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Article

Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils

by
Mengmeng Zhang
1,2,
Guoyuan Zou
2,
Zhiping Yang
1,
Yanmei Li
2,
Na Sun
2,
Shangqiang Liao
2,
Lianfeng Du
1,2,* and
Shunjiang Li
1,2,*
1
College of Resources and Environment, Shanxi Agricultural University, Jinzhong 030800, China
2
Institute of Plant Nutrition, Resources and Environment, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China
*
Authors to whom correspondence should be addressed.
Processes 2025, 13(4), 1010; https://doi.org/10.3390/pr13041010
Submission received: 21 February 2025 / Revised: 11 March 2025 / Accepted: 20 March 2025 / Published: 28 March 2025
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
Heavy metals are a common type of contaminant found in soil. However, knowledge gaps still exist in the characteristics of the heavy metals in facility farmland soils. Therefore, a differential analysis was conducted on the heavy metal (Cd, Cr, Pb, As, Cu, Zn, and Ni) content in soils that were amended with different types of fertilizers (T1, organic fertilizer; T2, organic fertilizer and chemical fertilizer). Regression analysis was employed to demonstrate the trends of metal content changes with cropping durations (5, 10, 15, and 20 years). The Pearson correlation analysis was applied to investigate the correlation between soil nutrients and soil metals, and to determine the key factors influencing heavy metal concentration through the random forest model. The results indicated that the average concentration of Cd (0.148 mg·kg−1) was higher than the soil background value but lower than the potential ecological risk threshold (0.3 mg·kg−1, 6.5 < pH ≤ 7.5). The Cd content in soil where only organic fertilizer was applied was 0.118 mg·kg−1, while the Cd content in soil where both organic fertilizer and chemical fertilizer were applied was 0.148 mg·kg−1. There was no significant difference in Cd content between the two fertilization types. The regression analysis indicated a linear increasing trend in Cd content as the cropping duration increased. The Pearson correlation analysis showed that there were also significant correlations between pH and both As and Ni; between SOM (soil organic matter) and Cr, Cu, and Zn; as well as between Cd and EC (electrical conductivity). The random forest model exhibited high prediction accuracy when explaining the most significant factors influencing the concentrations of Cd, Cr, Pb, As, Cu, Zn, and Ni, with overall correlation coefficients (r) of 0.009, 0.44, 0.33, 0.04, 0.32, 0.61, and 0.65, respectively. Using Hakanson’s method to conduct an ecological risk assessment on the soil in the study area, it was found that the overall potential ecological risk level was relatively low. However, the proportion of moderate to high ecological risk associated with Cd elements was close to 40%, which requires special attention.

Graphical Abstract

1. Introduction

The rapid development of urbanization has exacerbated ecological risks in agriculture, with human activities such as industrial production, transportation, sewage irrigation, application of organic fertilizers, chemical fertilizers, and pesticides all contributing directly or indirectly to the accumulation of heavy metals in soil [1,2]. Compared to pollution in water bodies and air, soil heavy metal pollution is more concealed. Once heavy metals enter the soil, they are not easily degraded by microorganisms [3], and they are difficult to transport by runoff erosion, leading to their continuous accumulation in the soil [4]. When heavy metals accumulate to a certain level, they can lead to environmental deterioration, contaminate rivers and groundwater sources, and cause a decline in soil fertility, resulting in reduced crop yields. Furthermore, they may directly poison plants and indirectly impact human health [5,6,7,8,9]. This has become an urgent soil environmental issue that demands immediate attention and for relevant investigations to be conducted in some countries [10,11,12,13,14]. Among the 32 soil samples collected from India, the concentrations of six heavy metals (As, Cr, Cu, Zn, Ni, and Pb) in most urban soils were higher than the geochemical background values (Grade I) and the Canadian soil quality guideline values (Grade II) [15]. In the facility farmlands of northern Jordan, some facility farmland soils failed to meet soil quality standards, particularly for Cd, Pb, and Ni content, with average concentrations of 0.81 mg·kg−1, 53.0 mg·kg−1, and 49.3 mg·kg−1, respectively. The Cd posed the highest total hazard index (72.27–82.67%), followed by Pb (11.49–14.87%) [16]. The survey revealed that the cadmium concentration in farmland soil in Shenyang, China (0.246 ± 0.156 mg·kg−1) exceeded the standard (HJ/T 333-2006) by a proportion of 39.29% [17]. In the Wuwei region of Gansu Province in China, there was a cumulative trend of Cd, Cu, and Zn elements in facility farmland soil, with concentrations 60%, 23%, and 14% higher, respectively, than those in cultivated soil [18]. However, knowledge gaps still exist in terms of the characteristics of heavy metal in facility farmland soils.
Facility farmlands, characterized by semi-enclosed environments with high temperatures, humidity, and fertilization levels, along with continuous cultivation, have led to numerous irreversible environmental issues, including heavy metal pollution [19,20]. Among these heavy metals, Cd contamination in facility farmland soil is particularly severe. In southern, northern, and northwestern China, the exceedance rates for Cd were 41.7%, 54.5%, and 11.1%, respectively [21]. The farmland soil in Baiyin District, Gansu Province, was heavily contaminated with Cd, with 100% of samples exceeding the risk management threshold for farmland [22]. In Beijing and the Xiangfen County of China in Shanxi Province, the average Cd concentrations were higher than the soil background values and approached the limit set by the Environmental Quality Evaluation Standards for Edible Agricultural Products Production Areas (0.40 mg·kg−1), indicating a certain degree of accumulation risk [23]. Among 149 soil samples collected from four major vegetable-producing regions in Shandong Province, it was found that the cadmium contamination level was the highest, reaching 9.40% [24]. In San Luis Potosí, México, Cd concentrations ranged from 3.72 to 7.46 mg·kg−1, which were below Canadian agricultural standards [25]. In facility farmland samples from Çanakkale, Turkey, Cd concentrations ranged from 0.68 to 1.07 ug·g−1, within the acceptable range for agricultural soil [26]. In facility farmland soil in Spain, Cd concentrations (0.1–1.9 mg·kg−1) were three times higher than those in cultivated soil [27]. It is vital to understand the characteristics of Cd enriched in facility farmland soil.
Facility farmland represents a modernized approach to agricultural production that minimizes external environmental disturbances and achieves a high degree of intensification. Since the 1990s, China’s facility farmland has undergone rapid development [28], becoming a pillar industry of China’s modern agricultural development. According to the National Plan for the Construction of Modern Facility Agriculture (2023–2030) released by the Ministry of Agriculture and Rural Affairs, China’s facility farmland crop planting area reached approximately 26.7 million hectares in 2021, ranking first in the world [29]. Beijing, as China’s capital and political–cultural center, is also one of the largest cities in the world. By the end of 2022, its facility agricultural area reached 32,496.0 hectares, accounting for 22.11% of the total cultivated land area (Beijing Statistical Yearbook 2023) [30]. With rapid urban development, the impacts on soil environmental quality changes are accelerating, and the pressure on soil environmental pollution in facility farmland is also increasing. Consequently, soil environmental quality has attracted increasing attention from the public. To ensure the ecological environmental safety and sustainable development of facility farmland in Beijing’s suburbs, strengthening soil environmental quality monitoring is of paramount importance.
This study takes typical facility farmland in northern China as the research object, investigating the content of soil nutrients and the accumulation of heavy metals. Specifically, the objectives of this study are as follows: (1) determining the content of heavy metals in soil in northern China and evaluating the level of soil pollution; (2) clarifying the distribution of heavy metal content in soil under different fertilization types and planting years; (3) and identifying the key factors influencing the heavy metal content in facility farmland soil. We hypothesize (1) that the Cd element is a heavily enriched heavy metal in facility agriculture soils in northern China and (2) that coupling organic fertilizer and soil nutrients are significant factors influencing heavily enriched heavy metals.

2. Materials and Methods

2.1. Sampling Area and Method

This study was conducted in the northern region of the North China Plain (115.7–117.4° E, 39.4–41.6° N), which has a temperate semi-humid continental monsoon climate with abundant sunshine and four distinct seasons [31]. The annual average temperature there is about 11.8 °C, and the average annual rainfall about 600 mm [32,33]. The main soil types are cinnamon soil and fluvo-aquic soil, with the soil pH being neutral or weak alkaline [33]. The production bases for facility agriculture were mainly located in the plain areas of the region. Before being renovated, the facility farmland was all used as large fields, mainly for planting wheat and corn.
From May to July 2023, a total of 69 soil samples (0–20 cm) were collected from facility farmlands in the research region. During the sampling process, special locations such as field ridges, roads, ditches, fertilizer piles, and manure heaps were avoided based on the conditions of the plots. Each facility farmland was designated as a sampling unit, and soil samples were collected using an “S”-shaped five-point sampling method within each facility farmland. After thoroughly mixing the soil samples, the quartering method was used to obtain a portion of the mixed soil sample. The mixed samples were then placed in clean self-sealing bags, properly labeled, and brought back to the laboratory for impurity removal, air-drying, crushing, and sieving through nylon sieves of 2.0, 1.0, and 0.149 mm. At the same time, information such as the planting years (5, 10, 15, and 20 years) and fertilization types (T1, organic fertilizer; T2, organic fertilizer and chemical fertilizer) was also recorded. The types of organic fertilizer included sheep manure, cow manure, and chicken manure, and some of the organic fertilizers applied in this region were sourced from the surrounding area of the sampling site (self-reported by farm managers/owners based on their records).

2.2. Sample Measurement Method

In this study, seven heavy metal elements were determined. The Pb and Cd in soil were determined using graphite furnace atomic absorption spectrometry (Agilent Technologies, Inc., Agilent 240FS, Santa Clara, CA, USA) (GB/T17141-1997) [34], As was determined with atomic fluorescence spectrometry (Agilent 240FS) (HJ491-2019) [35], and chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni) were determined by flame atomic absorption spectrometry (Agilent 240FS) (HJ491-2019). The soil organic matter (SOM) concentrations were determined using the potassium dichromate oxidation volumetric method [36]; the total nitrogen (TN) was measured with the Kjeldahl distillation method (KDY-9820, Beijing, China) [37]; available P (AP) was measured using the 0.5 mol·L−1 sodium bicarbonate extraction-spectrophotometric method (DR6000, Loveland, CO, USA) [38]; and available K (AK) was extracted with 1 mol·L−1 NH4OAc and measured by atomic absorption spectrometry [39]. The soil pH was measured with a pH meter (Mettler S220, Greifensee, Switzerland) using a 1:5 soil-to-water ratio [40]. Soil electrical conductivity (EC) was measured using the electrode method with a soil-to-water mass ratio of 1:5 [40].

2.3. Ecological Risk Assessment

The potential ecological risk index method proposed by Hakanson (1980) comprehensively considers the content, toxicity, and ecological effects of heavy metals, along with the synergistic effects of multiple heavy metals [41]. It is one of the most commonly used methods for ecological risk assessment [42]. The potential ecological risk index (Ei) of a single heavy metal element and the comprehensive potential ecological risk index (RI) were calculated as follows [43]:
R I = ∑ E I = ∑   ( T i × C i C n )
where Ti is the toxic response factor for a given substance (Cd = 30, Cr = 2, Pb = 5, As = 10, Cu = 5, Zn = 1, Ni = 5) [44], Ci represents the heavy metal content in the topsoil, and Cn is the regional background heavy metal value in the topsoil [45]. The risk classification standards are listed in Table 1.

2.4. Variable Importance Determination

The random forest (RF) model is an ensemble machine learning algorithm based on classification and regression that can be used to analyze complex nonlinear relationships between independent and dependent variables. It utilizes the bootstrap sampling method to randomly extract smaller samples from the original data to be used as the training set and then builds decision trees based on these data. The sampling and training steps are repeated to establish a large number of decision trees, eventually forming a random forest [46]. The RF regression model can be used to model all covariates and determine their importance. The increase in mean squared error (IncMSE) was selected as the metric to quantify the variable importance. IncMSE involves randomly shuffling the values of a feature in the dataset and measuring the increase in the mean squared error (MSE) of the model’s predictions. The larger the value, the greater the importance of the variable [47]. In this study, RF was employed to identify the key factors (Cd, Cr, Pb, As, Cu, Zn, Ni, TN, TP, AP, AK, pH, EC, and SOM) influencing the concentrations of heavy metals in the soil of facility farmland.

2.5. Data Analysis

All data were processed and analyzed using Microsoft Excel 2021 (v17.0) and IBM SPSS Statistics version 27, using difference analysis to study the impact of different fertilization types on the distribution of heavy metals in soil. The autocorrelation of the soil nutrient content (SOM, TN, TP, AP, AK) and its correlation with heavy metal content were analyzed using Pearson’s correlation analysis. RF was used to model all the covariates and determine the key factors influencing the heavy metal content in the soil of facility farmlands. Additionally, Origin 2021, Prism 9.5.1, and R 4.3.3 software were used for graphing.

3. Results

3.1. Characteristics and Risks of Heavy Metal Content in Surface Soil (0 to 20 cm)

Table 2 lists the background values of seven heavy metals in soil. This study found that the concentrations at 66.67%, 100%, 95.65%, 95.65%, 100%, 0%, and 0% of the sampling sites exceeded the soil background values for Cd, Cr, As, Cu, Zn, Pb, and Ni, respectively. However, the average contents of the Cd, Cr, Pb, As, Cu, Zn, and Ni elements remained below the risk screening values (GB 15618-2018) [48]. The coefficients of variation (CVs) for the seven elements were as follows: Ni (52%), Cd (37%), Cr (37%), As (21%), Cu (21%), Zn (20%), and Pb (17%). Notably, CV > 36% was classified as high variability, while 16% < CV < 36% was classified as moderate variability. Among them, Cd, Cr, and Ni have high variation, while As, Cu, Zn, and Pb had moderate variation. A higher CV indicated a greater likelihood of anthropogenic influence and less even spatial distribution. The kurtosis values for Cd, As, and Cu exceeded 1, indicating steeper normal distribution curves and extreme values. The skewness values for Cd, Cr, and As were close to 1, displaying right-skewed distributions and indicating the existence of higher abnormal values.
The average EI of the seven heavy metal elements in the 0 to 20 cm soil layer were in the following order: Cd (37.4) > As (14.9) > Cu (7.9) > Cr (5.3) > Pb (4.3) > Ni (3.4) > Zn (1.7), indicating that all seven elements belonged to the category of having a slight pollution risk (Figure 1). Remarkably, the range of the Cd EI was from 14.9 to 84.2, in which slight ecological risks accounted for 60.87%, and moderate and strong ecological risks accounted for 37.68% and 1.45%, respectively. From the perspective of comprehensive risk, the average RI of the heavy metals in the 0 to 20 cm soil layer was 74.8; the overall risk level was relatively light.

3.2. Effects of Fertilizer Type and Planting Year on Soil Heavy Metal Content

The fertilization type had various effects on the heavy metal concentrations in the study (Figure 2). There were significant differences (p < 0.05) in the contents of Cr, As, Cu, Zn, and Ni between the two types of fertilized soils, T1 and T2. The average contents of Cr, As, Cu, Zn, and Ni in the T1 type of fertilized soil were 98.40 mg·kg−1, 11.03 mg·kg−1, 29.32 mg·kg−1, 97.20 mg·kg−1, and 25.88 mg·kg−1, respectively. There were no significant differences in the Cd and Pb soil contents among the two types of fertilization (p > 0.05). The Cd contents in T1 and T2 soils were 0.118 mg·kg−1 and 0.148 mg·kg−1, respectively.
In the 0 to 20 cm soil layer, the contents of Cd and Cr increased with increasing planting years, whereas the contents of Cu, Zn, and Ni decreased with increasing planting years; there was a significant negative correlation between the copper element and planting years (p < 0.01) (Figure 3). No significant correlation was found between the concentrations of lead (Pb) and arsenic (As) in the soil of facility farmland and the planting years. It was also found that the concentration of heavy metals in the soil of facility farmland tended to decrease slightly after a certain period of cultivation. The average concentrations of Cd and Pb in the soil reached their peaks in the 11th year of cultivation, which were 0.211 mg·kg−1 and 24.25 mg·kg−1 respectively. In the fourteenth year, the average content of Cr in the soil was the highest, which was 121.25 mg·kg−1. In the fifth year of planting, the average contents of Cu and Zn in the soil were the highest, which were 49.20 mg·kg−1 and 124.60 mg·kg−1, respectively.

3.3. Correlation Analysis Between Heavy Metal Concentrations and Soil Nutrients

In this study, the average value and range of pH were 7.53 and 6.67–8.09 in soil from 0 to 20 cm. The content ranges of TN, TP, AP, AK, and SOM were 0.20–1.81 g·kg−1, 0.02–0.33%, 34.61–614.20 mg·kg−1, 56.10–1449.10 mg·kg−1, and 13.67–76.26 g·kg−1, respectively. Meanwhile, the average values were 0.78 g·kg−1, 0.10%, 264.37 mg·kg−1, 508.94 mg·kg−1, and 42.05 g·kg−1, respectively (Table 3). Respectively, 39% of TP, 97% of AP, 85% of AK, and 49% of SOM sites had very abundant nutrient contents. Additionally, 55% of the surveyed sites had TN content levels that were moderate or above.
In the 0–20 cm soil layer, TN, TP, AP, AK, and SOM exhibited significant positive correlations with Cu and Zn (Figure 4). The pH value showed a significant positive correlation with As and Ni (p < 0.05). The EC value had a significant positive correlation with Cd and Pb (p < 0.05). TP demonstrated a significant negative correlation with Cr and Ni (p < 0.05). AP had a significant negative correlation with As (p < 0.05). SOM exhibited a significant negative correlation with Cr (p < 0.05). Changes in soil nutrient content were related to the changes in heavy metal content. Since soil nutrient content primarily originated from fertilization, the application of organic and chemical fertilizers may have been an important reason for the increase in heavy metal content in the soil of facility farmlands.

3.4. Key Factors Influencing Heavy Metal Concentrations in Farmland Soil

The overall accuracy correlation coefficients (r) of the models for Cd, Cr, Pb, As, Cu, Zn, and Ni in the 0 to 20 cm soil were 0.009, 0.44, 0.33, 0.04, 0.32, 0.61, and 0.65, respectively, showing relatively high prediction accuracies (Figure 5). The two most important factors explaining the Cd content were Zn (40.63%) and EC (30.79%). The top three factors explaining the Cr content were TP (44.73%), AP (18.18%), and Cd (16.00%). The most important factors explaining the Pb content were Cu (42.73%), Ni (35.48%), Zn (22.34%), and AP (21.36%). Meanwhile, As was mainly influenced by Pb (40.73%), pH (21.39%), and Cu (19.89%), whereas Cu was mainly influenced by Zn (63.90%), TP (17.56%), and Cr (17.48%). The three most important factors explaining the Zn content were TP (80.37%), Zn (49.56%), and AK (33.54%). The three most important factors explaining the Ni content were Cr (27.56%), Pb (24.10%), and pH (16.77%). Overall, the variations in heavy metal content in the 0 to 20 cm soil layer were significantly influenced by fertilization (as heavy metals and nutrients commonly coexist in certain fertilizers, they are applied to the soil together, affecting the heavy metal content of the soil).

4. Discussion

The findings of this study indicate that there was a significant accumulation of Cd in the surface soil of facility farmland (Table 4). This was consistent with other research findings, which showed that the cadmium content in farmland soil in Wuqing District, Tianjin, was higher than the local soil background value, indicating a clear accumulation phenomenon [49]. In two districts of Bangladesh, the Cd content in soil exceeded the local soil background value of 0.3 mg·kg−1, suggesting a significant influence from human activities [50]. Using the Hakanson potential ecological risk index, it was found that some facility farmland soils are contaminated with Cd. The accumulation of Cd in topsoil still persists within the surveyed area, and there is a trend of decreasing numbers of sites with moderate to high ecological risk levels [51,52]. However, Cd is one of the major contaminants in the soil of facility farmland in northern China. Studies have shown that its main sources include fertilizer application, pesticide use, atmospheric deposition, and irrigation water. Due to the long-term use of film-covered, semi-enclosed planting modes in facility farmland, atmospheric deposition is not the primary factor influencing the Cd content in the soil [53]. Between 2015 and 2019, compared to 2005–2009, the input of Cd from irrigation water into farmland soil in north China decreased, while the input of Cd from livestock and poultry manure increased by 18.6% [54,55]. The amount of Cd that entered into soil from livestock and poultry manure would be 3.2 to 122.4 g ha−1 yr−1; the highest concentration of Cd in the topsoil (0–20 cm) would increase 0.049 mg·kg−1 per year [56,57,58]. Pesticide application can also affect the accumulation of Cd in soil, but it is not the primary factor [59].
There are significant differences between facility farmland and open field farmland in the production process. It operates under a closed condition characterized by long-term film mulching, high temperature, high humidity, high input, and high output, representing a planting mode with intense human interference. Under different fertilization types, Cd, Cr, As, Cu, and Zn all exhibit varying degrees of accumulation, which may be related to the improper use of fertilizers [60]. In the study area, the base fertilizers for farmland mainly included sheep manure, commercial organic fertilizer, and chemical fertilizer. Research has shown that long-term fertilization can affect soil physicochemical properties, thereby influencing heavy metal content [61]. Heavy metals entering the ecosystem may lead to accumulation in the food chain, and the concentration of heavy metals in food has been proven to be closely related to the concentration of heavy metals in soil [62]. There were certain differences in the heavy metal content of livestock and poultry manure from different sources [63,64,65]. The over-limit rates of Cd, Cr, Pb, and As in chicken manure were 0%, 3.6%, 0.4%, and 6%, respectively, while the over-limit rate of As in cow manure was 1.2%. The over-limit rates of Cd, Cr, Pb, and As in sheep manure were all 0% [66]. According to the Chinese industry standard for organic fertilizers (NY 525-2012) [67], the over-limit rate of chromium in livestock and poultry manure was 2.76% [65]. It can be seen that livestock and poultry manure were a major source of heavy metals in the soil of north China [68], and they are also one of the main factors leading to the accumulation of metal elements in farmland soil.
Table 4. Organic fertilizer raw materials and commodities, and organic fertilizer heavy metal index test samples (mg·kg−1) [66,69].
Table 4. Organic fertilizer raw materials and commodities, and organic fertilizer heavy metal index test samples (mg·kg−1) [66,69].
Index Organic FertilizerCommercial Organic Fertilizer
Sheep Manure (n = 63)Cow Manure (n = 245)Chicken Manure (n = 285)Sheep Manure (n = 40)Sheep/Cow Manure (n = 2)
Cdrange0–1.70–2.30–2.30.005–0.0700.022–0.045
Average0.4 ± 0.30.3 ± 0.40.3 ± 0.50.0420.034
Crrange0–590.6–106.90–28961.76–28.407.29–17.20
Average20.4 ± 10.720.7 ± 16.366.3 ± 308.512.4012.20
Pbrange2.7–46.10.3–40.90–51.70.93–38.89.33–14.70
Average11.3 ± 88.7 ± 7.19.2 ± 8.911.8012.02
Asrange0.5–12.50.1–32.60.1–55.80.30–11.42.30–6.50
Average5.5 ± 2.84.3 ± 3.46.0 ± 6.23.444.39
Curange0–75.411.6–36.313.4–41.24.00–123.112.2–30.3
Average18.8619.721.5516.421.23
Znrange0–272.638.6–319.846.0–242.014.0–15455.4–82.6
Average95.1690.51114.2468.868.9
As can be seen from Figure 4, Cd shows an overall upward trend within the planting years. Simultaneously, studies have found that the heavy metals in the soil of facility farmland exhibit a slight decreasing trend after a certain number of planting years, which may be attributed to changes in the soil’s physicochemical properties during long-term planting [61,70]. Numerous studies have also indicated that the accumulation of heavy metals in soil tends to increase with the prolongation of planting years, which is associated with the excessive application of metal-containing organic and chemical fertilizers [71], and is also related to the extensive use of pesticides containing metals, as well as the direct application of livestock and poultry manure (such as chicken manure and sheep manure) back to the farmlands [72,73].
The Pearson correlation results in this study also indicate that there were significant correlations between the total amounts of Cu, Zn, and Cr in the soil of facility farmland and SOM. There was also a significant correlation between As and Ni and pH (p < 0.05). Previous studies have also shown that soil nutrients (such as soil pH, SOM, TN, and TP) were correlated with soil heavy metal concentrations [74,75]. Further consideration of the relatively limited sources of SOM, TN, TP, and AP in facility cultivation, mainly through organic fertilizers and chemical fertilizers, is essential. The analysis of the major inputs in facility farmland soil in this study further confirms that Cd, Cr, Cu, and Zn contents in organic and chemical fertilizers are relatively high, contributing prominently to the accumulation of heavy metals in facility soil. Additionally, studies by Xu Minggang et al. [76] have shown that the long-term application of organic fertilizers made from livestock and poultry manure not only increases soil nutrient content but also leads to an increase in soil heavy metal content. Therefore, scientifically and reasonably processing livestock and poultry manure and selecting safe organic/chemical fertilizers are effective ways to slow down the degradation rate of facility farmland soil quality and ensure the safe and sustainable development of facility agriculture.

5. Conclusions

This study validated our hypothesis that Cd is a heavy metal that accumulates to a high degree in the soils of facility farmland in northern China. Compared to soil background values, the accumulation of Cd in facility farmland soils was the most severe, and the concentration of Cd in some of these farmland soils had reached levels that may pose potential risks to both the ecosystem and human health.
Heavy metal-containing chemical fertilizers and organic fertilizers were the primary sources of Cd, Cr, As, Cu, and Zn in soil. The type and quantity of the fertilizers applied were significant factors contributing to the differences in heavy metal accumulation in soil under the various fertilization practices studied.
Regression analysis also indicated that Cd content increases linearly with the number of years of cultivation. Therefore, the duration of cultivation is an important factor influencing soil nutrient and heavy metal accumulation in greenhouse farmland. Considering the close relationship between certain soil nutrient indicators and heavy metal content in soil, as revealed by Pearson correlation analysis, as well as the most significant factors influencing heavy metal (Cd, Cr, Pb, As, Cu, Zn, and Ni) content as identified through the random forest model, it is evident that long-term fertilization not only increases soil nutrient content but also leads to an increase in soil heavy metal content.
The results indicate that the long-term excessive use of chemical fertilizers and organic fertilizers containing high concentrations of heavy metals can lead to the accumulation of heavy metals in soil. Therefore, to maintain high soil quality and achieve sustainable utilization, it is necessary to control the quality of chemical fertilizers and organic fertilizers in order to manage the levels of heavy metals in facility farmland soil.

Author Contributions

Writing—original draft, writing—review and editing, conceptualization, data curation, methodology, investigation, formal analysis, M.Z.; investigation and data curation, G.Z., Z.Y., Y.L., N.S. and S.L. (Shangqiang Liao); conceptualization, funding acquisition, supervision, writing—review and editing, L.D. and S.L. (Shunjiang Li). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the study was financially supported by the grants from the National key research and development program of China (Project No.: 2023YFD1700104), and the earmarked fund for CARS (Project No.: CARS-02-23).

Data Availability Statement

Data are available from the corresponding author upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 13 01010 g001
Figure 1. Potential ecological risk index diagram of heavy metals in facility farmland soil.
Figure 1. Potential ecological risk index diagram of heavy metals in facility farmland soil.
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Figure 2. Heavy metal content in facility farmland soils under different fertilization types. Note: T1, organic fertilizer; T2, organic fertilizer and chemical fertilizer. Different letters within the same heavy metals indicate significant differences (p < 0.05).
Figure 2. Heavy metal content in facility farmland soils under different fertilization types. Note: T1, organic fertilizer; T2, organic fertilizer and chemical fertilizer. Different letters within the same heavy metals indicate significant differences (p < 0.05).
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Figure 3. The change in heavy metal content in facility farmland soils with planting year.
Figure 3. The change in heavy metal content in facility farmland soils with planting year.
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Figure 4. Correlation plot of the soil heavy metal content with soil nutrients. Note: * correlation is significant at the 0.05 level, ** correlation is significant at the 0.01 level. TN (total nitrogen), TP (total phosphorus), AP (available phosphorus), AK (available potassium), SOM (soil organic matter), and EC value (electrical conductivity). (A) Comprehensive Correlation Analysis of Soil Parameters Including Physical-Chemical Properties, Nutrient Levels and Heavy Metal Contents; (B) Correlation Analysis of Soil Physical-Chemical Properties, Nutrient Levels and Heavy Metal Contents.
Figure 4. Correlation plot of the soil heavy metal content with soil nutrients. Note: * correlation is significant at the 0.05 level, ** correlation is significant at the 0.01 level. TN (total nitrogen), TP (total phosphorus), AP (available phosphorus), AK (available potassium), SOM (soil organic matter), and EC value (electrical conductivity). (A) Comprehensive Correlation Analysis of Soil Parameters Including Physical-Chemical Properties, Nutrient Levels and Heavy Metal Contents; (B) Correlation Analysis of Soil Physical-Chemical Properties, Nutrient Levels and Heavy Metal Contents.
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Figure 5. Importance ranking of the factors influencing heavy metals in facility farmland soils.
Figure 5. Importance ranking of the factors influencing heavy metals in facility farmland soils.
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Table 1. Classification of pollution grades for potential ecological risk assessment.
Table 1. Classification of pollution grades for potential ecological risk assessment.
Potential Ecological Risk IndexPotential Ecological Risk Level
SlightModerateStrongVery StrongExtremely Strong
EI<4040–8080–160160–320≥320
RI<150150–300300–600≥600-
Table 2. Heavy metal content characteristics of facility farmland soils.
Table 2. Heavy metal content characteristics of facility farmland soils.
Heavy MetalAverage (mg·kg−1)Max (mg·kg−1)Min (mg·kg−1)Median (mg·kg−1)Standard DeviationCoefficient of Variation (%)SkewnessKurtosisNatural Background Value (mg·kg−1)
Cd0.1480.3340.0590.1380.055370.9491.1590.119
Cr78.89174.5032.8073.6028.93370.750.4629.8
Pb21.0731.2011.2021.103.65170.080.5724.6
As10.5317.906.5010.402.20210.711.527.09
Cu29.4949.2014.0029.406.09210.361.1318.7
Zn98.90150.2060.5095.1020.13200.36−0.5057.5
Ni18.0141.800.7018.009.41520.07−0.6926.8
Note: Natural background value [45].
Table 3. Soil nutrient content characteristics of facility farmlands.
Table 3. Soil nutrient content characteristics of facility farmlands.
Soil NutrientsTN (g·kg−1)TP (%)AP (mg·kg−1)AK (mg·kg−1)SOM (g·kg−1)
Average0.780.10264.37508.9442.05
Max1.810.33614.201449.1076.26
Min0.200.0234.6156.1013.67
Standard deviation0.320.06126.51310.0413.95
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Zhang, M.; Zou, G.; Yang, Z.; Li, Y.; Sun, N.; Liao, S.; Du, L.; Li, S. Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils. Processes 2025, 13, 1010. https://doi.org/10.3390/pr13041010

AMA Style

Zhang M, Zou G, Yang Z, Li Y, Sun N, Liao S, Du L, Li S. Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils. Processes. 2025; 13(4):1010. https://doi.org/10.3390/pr13041010

Chicago/Turabian Style

Zhang, Mengmeng, Guoyuan Zou, Zhiping Yang, Yanmei Li, Na Sun, Shangqiang Liao, Lianfeng Du, and Shunjiang Li. 2025. "Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils" Processes 13, no. 4: 1010. https://doi.org/10.3390/pr13041010

APA Style

Zhang, M., Zou, G., Yang, Z., Li, Y., Sun, N., Liao, S., Du, L., & Li, S. (2025). Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils. Processes, 13(4), 1010. https://doi.org/10.3390/pr13041010

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