INTRODUCTION

Cadmium (Cd) is a highly toxic heavy metal with high mobility from soil to crops. It can enter the human body through the food chain and harm human health [5, 11, 14, 15], and is an important kind of heavy metal pollutant [12]. In recent years, wastewater irrigation, manure, the unreasonable use of agricultural chemicals, and industrial “three wastes” emissions have caused serious Cd pollution in China’s farmland soils. Data from the National Soil Pollution Investigation Bulletin released in April 2014 showed that the overall over-standard amount of Cd in soil in China was 16.1%, with the over-standard amount of inorganic substances accounting for 82.8% of all the over-standard amounts. The over-standard amount of Cd was the highest, reaching 7.0% [3]. Therefore, Cd pollution is the most prominent heavy metal pollution problem in soil in China.

Soil Cd pollution is relatively common in southern China. Studies showing the change of Cd form in acidic soils have been reported [16], but there are few related studies on calcareous soils in northern China. Calcareous soil has weak buffering capacity to acid and alkalinity, so the excessive application of chemical nitrogen fertilizer results in decreasing soil pH year over year [7]. Combined with the acidification effect of the plant rhizosphere on soil [21], Cd pollution is becoming increasingly common. In Beijing, for example, more than 40% of monitoring sites have “mild” or “moderate” pollution [22].

The existing form of Cd in soil is affected by many factors such as soil pH, CEC, organic matter, soil texture, and soil type; soil pH is an important factor affecting the availability of Cd [23]. A large number of studies have shown that acidity and alkalinity can affect the release of heavy metals from soil, and pH is an important factor affecting the state of heavy metals in soil and the absorption by plants [6]. The availability of Cd in soil and the uptake of Cd by plants decreases with increasing pH [25]. However, some studies indicate that there is not a simple overall linear relationship between soil pH and available Cd content; however, a linear relationship does exist within a corresponding pH range [29]. For example, a study by Chen Nan indicated that when soil pH increases from 4 to 10, exchangeable Cd content decreases by 44.12%, and reducible Cd content and residual Cd content increases by 106.25% and 154.55%, respectively [2]. Kang Liusheng indicated that when soil pH > 6.5, available Cd content showed a downward trend with the increase of soil pH; however, at soil pH < 6.5, available Cd content did not decrease with increasing pH [10]. Along with the Cd pollution situation, soilless cultivation technology has been widely used in China because of increasingly scarce land resources [1, 13]. At present, the common culture substrate is a combination of organic and inorganic materials. Organic raw materials are generally peat, crop straw, and waste from processing of agricultural products, while inorganic raw materials are more diverse, including perlite, vermiculite, and slag. However, with the increase in the amount of substrate material and service life, Cd introduced through fertilizer increasingly accumulates, presenting a risk of pollution. Most studies on the effect of pH on Cd availability are focused on acidic soils in southern China, while the correlation between pH and available Cd in calcareous soils and culture substrate has not been reported. In this study, the effect of pH on available Cd in calcareous soil and substrate was studied through simulation experiments, providing support for controlling Cd pollution to crops by controlling the pH.

Table 1.   Variance analysis of available Cd in soil
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OBJECTS AND METHODS

Test materials. The tested soil was obtained from Hongke Farm in Fangshan District, Beijing. The soil was a calcareous cinnamon soil, pH 7.8, organic matter content 1.92%, total Cd content 0.08 mg kg–1, and available Cd content 0.02 mg kg–1, calcium carbonate 35.6 mg kg–1. The experimental culture substrate was peat, vermiculite, and perlite mixed at a ratio of 6 : 3 : 1, pH 5.1, organic matter content of 31.12%, total Cd content of 0.01 mg kg–1, and available Cd content of 0.002 mg kg–1.

Experimental design. The experiment was conducted in the greenhouse of Beijing Academy of Agriculture and Forestry Sciences from April to November 2021. Air-dried soil or culture substrate (1000 g) was weighed into a pot, and the material was sifted to 2 mm. The soil treatments were labeled: T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, TCK, and the culture substrate treatments were labeled: J1, J2, J3, J4, J5, J6, J7, J8, J9, J10, J11, J12, J13, J14, J15, JCK. Each treatment was repeated three times, for a total of 48 pots each of soil and culture substrate. Filter paper and non-woven cloth was placed into each pot to prevent leakage and washout of the material when adding water. A tray was added to the bottom of each pot to collect the seepage solution, which was then poured back into the pot.

A CdCl2 solution was made by adding 0.0594 g CdCl2 to 10 L water. This solution (168.4 mL) was measured into each pot and mixed with the soil and culture substrate (i.e., the amount of exogenous Cd added was 1.0 mg kg–1). During the equilibrium period for exogenous Cd, 70% of the maximum field water capacity of the sample was maintained for 100 d by weighing every 24 h and adjusting with deionized water. The soil or substrate samples were dried and screened, and the pH was measured. The pH was then adjusted to 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10.0 with 1 mol L–1 HCl or NaOH. During this period, samples were taken intermittently, and the pH was adjusted to the preset value above (with an upper and lower range of 0.5). After 90 d of equilibrium, the pH was kept stable and the samples were dried naturally. After grinding and passing through a nylon sieve, the samples were analyzed [26]. The pH was not adjusted in samples TCK and JCK.

Analysis items and methods. The pH of the soil samples was determined by sieving 10 g of air-dried soil sample through a 2-mm aperture sieve, placing in a 50-mL beaker, adding 25 mL deionized water (water : soil 2.5 : 1), stirring 5 min, standing 3 h, and measuring pH with potentiometer. The culture substrate pH was determined with the same procedure as the soil. The organic matter of the soil samples and culture substrate samples was determined by the high temperature external thermal potassium dichromate oxidation-volumetric method. The soil or culture substrate (0.2 g) was passed through a 0.149-mm sieve into a test tube, and 10.00 mL 0.4 mol L−1 potassium dichromate-sulfuric acid solution was added. The sample was placed in an oil bath at 185–190°C, controlling the sample temperature at 170–180°C. The liquid was boiled for 5 min, and the liquid and soil residue were transferred to a 250 mL triangle bottle and adjusted to 50–60 mL. Three drops of phenanthroline indicator were added and the remaining potassium dichromate was titrated with a standard solution of ferrous sulfate. The solution color changed from orange-yellow to blue-green to brown-red. The available Cd in soil was determined by graphite furnace atomic absorption spectrophotometry with DTPA extraction. Air-dried soil samples (5.0 g) passed through a 2-mm nylon sieve were weighed and placed in a 100 mL stoppered conical flask. DTPA extraction agent (25.0 mL) was added, and the sample was placed into a horizontal oscillator at room temperature (25 ± 2°C) and shaken at 180 rpm for 2 h. After filtration (discarding the initial 5–6 mL of filtrate), Cd in the filtrate was analyzed with a graphite furnace atomic absorption spectrophotometer. Total Cd was determined by graphite furnace atomic absorption spectrophotometry with hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid digestion. The samples were ground with agate mortar until they passed through 0.149 mm nylon sieve. The sample (0.3 g) was placed in a 50-mL Teflon crucible, wetted with water and 5 mL HCl (36%), and heated at low temperature to evaporate to approximately 2–3 mL. Nitric acid (5 mL, 68%), 4 mL hydrofluoric acid (40%), and 2 mL perchloric acid (70%) were added, then the sample was heated at moderate temperature (60–80°C), covered. After 1 h, the cover was removed to allow perchloric acid smoke to escape, and the cover was replaced to allow the black organic carbide to fully decompose. After the black organic matter in the crucible disappeared, the contents were steamed until sticky. This process was repeated according to the digestion method. Nitric acid (1 mL, 2.67 mol/L) and 3 mL diammonium hydrogen phosphate solution was added to the residue, and the volume was adjusted to 25 mL. The solution was analyzed with a graphite furnace atomic absorption spectrophotometer.

Data processing. Calculations and plots were performed with Microsoft Excel 2016 and analyzed by SPASS22.

RESULTS

Effect of pH on available Cd in soil. As shown in Fig. 1, the total Cd in the soil was 1.0 mg kg–1, the pH was 3.40–8.97, and the available Cd varied from 0.33 to 0.53 mg kg–1. With increasing pH, the content of available Cd in soil decreased. According to regression statistics and variance analysis, the P-value was less than 0.05, the scatter plot curve was Y = −0.0375x + 0.662, correlation coefficient was R = 0.9454, and Significance F was 3.35 × 10–8, indicating a significant negative correlation consistent with the results of Yi Zhenxie [27], Park [19] and Lu [17]. The data from all studies showed that the content of available Cd in calcareous soil could also be reduced by increasing pH.

Fig. 1.
figure 1

Available Cd content in soil at different pH values.

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Effect of pH on available Cd in culture substrate. As shown in Fig. 2, when the pH of the tested culture substrate was in the range of 3.03 to 9.47, the available Cd content varied from 0.21 to 0.45 mg kg–1. In contrast to the trends seen in the soil, in the culture substrate, increasing pH increased the available Cd content that became stable after reaching a high point. The polynomial regression equation R2 = 0.8635 was used to graph the pH and the available Cd content; the trend was a downward parabola. The analysis of the available Cd in the culture substrate in the pH range of 4.37 to 9.33 showed that there was no significant difference in the available Cd in the culture substrate under different pH conditions; therefore, within this pH range, available Cd content cannot be reduced in the culture substrate by changing the pH. In the pH range of 3.03 to 4.37, there was a significant positive correlation between pH and available Cd content in the culture substrate (slope of 0.1706), indicating that in this pH range, the available Cd content in the culture substrate rapidly increases with increasing pH (Fig. 3).

Fig. 2.
figure 2

Content of available Cd in culture substrate with different pH values.

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Fig. 3.
figure 3

Content of available Cd in culture substrate at pH 3.03 to 4.37.

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DISCUSSION

Differences of Cd availability between cultivated substrate and soil. When 1.0 mg kg–1 Cd was added in the pH range of 3.0 to 9.0, the highest value of available Cd in the culture substrate was 0.45 mg kg–1, which was lower than that in the soil; the lowest value was 0.21 mg kg–1, which was also lower than that in the soil. This indicates that the available Cd content of substrate is lower than that of soil. Studies have shown that organic matter in soil is an important factor affecting the availability of Cd [9]. The solubility of organic matter will decrease with decreasing pH, and the complexation ability of Cd will also decrease [26]. Compared with soil, organic matter in the substrate is significantly higher than in the soil. Over a certain range, higher organic matter results in greater adsorption capacity for heavy metals, and increasing organic matter can promote the conversion of carbonate-bound Cd to organic-bound Cd [8]. In this study, the organic matter content of the culture substrate was significantly higher than that of the tested soil, and the experimental data also showed that the high organic matter content reduced the available Cd content.

Effect of pH on of available Cd content in the culture substrate. The pH of the substrate ranged from 3.03 to 4.37, and there was a significant positive correlation between pH and the content of available Cd, but lower pH was not conducive to plant growth. When the pH was between 4.37 and 9.47, there was no significant difference in the available Cd content in the substrate, indicating that available Cd is not influenced by changing the pH of the culture substrate in this range. The same culture substrate was used in this experiment, but its pH was changed. Further studies are required to determine if the change of pH will affect organic matter, Eh, CEC, anions/cations, or other factors that synergically affect effective state of Cd in the culture substrate, thus changing the nonlinear representation of the effective state Cd content in the culture substrate.

Other factors affecting available Cd content. Studies have shown that increasing soil pH will increase the negative charge on the surface of clay minerals, hydrated oxides, and organic matter in the soil, improving the adsorption capacity and reducing the bioavailability of Cd [26]. However, other studies have shown that the effect of soil pH on the bioavailability of Cd in soil may not be a single increasing relationship [18]. Our experimental data has shown that increasing pH has a significant negative correlation with available Cd content in the soil, and a significant positive correlation between pH and available Cd content in the culture substrate at a pH range of 3.03 to 4.37. This was a simulation experiment, and plants will also influence the effective state of Cd (root exudates, plant absorption, and dilution effect) in the complex soil-water-plant system; soil types [24], microorganism [17], Eh [20], CEC [4], soil enzymes [28], and other factors will also affect the morphological changes of Cd. This study only investigated the available Cd in soil and culture substrate at different pH values, which does not fully reflect the available Cd in soil or culture substrate in the actual planting system. Therefore, plants should be included in the research system in future studies to reflect the actual situation more objectively.

CONCLUSIONS

At a pH range of 3.40–8.97, the content of available Cd in calcareous soil decreased with increasing pH, showing a significant negative correlation. At a pH range of 3.03–9.47, the content of available Cd in the culture substrate increased with the increase of pH and tended to be stable after reaching a high.