Translocation and accumulation of heavy metals from the rhizoshphere soil to the medicinal plant (Paeonia Lactiflora Pall.) grown in Bozhou, Anhui Province, China

ABSTRACT

The concentration of heavy metals (HMs) in soil-plant systems was an important indicator of soil heavy metal pollution, and the migration and transportation abilities of HMs from soil to plants were of great significance for evaluating food safety and reducing health risks. This study was conducted to investigate the contamination and translocation of HMs in the soil-Paeonia lactiflora system in Bozhou, Anhui Province, China. The soil pollution was assessed by calculating the Geoaccumulation index (Igeo), Potential ecological risk index (RI), and Pollution load index (PLI). The transfer ability of HMs was evaluated by using the bioconcentration factor (BCF) and translocation factor (TF). The results showed that Fe, Mn , Cr , and As were the priority pollutants in soil. Paeonia lactiflora presented high potential to translocate Zn, Cu, and Fe from soil to roots, and had strong ability to translocate HMs except for Cd from roots to stems and leaves

KEYWORDS:

• Heavy metals
• soil
• paeonia lactiflora
• translocation
• pollution

1. Introduction

Paeonia lactiflora Pall. (Chinese herbaceous peony) is a famous perennial herb that is mainly cultivated in East Asia, such as China, Korea, and Japan [1,2]. Its root, also known as Paeoniae Radix Alba, is a traditional Chinese medicinal material, mainly has the effects of calming liver and relieving pain, nourishing blood and regulating menstrual cycle, collecting Yin and antiperspirant (Chinese Pharmacopoeia, 2020). In China, the efficacy and attending functions of Paeonia lactiflora were recorded in the famous book of ‘Shennong Materia Medica Classic’ (Eastern Han Dynasty, A.D. 25–220), indicating that Paeonia lactiflora has been used in medicine for more than 2,000 years [2]. Paeonia lactiflora is suitable for mild temperature and full sunshine. It was historically produced in Bozhou (Anhui Province), Hangzhou (Zhejiang Province) and Zhongjiang (Sichuan Province) in China. Paeonia lactiflora cultivated in Bozhou (short for ‘Bo Baishao’) has been included in Chinese Pharmacopoeia (2020) as one of the four authentic medicinal materials in Anhui Province due to its large planting amount, better quality and better curative effect than other regions.

Heavy metals (HMs) are a class of elements with densities greater than 5 g/cm³ in the periodic table [3,4]. Their sources in soil include both natural sources caused by long-term geological process and parent material weathering and anthropogenic sources, such as fertilizer application, wastewater irrigation, traffic emissions, fossil fuel burning and other industrial, agricultural, and business activities [4–6]. In recent years, with the continuous development of industrialization, agriculture and urbanization, heavy metals released by excessive human activities continue to accumulate in soil, resulting in serious soil heavy metal pollution, which has attracted worldwide attention [6–8]. Heavy metals enriched in soil are characterized by non-biodegradability, enrichment, and toxicity, which in turn cause soil resource degradation and ecosystem deterioration [6,9]. Even more, the pollution of heavy metals on agricultural soil is of particular concern because agricultural soil is the main source of nutrients required for plant growth, and harmful substances such as heavy metals in soil can also be transferred to plants [7,10]. The accumulation of heavy metals in plants, especially edible plants (such as crops and medical plants) not only affects plant growth but also poses a great risk to human health through the food chain. According to the National Soil Pollution Survey Communique, jointly released by the Ministry of Environmental Protection and the Ministry of Land and Resources, 16.1% of China’s agricultural soil were contaminated to varying degrees, while inorganic pollution types, represented by heavy metals (Cu, Pb, Zn, Ni, As, Cd, Hg, and Cr) accounted for 82.8% of all sites exceeding the national standard level [3,6,9,11]. It is an effective method to explore the impact of soil heavy metals on plants by studying the distribution characteristics of heavy metals in the soil-plant system. At present, a large number of researches focus on the effects of heavy metals in soil on wheat [9], rice [12], corn [13], vegetables [14] and other plants [11], while there are few studies on the soil-medicinal plant system. Therefore, in this study, the migration and transformation characteristics of heavy metals in a soil and medicinal plant system (Paeonia lactiflora Pall.) was investigated.

In this study, a total of 14 Paeonia lactiflora plants and corresponding rhizosphere soil samples were collected from Bozhou City, Anhui Province, China. The main objectives of this study were to investigate (1) the concentrations of Al, Fe, Mn, Cu, Pb, Zn, Cr, As and Cd in soil and different organs (root, stem, leaf) of Paeonia lactiflora; (2) the accumulation characteristics of these nine heavy metals in soil samples; (3) the transfer capacity of heavy metals among soil and different parts of Paeonia lactiflora; and (4) the correlation between heavy metals in soil and different parts of Paeonia lactiflora.

2. Materials and methods

2.1 Study area

Bozhou City is located in the northwest of Anhui Province, China, with a total area of 8,429 square kilometers (km²). This study observes the Qiaocheng District in Bozhou City as the main study area, which is about 50 kilometers wide from east to west, and about 70 kilometers long from north to south (115°32’24“E − 116°01’35”̋E, 33°42’48“N − 34°04’56“N, Figure 1). Qiaocheng District covers a total area of 2,226 km², of which 1,320 km² is cultivated land. All the Paeonia lactiflora plants and soil samples were collected from this area.

Figure 1. Sample sites in Bozhou, Anhui Province, China.

2.2 Sample collection and preparation

In total, 14 Paeonia lactiflora plants (Chinese herbaceous peony) were uprooted together with their stems and leaves from Qiaocheng District of Bozhou City, Anhui province, China, in May 2021 (Figure 1). Then, after gently shaking to remove clumps of soil, the rhizosphere soil of each Paeonia lactiflora plant was collected with a brush. All the Paeonia lactiflora plants and soil samples were packed in sealed and labeled plastic bags and transported back to the laboratory for preparation.

In the laboratory, the Paeonia lactiflora plants were completely washed with tap water to remove adhering dust and soil, then washed several times with deionized water, and sequentially air-dried at room temperature. Then, the Paeonia lactiflora plants were separated into the roots (labeled as R1-R14), stems (labeled as S1-S14), and leaves (labeled as L1-L14) using a stainless steel knife. All the dried plant samples were ground into fine powder by an agate ball-grinder using 10-mm agate ball and then sieved through a 100-mesh sieve. Soil samples were also naturally air-dried and passed through a 2-mm nylon sieve to remove remnants of roots, twigs, stones, bricks, and other non-soil components. Then, the soil samples were ground into powder until all samples passed through the 100-mesh nylon sieve and labeled as T1-T14 for soil samples.

The powdered soil samples and roots, stems, and leaves of Paeonia lactiflora plants were all stored in sealed polyethylene containers, and kept in refrigerator at 4°C for preservation before further chemical assay.

2.3 Experimental determination

2.3.1 Soil physicochemical properties

The soil pH and organic matter (OM) of each soil sample were determined by standard methods. Soil pH was measured using the glass electrode method (NY/T 1377–2007) by soaking the pH probe in the soil suspension with a ratio of 1:2.5 of soil to ultrapure water (w/v). The content of the organic matter in each soil sample was measured using the potassium dichromate oxidation method (NY/T 1121.6–2006) through the oxidation of OM using K₂Cr₂O₇ in an acid medium.

2.3.2 Total concentrations of heavy metals

An aliquot of 40.0 mg of each powdered soil sample and roots, stems, and leaves of Paeonia lactiflora plants was weighted into the clean and dry labeled Teflon bottles. Each part of Paeonia lactiflora was digested with 4 mL of concentrated nitric acid (HNO₃), while an acid mixture of concentrated nitric acid (HNO₃) and hydrofluoric acid (HF) with a ratio of 1:1 (3 mL: 3 mL) were added into soil samples. Thereafter, all the samples were digested using the oven digestion system with the temperature of 180°C for 12 h. For soil samples, after digestion for 12 h, the residual acids in the Teflon bottles were expelled in an acid capture instrument for approximately 30–45 min at 150°C. Then, an additional of 2 mL HNO₃ and 1 mL of hydrogen peroxide (H₂O₂, 30%) were added and digested again in the oven at the temperature of 150°C for another 6 h. Finally, after cooling, all the digested soil samples, and digested root, stem and leaf samples of Paeonia lactiflora were filtered with 0.45 μm filter papers, and were diluted with distilled deionized water to 25 mL.

Total concentrations of HMs (Al, Fe, Mn, Cu, Pb, Zn, Cr, As, and Cd) in roots, stems, and leaves of Paeonia lactiflora, and that of Mn, Cu, Pb, Zn, Cr, As, and Cd in soil samples were determined by inductively coupled plasma-mass spectrometry (ICP-MS) (ICAPQ, Thermo, U.S.A). Due to the high content of Al and Fe in soils, the total concentrations of Al and Fe in soil samples were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) (ICAP 7200, Thermo, U.S.A) [15,16].

2.3.3 Extractable concentrations of heavy metals in soils

The total content of heavy metals in soils is a poor indicator of metal mobility and bioavailability to plants [17,18]. These properties of heavy metals in soils are based on their different chemical forms, which can be obtained through leaching of the soil using chemical extractants [18,19]. Diethylene triamine pentaacetic acid (DTPA), an efficient chelating agent with strong chelation, can quickly form water-soluble complexes with most of heavy metals. DTPA can release soluble, exchangeable, adsorptive, and organically bound metal fractions, and may also release some metals fixed by oxides [20]. Thus, in the present study, to estimate the mobility and bioavailability of HMs in soils, the diethylene triamine pentaacetic acid (DTPA) method (NT/T 890–2004) was used to extract the available concentrations of Fe, Mn, Cu, Pb, and Zn in soil samples. An aliquot of 10.00 g of dried soil samples (<2 mm) were placed into 150 mL labeled centrifuge tubes, and 20 mL of DTPA extractant was added. Then, all the samples were shaken for 2 h at a frequency of 200 r/min. Thereafter, the samples were centrifuged for 10 min at 4500 r/min, and the supernate of each sample was filtered through 0.45 μm filter paper into 25 mL labelled volumetric flask for further determination. The DTPA-extractable concentrations of Fe, Mn, Cu, Pb, and Zn in the filtrate were detected by flame atomic absorption spectroscopy (FAAS) (AA−6880, Shimadzu, Japan).

During the experiment, before use, all the glassware and plastic containers were soaked in 10% HNO₃ for at least 24 h, and then thoroughly washed with deionized water [9]. In addition, the concentrated nitric acid, hydrofluoric acid, and hydrogen peroxide used in the digestion were all of guaranteed reagent grade.

2.4 Quality control

A quality control program was conducted which included procedural blanks, replicate samples, and standard reference material (GBW 10,027 (GSB−18) for plants, GBW07403 (GSS−3) for soils, and GBW 07460 (ASA−9) for DTPA method). The determined heavy metal concentrations showed good agreement with the standard reference material, and the recoveries varied from 73.29% to 101.0% for plant samples, 85.70% to 112.2% for total concentrations of HMs in soil samples, and 83.88% to 113.9% for available concentrations of HMs in soils. The relative standard deviations (RSD) of the randomly selected replicate samples ranged from 0.08% to 8.68%.

2.5 Assessment of potential heavy metal accumulation

In this study, to assess the heavy metal accumulation in soils, the geoaccumulation index (Igeo), potential ecological risk index (RI), and pollution load index (PLI) were used.

2.5.1 Geo-accumulation index

The geoaccumulation index (Igeo) was calculated to evaluate the heavy metal contamination in soils using the following equation [21]:

(1) ${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}={log}_2(\frac{C_{\mathrm{i}}}{1.5{\mathrm{C}}_{{ }_0}^{\mathrm{i}}})$(1)

Where C_i is the measured total concentration of a single heavy metal i in soil samples, and${\mathrm{C}}_0^{\mathrm{i}}$is the geochemical background values of the heavy metal i. In this study, the soil background values of heavy metals in (North) Anhui Province were used [22,23]. The constant 1.5 is a correction factor that accounts for possible natural variations due to the lithogenic effects [24,25].

The geoaccumulation index (Igeo) included the following seven categories: practically unpolluted (Igeo ≦ 0, Class 0), unpolluted to moderately polluted (0 <Igeo ≦1, Class 1), moderately polluted (1 <Igeo ≦2, Class 2), moderately to heavily polluted (2 <Igeo ≦3, Class 3), heavily polluted (3 <Igeo ≦4, Class 4), heavily to extremely polluted (4 <Igeo ≦5, Class 5), and extremely polluted (Igeo ≧5, Class 6) [24,26].

2.5.2 Potential ecological risk index

The potential ecological risk index (RI) is a widely used method to assess the risk of heavy metal elements in soils and first proposed by Hakanson [27]. This index is calculated based on the following formula:

(2) $\mathrm{C}{\mathrm{F}}_{\mathrm{i}}=\frac{{\mathrm{C}}_{\mathrm{i}}}{{\mathrm{C}}_0^{\mathrm{i}}}$(2)

(3) ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}={\mathrm{T}}_{\mathrm{r}}^{\mathrm{i}}\times \mathrm{C}{\mathrm{F}}_{\mathrm{i}}$(3)

(4) $\mathrm{R}\mathrm{I}=\sum _{i=1}^n{E}_r^i$(4)

Where$\mathrm{C}{\mathrm{F}}_{\mathrm{i}}$is the contamination factor of an individual heavy metal i in soil samples, ${\mathrm{E}}_{\mathrm{r}}^i$describes the potential ecological risk coefficient of the heavy metal i, where r is the abbreviation of risk, and is not a variable; $T_r^i$ is its ‘toxic-response’ factor, indicating the toxicity level of heavy metals to organisms. In this study, the$T_r^i$of Mn, Cu, Pb, Zn, Cr, As, and Cd is 1, 5, 5, 1, 2, 10, 30, respectively [28]. In addition, RI represents the potential ecological hazard index of the total heavy metals in soil samples [29]. The classifications of ${\mathrm{E}}_{\mathrm{r}}^i$and RI are presented as follows: low risk (${\mathrm{E}}_{\mathrm{r}}^i$≦40; RI≦150), moderate risk (40<${\mathrm{E}}_{\mathrm{r}}^i$≦80; 150≦300), considerable risk (80<${\mathrm{E}}_{\mathrm{r}}^i$≦160; 3000), high risk (160<${\mathrm{E}}_{\mathrm{r}}^i$≦320; RI > 600), extreme risk (${\mathrm{E}}_{\mathrm{r}}^i$ >320) [30,31].

2.5.3 Pollution load index

The pollution load index (PLI) is used to indicate the contamination level of the total heavy metals in soil samples and is defined as follows [32,33]:

(5) $\mathrm{P}\mathrm{L}\mathrm{I}=\sqrt[\mathrm{n}]{C{F}_1\times C{F}_2\times \cdots \times C{F}_n}$(5)

Where n is the number of the analyzed HMs. The PLI divided the soil samples into four categories: no contamination (PLI ≦ 1), moderate contamination (1 <PLI ≦ 2), heavy contamination (2 < PLI ≦3), and extremely heavy contamination (PLI >3) [24].

2.6 Transfer factor

In order to evaluate the potential capability of heavy metal transfer from soils to Paeonia lactiflora, the bioconcentration factor (BCF) and translocation factor (TF) were calculated using the following equations [9,11,34,35]:

(6) $\mathrm{B}\mathrm{C}{\mathrm{F}}_{\mathrm{t}\mathrm{o}tal}=C_{Root(i)}/C_{total(i)}$(6)

(7) $\mathrm{B}\mathrm{C}{\mathrm{F}}_{\mathrm{a}\mathrm{v}ailable}=C_{Root(i)}/{\mathrm{C}}_{\mathrm{a}vailable(i)}$(7)

(8) $\mathrm{T}{\mathrm{F}}_{\mathrm{R}\mathrm{S}}={\mathrm{C}}_{\mathrm{S}\mathrm{t}\mathrm{e}\mathrm{m}(\mathrm{i})}/{\mathrm{C}}_{\mathrm{R}\mathrm{o}\mathrm{o}\mathrm{t}(\mathrm{i})}$(8)

(9) $\mathrm{T}{\mathrm{F}}_{\mathrm{S}\mathrm{L}}=C_{Leaf(i)}/{\mathrm{C}}_{\mathrm{S}\mathrm{t}\mathrm{e}\mathrm{m}(i)}$(9)

Where BCF_total represents the transfer capacity of a given heavy metal i from soil to root, calculated on the basis of the total (C_total(i)) concentrations of the heavy metal i in soil, and BCF_available is the transfer factor of a given heavy metal i from soil to root, calculated on the basis of the available (C_available(i)) concentrations of the heavy metal i in soil. In addition, TF_RS and TF_SL express the transfer capacity of a given heavy metal i from root to stem, and from stem to leave, respectively. C_Root(i), C_Stem(i), and C_Leaf(i) are the total concentrations of the heavy metal i in roots, stems, and leaves of Paeonia lactiflora, respectively.

2.7 Statistical analysis

In this study, correlation analysis was conducted to explore the relationship among soil pH, OM, and heavy metals in soils and each parts of Paeonia lactiflora. Data collection, statistics, and correlation analysis were conducted using Microsoft Office 2016 and the statistical software package SPSS 27 for Windows (IBM Corporation, U.S.A). In addition, the figures in this study were drawn using CorelDraw 2021 (Corel Corporation, Canada) and Origin 9.1 (Origin Lab, U.S.A), respectively.

3. Results and discussion

3.1 Soil physicochemical characteristics

Description of soil physicochemical parameters (pH and OM) of the 14 soil samples collected from Bozhou city are shown in Table 1. The soil pH values ranged from 8.06 to 8.32, with an average of 8.19, indicating slightly alkaline characteristics of the soils. The content of organic matter (OM) in the studied soil, varied from 10.50 to 29.87 g/kg, with an average of 17.00 g/kg.

Table 1. Statistical description of total and DTPA extracted heavy metal concentrations in soils and their background values (mg/kg).

		Max	Min	Mean	SD^a	CV(%)^b	Background value^c
Soil physicochemical properties	pH	8.32	8.06	8.19	0.08	0.92
	OM	29.87	10.50	17.00	6.39	37.58
Total	Al	67264	34434	57540	7946	13.81	67200
concentration	Fe	41200	18495	33024	5644	17.09	31400
	Mn	742.60	334.08	585.78	106.50	18.18	462
	Cu	30.74	12.28	23.13	5.38	23.24	26
	Pb	30.15	13.44	22.71	3.79	16.69	25
	Zn	70.25	36.24	57.27	9.23	16.11	66
	Cr	597.44	239.00	411.62	129.28	31.41	71
	As	15.51	7.40	12.63	2.28	18.02	10.7
	Cd	0.20	0.11	0.17	0.02	13.32	0.17
DTPA	Fe	64.18	43.97	50.02	5.75	11.50
	Mn	17.89	17.64	17.78	0.08	0.47
	Cu	3.09	1.44	1.98	0.56	28.47
	Pb	4.19	2.36	3.13	0.62	19.94
	Zn	2.19	1.15	1.63	0.33	20.13

^aStandard deviation (SD).

^bCoefficient variation (CV, %).

^cSoil background values in Anhui Province for Al, Fe, and Mn; soil background values in North Anhui Province for Cu, Pb, Zn, Cr, As, and Cd.

3.2 Characteristics of heavy metals in soils

The total concentrations of Al, Fe, Mn, Cu, Pb, Zn, Cr, As, and Cd in these soil samples were quite different (Table 1) and ranged from 34,434 to 67,264 mg/kg, 18495 to 41,200 mg/kg, 334.08 to 742.60 mg/kg, 12.28 to 30.74 mg/kg, 13.44 to 30.15 mg/kg, 36.24 to 70.25 mg/kg, 239.00 to 597.44 mg/kg, 7.40 to 15.51 mg/kg, 0.11 to o.20 mg/kg, respectively. The order on the basis of the average concentrations of HMs in soils was Al > Fe > Mn > Cr > Zn > Cu > Pb > As > Cd. This result is similar to that obtained in Hebei Province, China, which is close to Anhui Province in geological location [36]. In comparison with their respective soil background values in Anhui Province, the HMs were enriched to different degrees. Among them, the average values of Fe, Mn, Cr, and As were, respectively, 1.05, 1.27, 5.80, and 1.18 times higher than their corresponding soil background values (Table 1), showing that Cr was the most enriched element.

Moreover, results of the geoaccumulation index (Igeo), potential ecological risk index (RI), and pollution load index (PLI) were shown in Table 2. Based on the average value of the geo-accumulation index (Igeo), Cr (Igeo = 1.87) was the only moderately polluted element in the soil, while the other heavy metals (Al, Fe, Mn,Cu, Pb, Zn, As, and Cd) were categorized in the non-polluted scope (Igeo <0). In general, the results of assessing the contamination level of the total heavy metals in soil samples through the pollution load index showed that the studied soils presented moderate contamination (1 <PLI ≦ 2), with an average PLI value of 1.20, of which Cr was the largest contributor. Nevertheless, in this study, HMs assessed by potential ecological risk index had no potential risk to the soil environment (${\mathrm{E}}_{\mathrm{r}}^i$≦40, and RI≦150). Due to the high enrichment of Cr in comparison with its soil background value, and its high Igeo value, Cr was considered as the priority pollutant of the soil-Paeonia lactiflora system in this study, and might be affected by human activities. Therefore, special attention should be paid to soil Cr to prevent further accumulation in the soil.

Table 2. Results of the geoaccumulation index (Igeo), potential ecological risk index (${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ and RI), and pollution load index (PLI).

Igeo

${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$

RI

PLI

Max

Min

Mean

Max

Min

Mean

Range

Mean

Range

Mean

Al

−0.58

−1.55

−0.82

38.65–71.33

63.94

0.17–1.39

1.2

Fe

−0.19

−1.35

−0.54

Mn

0.10

−1.05

−0.27

1.61

0.72

1.27

Cu

−0.34

−1.67

−0.79

5.91

2.36

4.45

Pb

−0.33

−1.50

−0.76

6.03

2.69

4.54

Zn

−0.49

−1.45

−0.81

1.06

0.55

0.87

Cr

2.48

1.16

1.87

16.83

6.73

11.60

As

−0.05

−1.12

−0.37

14.50

6.92

11.80

Cd

−0.40

−1.30

−0.66

34.80

18.67

29.42

3.3 Concentrations of DTPA extractable heavy metals

The bio-available concentrations of heavy metals extracted by DTPA in soils were also shown in Table 1. It is reported that the toxicity assessment of heavy metals based on their bio-available concentrations in the soil is better than the assessment of total metal concentrations [19,20]. DTPA extraction is a widely used and effective method to evaluate the heavy metal bioavailability in soils [37]. In the present study, the content of Fe, Mn, Cu, Pb, and Zn extracted by DTPA were 43.97–64.18, 17.64–17.89, 1.44–3.09, 2.36–4.19, 1.15–2.19 mg/kg, respectively. On average, the proportion of metals extracted by DTPA was in a descending order as follows: Pb (13.78%) > Cu (8.57%) > Mn (3.04%) > Zn (2.84%) > Fe (0.15%), indicating the highest bioavailability of Pb in soil. The bio-available content of Cu, Pb, and Zn in this study was comparable with a previous study [38], but far lower than that in soil irrigated with contaminated water [5]. Due to the lack of the standard limits of bioavailable heavy metals in Chinese soil, the pollution degree based on DTPA extractable heavy metals was not evaluated [5].

Besides, correlation analysis was conducted between the total and DTPA extractable concentrations of heavy metals to evaluate their relationship (Table 3). First, significant correlations were shown between each two of HMs (Al, Fe, Mn, Cu, Pb, Zn, Cd and As) in the soil, with the correlation coefficients ranging from 0.790 to 0.993 (P < 0.01). The cross-correlations indicated the same sources of these heavy metals in the soil. However, Cr in the soil showed a negative correlation with the other eight HMs. Second, the content of Cu extracted by DTPA displayed significant positive correlation with its total concentrations (r = 0.809, P < 0.01) in the soil, indicating that the mineralogical source and pedological processes of the available Cu was the same as their total content [20]. However, with respect to Fe, Mn, Pb, and Zn, the relationship between their total and DTPA extractable content were poor.

Table 3. Correlation coefficient of total and DTPA extracted heavy metals in soils.

	Al	Fe	Mn	Cu	Pb	Zn	Cr	As	Cd	DFe	DMn	DCu	DPb	DZn
Al	1
Fe	.971**	1
Mn	.966**	.976**	1
Cu	.880**	.917**	.906**	1
Pb	.953**	.957**	.957**	.899**	1
Zn	.912**	.926**	.928**	.974**	.912**	1
Cr	−0.141	−0.218	−0.369	−0.351	−0.256	−0.309	1
As	.955**	.973**	.993**	.924**	.935**	.935**	−0.386	1
Cd	.878**	.899**	.855**	.790**	.900**	.849**	−0.041	.831**	1
Fe-DTPA	−0.221	−0.326	−0.195	−0.416	−0.211	−0.262	−0.001	−0.254	−0.150	1
Mn-DTPA	0.032	0.118	0.227	0.199	0.196	0.189	−.669**	0.209	0.138	0.336	1
Cu-DTPA	.635*	.758**	.796**	.809**	.748**	.757**	−.706**	.809**	.607*	−0.303	.576*	1
Pb-DTPA	0.389	0.497	.562*	.555*	0.520	.550*	−.610*	.550*	0.449	0.114	.833**	.834**	1
Zn-DTPA	−0.134	−0.101	0.013	−0.105	−0.005	−0.002	−0.418	−0.042	0.003	.618*	.717**	0.248	.637*	1

*Correlation is significant at P < 0.05 (2-tailed).

**Correlation is significant at P < 0.01 (2-tailed).

3.4 Heavy metals in different tissues of Paeonia lactiflora

The concentrations of HMs in different parts (roots, stems, and leaves) of Paeonia lactiflora were shown in Table 4. The average concentrations of HMs in stems and leaves adhered to the same order: Al > Fe > Zn > Mn > Cu > Cr > Pb > As > Cd, and the order of HMs in roots decreased in a similar pattern with that of HMs in soil: Al > Fe > Mn > Zn > Cr > Cu > Pb > As > Cd. By comparing the concentrations of HMs in different parts of Paeonia lactiflora, the average concentration of the nine heavy metals studied in leaves and stems were all higher than those in roots. Specifically, Al, Fe, Mn, Pb, Zn, Cr, and As were mainly concentrated in leaves, Cu and Cd were mainly accumulated in stems. Among the heavy metals studied, the Pb content in leaves and stems, respectively, was 17.49 and 9.25 times higher than that in roots, indicating a great transfer of Pb from root to stem and leaf. Furthermore, it is reported that air (atmospheric deposits) as well as soil is also an important source of heavy metals in plant leaves, which may lead to the higher content of heavy metals in leaves than other plant tissues [39]. As known, Pb is a semi-volatile element, and Pb from coal combustion, automobile exhaust and industrial activities tends to be attached to atmospheric fine particles and move over a long distance away from the emission sources [40]. It is reported that in the past few decades, with the rapid development of Bozhou’ economy, different types of industries have flourished in this city [41]. The highest concentrations of Pb in leaves were probably resulted from the atmospheric deposition as well as from the soil. More importantly, heavy metals accumulated in plant leaves can re-enter the soil as a result of plant leaf litter, which may pose potential risks to the soil ecosystem, and therefore need to be strengthened monitoring [5].

Table 4. Concentrations of heavy metals in root, stem, and leaf of Paeonia lactiflora, and their limits in CHMs (mg/kg).

	Root			Stem			Leaf			Standard limits
	Max	Min	Mean	Max	Min	Mean	Max	Min	Mean
Al	207.51	35.33	93.76	1118.49	77.11	375.92	1090.91	285.53	630.53
Fe	120.76	26.84	60.24	719.52	59.48	258.09	913.74	255.45	537.05
Mn	7.74	3.85	6.13	29.56	13.38	17.36	62.88	28.59	43.92
Cu	5.35	2.67	4.33	14.79	2.98	5.12	5.89	4.10	5.06	20
Pb	0.15	0.02	0.08	2.29	0.19	0.73	2.55	0.73	1.38	5
Zn	17.50	8.03	11.71	35.19	10.35	15.95	22.46	15.07	17.95
Cr	4.26	0.62	1.73	13.09	1.38	5.14	17.62	4.00	9.15
As	0.08	0.01	0.04	0.20	0.02	0.07	0.30	0.08	0.17	2
Cd	0.04	0.01	0.02	0.11	0.04	0.06	0.04	0.02	0.03	1

In addition, as the edible and medicinal parts of Paeonia lactiflora, heavy metal concentrations of Cu, Pb, As, and Cd in the roots were compared with the standard limits formulated by the Chinese Pharmacopoeia Commission (2020), which were 20, 5, 2, and 1 mg/kg, respectively. The results in this study showed that there were no elements with concentrations higher than the respective standard limits.

3.5 Transfer capacity of heavy metals

The transfer of HMs from soil to plants is an important pathway of human exposure through food chain [42]. Hence, the transfer capacity of HMs from the soils to different parts (root, stem, and leaf) of Paeonia lactiflora was evaluated using the bioconcentration factor (BCF) and translocation factor (TF), and the results were shown in Figure 2. The values of BCF and TF varied significantly among different HMs. In general, the bioconcentration factors (BCF_total) of HMs from soil to roots of Paeonia lactiflora were lower than 1 and descended in the following order: Zn > Cu > Cd > Mn > Cr > Pb > As > Fe > Al (Figure 2(a)). However, the bioconcentration factors (BCF_available) from soil to roots of Paeonia lactiflora calculated based on the concentrations of HMs in the soil extracted by DTPA were 7.39, 2.30, 1.19, 0.34, and 0.03 for Zn, Cu, Fe, Mn, and Pb, respectively (Figure 2(b)). This result suggested that Paeonia lactiflora was a plant with high potential to absorb Zn, Cu and Fe from the soil [11,43]. Additionally, the average translocation factors between different parts of Paeonia lactiflora were higher than 1, except for Cd with the average TF_SL value of 0.59 (Figure 2(c)). By analyzing the transfer factors (BCF and TF) of HMs from soil to root, root to stem and stem to leaf, the results showed that the transfer ability of most heavy metals was in the order of root to stem > stem to leaf > soil to root, indicating that the possibility of HMs transfer from soil to root was lower than the possibility of HMs transfer from root to stem and leaves. Our result was consistent with that in a soil-wheat system [9]. This might be due to more obstacles preventing the transfer of heavy metals from soil to the root, and once the metals entered the root, they would be transferred to the aboveground parts of the plant [42]. It is reported that soil properties, such as soil pH and organic matter, were important factors affecting HMs bioavailability and mobility [42,44]. The low BCF values of HMs in this study could be associated to the high soil pH (mean value of 8.19) in this study area.

Figure 2. Bioconcentration factor (BCF) and translocation factor (TF) of heavy metals (HMs) from soil to different parts of Paeonia lactiflora (a) BCF based on total concentrations of HMs from soil to root, (b) BCF based on concentrations of HMs extracted by DTPA from soil to root, (c) TF of HMs from root to stem, and from stem to leaf.

Among the heavy metals studied, the translocation factor of Pb from root to stem (TF_RS = 13.19) was the highest. The transfer factor of Pb from stem to leaf (TF_SL) and from soil to root (BCF_available) was 2.47 and 0.03, respectively. As described in section 3.3, the Pb content in leaves and stems was 17.49 and 9.25 times higher than that in roots, respectively, which might be due to the higher transfer capacity of Pb from root to stem, and from stem to leaf. In addition, studies have shown that the translocation of heavy metals from soil to plants has a good correlation with their bio-available concentrations in soil [19,38]. Among the heavy metals of Cu, Pb, Zn, Fe, and Mn, the proportion (13.78%) of Pb extracted by DTPA was the highest. However, a low bioconcentration factor (BCF_available) of Pb was found. The large amount of migration from root to stem could have resulted in the reduction of the Pb concentration in root, in turn leading to its low bioconcentration factor (BCF_available) from soil to root.

In addition, according to the average values of BCF_total and BCF_available, Zn and Cu showed the highest transfer capacity from soil to roots of Paeonia lactiflora, but displayed lower transfer capacity from root to stem, and from stems to leaves in comparison with the other HMs. In contrast, based on BCF_total, the proportion of Al and Fe transferred from soil to roots of Paeonia lactiflora was the least, while the transfer ability of Al and Fe was strong in the process of transferring from roots to stems and from stems to leaves. It has been reported that soil microorganisms, such as Bacillus subtilis and mycorrhizal fungi, can enhance metal solubility by increasing CO₂ release and decomposition of organic matter, and play different roles in different metal removal [45,46]. Therefore, the soil microorganisms may have affected the transfer capacity of HMs in soil.

3.6 Correlation analysis

The potential relationship among soil pH, OM, and heavy metals in soil and different parts of Paeonia lactiflora was discussed based on the results of correlation analysis (Table 5). First of all, root Cd was significantly positively correlated with soil pH (P < 0.05), and root Cr showed significant positive correlation with organic matter (P < 0.05). However, correlations among soil pH, OM and the other heavy metals in the roots of Paeonia lactiflora were not significant, suggesting that in addition to soil pH and organic matter, other soil properties, such as cation exchange capacity, and soil microorganisms might also influence HM transfer [10,45,46].

Table 5. Correlation coefficients of heavy metals between soil, root, stem and leaf of Paeonia lactiflora.

	Al-root	Fe-root	Mn-root	Cu-root	Pb-root	Zn-root	Cr-root	As-root	Cd-root
pH	0.133	0.164	−0.072	0.355	0.354	0.293	0.327	0.393	0.587
OM	0.336	0.508	0.265	0.409	0.051	0.373	0.577	0.405	−0.024
Al-soil	−0.056	−0.038	−0.504	.575*	0.300	−0.231	0.015	0.263	.559*
Fe-soil	−0.116	−0.136	−.604*	.538*	0.279	−0.340	−0.105	0.309	.699**
Mn-soil	−0.190	−0.157	−.669**	0.505	0.240	−0.302	−0.093	0.438	.677**
Cu-soil	−0.219	−0.236	−.618*	.554*	0.208	−.539*	−0.248	0.436	.615*
Pb-soil	−0.017	−0.011	−.557*	0.443	0.255	−0.302	−0.059	0.439	.623*
Zn-soil	−0.186	−0.173	−.588*	0.530	0.202	−0.383	−0.178	0.450	.597*
Cr-soil	.576*	0.483	.677**	0.035	0.172	0.330	0.410	−0.597	−0.449
As-soil	−0.235	−0.215	−.691**	0.518	0.263	−0.349	−0.149	0.392	.684**
Cd-soil	0.216	0.174	−0.342	0.502	0.343	−0.072	0.065	0.279	.633*
FeDTPA	0.189	0.412	0.135	−0.355	0.026	.657*	0.507	0.412	−0.361
MnDTPA	−0.038	0.086	−0.442	−0.065	0.089	−0.024	0.047	.777*	0.326
CuDTPA	−0.355	−0.347	−.819**	0.271	0.063	−.566*	−0.371	.792*	.806**
PbDTPA	−0.168	−0.051	−.651*	0.160	0.096	−0.308	−0.043	.959**	.643*
ZnDTPA	0.009	0.196	−0.176	−0.316	−0.134	0.270	0.186	.917**	0.185
	Al-stem	Fe-stem	Mn-stem	Cu-stem	Pb-stem	Zn-stem	Cr-stem	As-stem	Cd-stem
Al-root	0.085	0.103	0.366	−0.250	0.058	0.379	0.284	0.325	0.105
Fe-root	0.140	0.164	0.495	−0.196	0.073	0.452	0.292	0.303	0.180
Mn-root	−0.377	−0.333	0.283	−0.203	−0.385	0.030	0.159	−0.062	−0.218
Cu-root	0.012	0.010	0.275	−0.304	−0.091	0.098	−0.230	−0.194	0.410
Pb-root	0.173	0.216	0.245	−0.169	−0.029	0.159	−0.029	−0.070	0.133
Zn-root	−0.248	−0.258	0.281	−0.102	−0.269	0.006	−0.138	0.041	−0.099
Cr-root	0.010	0.095	.628*	−0.034	−0.083	0.444	0.161	0.044	0.297
As-root	.806*	.821*	0.444	.753*	.724*	0.659	0.450	0.669	0.609
Cd-root	.685**	.641*	0.202	−0.004	.642*	0.359	0.118	0.464	.651*
	Al-leaf	Fe-leaf	Mn-leaf	Cu-leaf	Pb-leaf	Zn-leaf	Cr-leaf	As-leaf	Cd-leaf
Al-stem	.625*	.678**	−0.419	0.428	.735**	0.161	.821**	0.509	0.491
Fe-stem	.629*	.681**	−0.429	0.466	.758**	0.240	.784**	.607*	.566*
Mn-stem	0.066	0.136	0.126	0.405	0.177	0.319	0.457	0.443	0.507
Cu-stem	0.123	0.158	−0.212	−0.277	0.153	−0.173	0.166	0.475	0.069
Pb-stem	0.497	.541*	−0.514	0.347	.743**	0.171	.701**	0.482	0.445
Zn-stem	0.234	0.321	−0.404	0.348	0.481	0.348	.579*	.577*	0.473
Cr-stem	0.280	0.374	−0.318	0.198	0.430	−0.062	.537*	0.431	0.295
As-stem	0.177	0.230	−0.484	0.228	.539*	0.113	0.518	0.207	0.289
Cd-stem	0.202	0.277	−0.299	0.334	0.350	0.332	0.523	0.387	.607*

Correlation is significant at P < 0.05 (2-tailed)

Correlation is significant at P < 0.01 (2-tailed)

Second, Mn and Cd in roots of Paeonia lactiflora are significantly correlated with most metal elements in the soil. Specifically, Mn in roots is negatively correlated with Fe, Mn, Cu, Pb, Zn, and As in soil, and positively correlated with Cr in soil. Cd in roots is significantly positively correlated with Al, Fe, Mn, Cu, Pb, Zn, As and Cd in soil. In addition, Cu in roots has significant positive correlation with soil Al, Fe, and Cu. However, the other heavy metals (Al, Fe, Pb, Zn, Cr and As) showed no significant correlation between soil and roots of Paeonia lactiflora. Moreover, Cu and Zn extracted by DTPA were significantly positively correlated with As and Cd in the roots of Paeonia lactiflora, while negatively correlated with Mn in the root. Third, the correlation results of heavy metals in stems and roots showed that most metals in stems of Paeonia lactiflora were not highly correlated with root metals, but Al, Fe, Cu, Pb and Cd in stems were positively correlated with As and Cd in roots. Fourth, there is a significant positive correlation between Al, Fe in the stems of Paeonia lactiflora and Al, Fe, Pb, Cr, As, and Cd in the leaves.

4. Conclusion

The accumulation and translocation of heavy metals from the rhizosphere soil to Paeonia lactiflora grown in Bozhou, Anhui Province, China, was discussed in this study. Combined geoaccumulation index, potential ecological risk index, and pollution load index were used to evaluate the heavy metal contamination and their environmental risk in the soil. The results showed that the soil was moderately contaminated by the nine HMs (Al, Fe, Mn, Cu, Pb, Zn, Cr, As, and Cd), and Cr was the largest contributor.

Distribution of HMs in Paeonia lactiflora showed that the metals in leaves and stems were higher than those in roots. Specifically, Al, Fe, Mn, Pb, Zn, Cr, and As were mainly concentrated in leaves, Cu and Cd were accumulated in stems. The highest concentrations of Pb in leaves were probably resulted from the atmospheric deposition as well as from the soil. Zn and Cu showed the highest transfer capacity from soil to roots of Paeonia lactiflora, but the proportion of Al and Fe transferred from soil to roots of Paeonia lactiflora was the least, which might be due to the high content of Al and Fe in soil. Moreover, Mn, Cu and Cd in roots of Paeonia lactiflora were significantly correlated with soil Mn, Cu and Cd, respectively. However, Al, Fe, Pb, Zn, Cr and As showed no significant correlation between soil and roots.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets analyzed during the current study are available from the corresponding author on reasonable request https://doi.org/0000-0002-8898-2779.

Additional information

Funding

This work was supported by the Natural Science Foundation of Anhui Province (2008085QD174), and the Natural Science Foundation of Anhui Province Universities (KJ2021ZD0142).