contaminated crops.

4.4. Hg Transfer in a Soil–Lettuce System (Prediction Model)

To develop technically sound policies for safe crop production on soils contaminated with heavy metals, researchers have developed empirical soil–plant metal transfer models based on soil and plant variables [12,28,44,47]. The primary advantages of these Freundlich-type soil–plant metal transfer models are their simplicity and applicability to real field conditions. However, the validity of these models is limited by the specific plant species and the range of soil total metal contents used in their development [29]. Several soil–plant Hg transfer models have been designed to estimate human Hg burdens from consuming various crops and vegetables [10,12,44]. Despite these advancements, the soil–lettuce Hg transfer characteristics and prediction models based on soil variables remain underexplored. In the current study, we used the physicochemical characteristics of the investigated soils and their respective lettuce Hg contents to derive a prediction model of Hg transfer in a soil–lettuce system. The results of stepwise multiple linear regression (SMLR) confirmed the findings of path analysis, highlighting soil total Hg, pH, amorphous aluminum (Amo Al), and amorphous iron (Amo Fe) as the most significant soil factors controlling total Hg content in lettuce (R2 = 0.82, p < 0.001). Similar prediction studies have been conducted by other researchers, with variations based on the choice of soil variables and plant species used. For instance, Ding and Zhang [10] studied the transference of Hg from soil to carrots and identified soil Hg, pH, and free Al oxides as the most significant variables controlling carrot Hg content. More recently, Hussain and Yang [12] investigated soil–pepper Hg transfer in 21 different soil types and found soil total Hg, available Hg, pH, and free Al/Fe oxides to be closely related to pepper fruit Hg content. In our study, both path analysis and SMLR were used to assess the relationships between lettuce Hg content and soil variables. The results of multiple regression confirmed the findings of path analysis, identifying soil Hg, pH, Amo Al, and Fe as the main soil variables controlling lettuce Hg content in the investigated soils.
Information on soil variables such as pH, organic matter (OM), clay content, free Al/Fe oxides, and soil total metal contents can be accessed from national soil investigation surveys. Therefore, the soil–lettuce Hg transfer model developed in this study can be employed to estimate area-specific human Hg burdens from lettuce consumption. However, further evaluation is necessary to determine the validity of the current experimental findings in real field environments and to assess the required adjustments for diverse cultivars.

5. Conclusions

The current study demonstrates that Hg phytoavailability and accumulation in soil–lettuce systems vary significantly with soil types and Hg contamination levels. Peak lettuce THg contents, which exceeded the safe limits for vegetable consumption, were observed in Hg-treated acidic soils (pH < 6.5). Although the generally low BCF values indicated minimal soil-to-lettuce Hg transfer during the growth period, relatively higher BCF values were found in acidic soils (pH < 6.5) compared to alkaline soils (pH > 7.5). A path analysis (PA) revealed that soil THg, pH, and amorphous (Amo) Al and Fe oxides had direct effects on lettuce’s THg contents. The stepwise multiple linear regression (SMLR) corroborated these findings, producing a prediction model in which soil THg, pH, and Amo Al and Fe oxides were the primary variables, explaining 82% of the variation in lettuce THg content. These findings highlight the importance of soil characteristics in influencing Hg transfer to crops and the need for careful management of Hg-contaminated soils to ensure the safe production of vegetables like lettuce. Moreover, the information generated from this investigation provides a theoretical basis for mitigating the environmental and health risks associated with Hg contamination in agricultural soils.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14071394/s1, Table S1: Soil location and parent material information; Table S2: Selective physicochemical properties of the experimental soils; Table S3. Soil Hg treatments and Hg limits in soil (Grade II) of the Soil Environmental Quality Standards in China (GB 15618-1995).

Author Contributions

J.Y. designed this study; S.U. performed the experiments and drafted the manuscript; S.H. helped in conducting the experiment and editing while J.Y., Y.N., X.X., A.I.D., T.K. and Y.F. gave technical support. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Top-Notch Young Talents Program of China (2022–2025) and the Agricultural Science and Technology Innovation Program of the Chinese Academy of Agricultural Sciences (2021–2025).

Data Availability Statement

The original data presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 14 01394 g001
Figure 1. Path analysis model of lettuce Hg content and soil properties. The direct effect (Pij) of a soil property on lettuce Hg is indicated by a unidirectional arrow, while a simple correlation coefficient (rij) among soil properties is indicated by a bidirectional arrow. The subscript indications are 1. Soil total-Hg, 2. Available-Hg, 3. Exchangeable-Hg, 4. Soil pH, 5. Organic matter (OM), 6. Clay, 7. Cation exchange capacity (CEC), 8. Amorphous Fe oxides (Amo Fe), 9. Amorphous Al oxides (Amo Al), and 10. Lettuce total Hg content (THg).
Figure 1. Path analysis model of lettuce Hg content and soil properties. The direct effect (Pij) of a soil property on lettuce Hg is indicated by a unidirectional arrow, while a simple correlation coefficient (rij) among soil properties is indicated by a bidirectional arrow. The subscript indications are 1. Soil total-Hg, 2. Available-Hg, 3. Exchangeable-Hg, 4. Soil pH, 5. Organic matter (OM), 6. Clay, 7. Cation exchange capacity (CEC), 8. Amorphous Fe oxides (Amo Fe), 9. Amorphous Al oxides (Amo Al), and 10. Lettuce total Hg content (THg).
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Figure 2. Soil total Hg concentration (µg kg−1) as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the column.
Figure 2. Soil total Hg concentration (µg kg−1) as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the column.
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Figure 3. (a). Lettuce total Hg concentration (µg kg−1) as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the column. (b). Lettuce total Hg concentration (µg kg−1) as affected by soil types. Significant differences (p ≤ 0.05) between soil types are shown by small letters above the bars.
Figure 3. (a). Lettuce total Hg concentration (µg kg−1) as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the column. (b). Lettuce total Hg concentration (µg kg−1) as affected by soil types. Significant differences (p ≤ 0.05) between soil types are shown by small letters above the bars.
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Figure 4. (a). Lettuce total Hg BCF as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the bars. (b). Lettuce total Hg (THg) BCF as affected by soil types. Significant differences between soil types are shown by small letters above the bars.
Figure 4. (a). Lettuce total Hg BCF as affected by soil types (n = 21) and Hg treatments; CK (control), low-Hg (Hg-I), and high-Hg (Hg-II). Means ± SE (n = 3) are shown. Significant differences (p ≤ 0.05) between Hg treatments within soil types are shown by small letters above the bars. (b). Lettuce total Hg (THg) BCF as affected by soil types. Significant differences between soil types are shown by small letters above the bars.
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Table 1. Soil total Hg concentration (µg kg−1) as affected by soil Hg treatments.
Table 1. Soil total Hg concentration (µg kg−1) as affected by soil Hg treatments.
Hg TreatmentSoil THg Concentration (µg kg−1)MeanSD
MinMax
CK (Control)15.98156.0254.10 c27.17
Hg-I (Low-Hg)281.041272.93606.58 b248.38
Hg-II (High-Hg)503.771991.901024.48 a314.18
Note: Significant differences (p ≤ 0.05) among treatment means are shown by small letters.
Table 2. Lettuce total Hg concentration (µg kg−1) as affected by soil Hg treatments.
Table 2. Lettuce total Hg concentration (µg kg−1) as affected by soil Hg treatments.
Hg TreatmentLettuce THg Concentration (µg kg−1)MeanSD
MinMax
CK (Control)0.861.461.06 c0.15
Hg-I (Low-Hg)5.319.927.17 b1.26
Hg-II (High-Hg)5.1814.0110.32 a2.01
Note: Significant differences (p ≤ 0.05) among treatment means are shown by small letters.
Table 3. Lettuce total Hg BCF as affected by soil Hg treatments.
Table 3. Lettuce total Hg BCF as affected by soil Hg treatments.
Hg TreatmentLettuce Leaf Hg BCFMeanSD
MinMax
CK (Control)0.0060.0380.020 a0.008
Hg-I (Low-Hg)0.0050.0250.012 b0.006
Hg-II (High-Hg)0.0050.0180.010 b0.004
Note: Significant differences (p ≤ 0.05) between treatment means are shown by small letters.
Table 4. Direct (diagonal values) and indirect (off-diagonal values) effects of soil properties on lettuce Hg concentration.
Table 4. Direct (diagonal values) and indirect (off-diagonal values) effects of soil properties on lettuce Hg concentration.
PropertySoil THgAv HgEx HgpHOMClayCECAmo FeAmo AlrR2U
Soil THg0.79 **−0.03−0.02−0.30−0.04−0.02−0.040.050.070.46 *0.82 **0.42
AvHg0.16−0.15−0.010.310.070.030.07−0.12−0.130.23
ExHg0.20−0.02−0.09−0.010.010.000.010.060.050.20
pH0.280.050.00−0.86 **−0.04−0.01−0.040.050.08−0.49 *
OM−0.11−0.07−0.010.180.140.03−0.01−0.02−0.19−0.12
Clay−0.21−0.090.000.190.090.060.08−0.10−0.14−0.15
CEC−0.13−0.040.020.14−0.080.040.15−0.10−0.15−0.19
Amo Fe−0.16−0.070.020.150.010.020.15−0.27 *0.140.13
Amo Al−0.17−0.040.030.160.090.060.120.12−0.47 *−0.14
Note: Soil THg: soil total Hg, AvHg: available Hg, ExHg: exchangeable Hg, OM: organic matter, CEC: cation exchange capacity, Amo Fe: amorphous Fe, Amo Al: amorphous Al, r: correlation coefficient, R2: determination coefficient, U: uncorrelated residue, ** p ≤ 0.01 and * p ≤ 0.05 (significance level).
Table 5. Stepwise multiple linear regression models of Hg transfer in a soil–lettuce system.
Table 5. Stepwise multiple linear regression models of Hg transfer in a soil–lettuce system.
Hg InputsPrediction ModelR2pSE
CK soilLog (lettuce THg) = 0.16 log (soil THg) − 0.360.320.0010.116
Hg-I + Hg-IILog (lettuce THg) = −1.55 log (soil pH) + 0.48 log (soil THg) − 0.1 log (Amo Al) − 0.07 log (Amo Fe) + 1.910.820.0000.085
CK + Hg-I + Hg-IILog (lettuce THg) = −1.44 log (soil pH) + 0.45 log (soil THg) − 0.1 log (Amo Al) − 0.07 log (Amo Fe) + 1.660.810.0000.082
Note: SE stands for standard error, R2 for determination coefficient, and p for probability or significance level.
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MDPI and ACS Style

Ullah, S.; Hussain, S.; Noor, Y.; Khanam, T.; Xia, X.; Darma, A.I.; Feng, Y.; Yang, J. Influencing Factors and Prediction Models of Mercury Phytoavailability and Transference in a Soil–Lettuce System under Chinese Agricultural Soils. Agronomy 2024, 14, 1394. https://doi.org/10.3390/agronomy14071394

AMA Style

Ullah S, Hussain S, Noor Y, Khanam T, Xia X, Darma AI, Feng Y, Yang J. Influencing Factors and Prediction Models of Mercury Phytoavailability and Transference in a Soil–Lettuce System under Chinese Agricultural Soils. Agronomy. 2024; 14(7):1394. https://doi.org/10.3390/agronomy14071394

Chicago/Turabian Style

Ullah, Subhan, Sajjad Hussain, Yousaf Noor, Tasawar Khanam, Xing Xia, Aminu Inuwa Darma, Ya Feng, and Jianjun Yang. 2024. "Influencing Factors and Prediction Models of Mercury Phytoavailability and Transference in a Soil–Lettuce System under Chinese Agricultural Soils" Agronomy 14, no. 7: 1394. https://doi.org/10.3390/agronomy14071394

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