Spatial distribution and migration of lead and zinc and the influence of parent materials in typical paddy soils of Hunan Province, China

doi:10.21203/rs.3.rs-2717776/v1

To establish a clear understanding of the distribution and migration characteristics of lead (Pb) and zinc (Zn) in paddy soil in Hunan Province, a representative sampling design was employed. Specifically, the study selected 63 sampling points from various soil formations with paddy soil development in the province, and 343 soil samples were collected from different soil profiles. These soil samples were analyzed for various parameters, such as total Pb and Zn content, pH, organic matter, soil particle size, cation exchange capacity, and free Fe2O3 content. The objective of the comprehensive study was to examine the spatial distribution of Pb and Zn, their migration patterns, and the impact of soil formation on their concentration levels. The results revealed that the surface layer of paddy soil had Pb and Zn content ranging from 17.62 to 114.07 mg/kg and 44.98 to 146.84 mg/kg, respectively. Notably, Pb was found to be more influenced by human activities than Zn, and the comprehensive quality impact evaluations for both Pb and Zn indicated safety. The horizontal distribution maps demonstrated that high-content sampling points were mainly located downstream of the Xiangjiang River. Moreover, the vertical distribution of samples showed four predominant trends, including surface enrichment, sub-surface enrichment, surface, and bottom layer enrichment, and fluctuation. Interestingly, it was observed that Zn had a stronger migration ability than Pb. Additionally, the soil formation was found to have the greatest impact on Pb and Zn content in the bottom soil layer.

Hunan Province

paddy soil

lead

zinc

spatial distribution characteristics

parent materials

The quality of agricultural soil plays a critical role in ensuring food security, and the concentration of heavy metals within it is of particular concern. According to a 2015 geochemical survey of arable land in China, heavy metal contamination in agricultural soils covered an area of 11,387 km²(Report on Geochemical Survey of Farmland in China, 2015). Studies have shown that there is a positive correlation between the contents of heavy metals in soil and plants to a certain extent(Koucim et al., 2021). The accumulation of heavy metals in agricultural soils leads to an increase of heavy metals in crops, which in turn can enter the human body through the food chain, posing a threat to human health. While many scholars have studied the contents and distribution characteristics of heavy metals in regional soils such as soil around mining areas and arable land in catchments, they mainly focus on typical areas in small regions. There have been few studies on the distribution characteristics of heavy metals in agricultural soils in a large region(Guo et al., 2017; Song et al., 2018). Kriging interpolation is a widely employed technique for elucidating spatial distribution patterns(Bai et al., 2019; Hou et al., 2017). However, sampling for the migration patterns of heavy metals in soil profiles is typically conducted based on soil depth rather than soil pedogenesis(Ji et al., 2009; Nan and Li, 2000). Soil pedogenesis, as a soil layer with distinctive genetic characteristics, can better reflect the vertical migration patterns of heavy metals in soil profiles. After entry into the soil, heavy metals undergo varying degrees of vertical migration under the influence of natural and anthropogenic factors, ultimately forming distinct vertical distribution characteristics.A study conducted by Zeng Siyan et al.(Zeng et al., 2021) revealed that the farmland left fallow due to heavy metal soil pollution was concentrated in Hunan and Henan provinces, indicating the presence of heavy metal contamination in agricultural soils in Hunan Province. Hunan province possesses abundant lead and zinc mineral resources, as exemplified by the tin mines in the province, which contribute to the presence of Pb and Zn in surrounding farmland soils(Wang et al., 2010). Agricultural soils in grain cultivation areas are prone to anthropogenic pollution, while paddy soils, which represent a significant portion of agricultural soils in Hunan province, occupy a crucial position in agricultural production activities. Therefore, it is essential to determine the spatial distribution and migration characteristics of Pb and Zn in paddy soils in Hunan province.

Some studies have suggested that zinc (Zn) is beneficial for rice growth by promoting protein and chlorophyll synthesis, as well as alleviating cadmium (Cd) stress. In Hunan Province, where Cd content is generally high in the soil, applying zinc fertilizers in a scientific manner can be advantageous for rice growth(Lilay et al., 2019; Wang et al., 2020). However, excessive Zn can have an inhibitory effect on rice growth. Lead (Pb) primarily enters the soil through pesticides and fertilizers. When the concentration of Pb is low, it does not significantly affect rice growth, but when it is excessive, it can impede photosynthesis and produce toxic effects on root systems, which can ultimately reduce rice yield. Previous studies have indicated high levels of Pb and Zn in agricultural soils near mining sites(Huang et al., 2017). However, there is limited research on the concentration of Pb and Zn in non-mining agricultural soils, particularly in Hunan Province. Additionally, the influence of parent material on the concentrations of Pb and Zn in paddy soils remains largely unexplored. Therefore, this study focuses on Hunan Province as the research area and collected soil samples according to soil genetic layers from 0-140cm. The concentrations of Pb and Zn, as well as soil properties, were analyzed, and the horizontal distribution and vertical migration of Pb and Zn under the influence of human activities and parent materials were investigated. This study is significant for understanding the migration and transformation of Pb and Zn in paddy soil, and it could provide fundamental data for scientific rice cultivation and food safety production.

General Situation of the Research Area

The province of Hunan is situated in the southern part of China, between 108°47’ E to 114°15’ E and 24°38’ N to 30°08’ N. The climate of the region is classified as middle subtropical monsoon climate, with high temperatures and increased precipitation during the summer season, and moderate weather with reduced precipitation in winter. The average annual precipitation exceeds 1000 mm, while the average annual temperature ranges between 16-19℃, creating favorable conditions for rice growth. The soil types in Hunan province are diverse, with acidic soil being the predominant type.

Sample Collection

Firstly, the soil map, the current land-use map and the administrative division map of Hunan Province were overlapped. Secondly, we found the most representative parent materials in this area, and then sampling was accomplished in the place where the paddy soils were most concentrated in different parent materials. The simple random distribution method was applied to set up the sampling sites. A total of 63 sampling sites were determined, and soil samples of 0-140cm were collected in each soil occurrence layer from top to bottom, with a total of 344 soil samples were collected. Each sample was collected with "s-shaped" sampling method, at least 5 points were selected for mixing, and 1 kg soil sample was reserved by quartering method. The spatial distribution of sampling sites showed in Fig. 1.

Sample Treatment and Determination

The samples were first processed by removing gravel and residual plants, and then dried in the laboratory. Subsequently, they were ground to three different mesh sizes (10 mesh, 60 mesh, and 100 mesh) for further analysis. The total concentration of Pb and Zn was determined using the method “Solid and sediment—Determination of aqua regia extracts of 12 metal elements—Inductively coupled plasma mass spectrometry” (HJ 803-2016). The standard soil samples used for testing were GBW07405 (GSS-5) and GBW07407 (GSS-7), and the quality of the results passed the test of standard soil samples. Other soil properties such as pH, organic matter, soil particle-size, CEC, and free iron oxides were determined using SOIL SURVEY LABORATORY METHODS.

Data Processing

The ICQQ (influence index of soil comprehensive quality) was used to evaluate the comprehensive quality of the soil, while the RAF (relative accumulation coefficient) and W (enrichment coefficient) were used to assess the enrichment characteristics of Pb and Zn in the surface soil. The K (otherness coefficient) was used to evaluate the variation in Pb and Zn contents in soil formed by different parent materials. Statistical analysis was conducted using SPSS 22 and Excel 2019. The semivariogram was fitted using GS+2.0, and the geostatistical analysis wizard of ArcGIS 10.5 was used for spatial interpolation. Figures were generated using Origin 2018.

Horizontal distribution of Pb and Zn in surface-level paddy soil.

Pb and Zn contents in surface-level paddy soil.

Table 1 and Fig. 2 present the statistical summary of Pb and Zn concentrations in the topsoil of paddy fields in Hunan Province. The average concentration of Pb was 36.91 mg/kg, slightly higher than the background value of soil in Hunan Province. The average concentration of Zn was 76.90 mg/kg, falling between the soil background values of Hunan Province and China. The maximum content of Pb was 1.14 times the risk screening value for Soil Environmental Quality--Risk Control Standard for Soil Contamination of Agricultural Land (GB15618-2018), whereas the maximum content of Zn did not exceed the risk screening value for soil contamination of agricultural land. This suggests that some paddy soils in the study area may have been affected by external disturbance, but no pollution occurred. The concentrations of Pb and Zn followed a non-normal distribution, with Pb mainly distributed between 20-50 mg/kg, and Zn mainly distributed between 60-100 mg/kg, as seen in Fig. 3 and Fig. 4. However, the concentration of Zn was normally distributed after logarithmic transformation, indicating that Pb content was more likely to be affected by human activities than Zn content. The quality grades of Zn in soil were classified according to the Specification of Multi-Purpose Regional Geochemical Survey(Specification for Multi-Target Regional Geochemical Survey, 2014), and the results were shown in Table 2. It was found that 33.33% of paddy soils in Hunan Province were at the rich level, 22.22% were at the relatively rich level, 19.05% were at the medium level, 15.87% were at the relatively deficient level, and 9.52% were at the deficient level. Soils with rich and relatively rich zinc levels can be referred to as zinc-rich soils. Only 55.55% of paddy soils were classified as zinc-rich soils. Zn is a necessary element that is beneficial for the growth of rice. Shortage of Zn in the soil could inhibit the normal development and growth of rice(Muthukumararaja and Sriramachandrasekharan, 2012). Therefore, in order to ensure the normal growth of rice, zinc fertilizer should be scientifically applied to paddy soils that are deficient in Zn.

Table 1 The contents of Pb and Zn in surface paddy soil

Heavy metals	max	min	mean	pH	CV	kurtosis	skewness	K-S test	Soil background value in Hunan Province^[16]	Soil background value in China^[17]	the risk screening values^[17]
Pb	114.07	17.62	36.91	5.97	52.58	5.166	2.167	non-normal distribution	27	26	100
Zn	146.84	44.98	76.90	5.97	27.22	0.836	0.826	lognormal distribution	94	72.40	200

Note: The unit of the content of Pb and Zn is mg/kg, the pH is dimensionless.

Table 2 Quality classification of Zn in soil

Grade of rich and deficiency	rich	relatively rich	middle	relatively deficient	deficient
criterion（mg/kg）	84-200	71-84	62-71	50-62	≤50
sampling point numbers	21	14	12	10	6

Table 3 displays the evaluation results of paddy soil quality in the study area. Of the 63 sampling points, 61 were found to be in a safe status: 20 sampling points had an ICQQ of 0, indicating a background status, and 41 sampling points had an ICQQ of 0 < ICQQ ≤ 1, indicating a cumulative status, but not exceeding the standard. Only two sampling points, XT10 and ZZ20, were in a slight pollution status. No sampling points were found to be in milder pollution, moderate pollution, or heavy pollution. The average ICQQ was 0.36, indicating that the paddy soil in the study area was in a safe status.

Table 3 ICQQ values of surface paddy soil

evaluation standard	safe（ICQQ≤1）	slight pollution1<ICQQ≤2）	Mild pollution（2<ICQQ≤3）	moderate pollution（3<ICQQ≤5）	heavy pollution（ICQQ＞5）
sampling point numbers	61	0	2	0	0

Spatial distribution of Pb and Zn in surface paddy soil.

To provide a clearer representation of the horizontal distribution of Pb and Zn concentrations in the topsoil of paddy soils, a classification system was developed based on their average concentration, soil background values of Hunan Province, and the risk screening values for agricultural land. The spatial interpolation analysis module of ArcGIS was used to perform spatial interpolation on the Zn concentration, and the resulting maps were used to visualize the distribution of Pb and Zn in paddy soils in Hunan Province. It should be noted that the data for Pb content did not conform to a normal distribution and therefore was not subject to spatial interpolation. Based on Figure 5, it can be inferred that the distribution of Pb in paddy soil is heterogeneous, with higher concentrations generally observed in the eastern, central, and southern regions, and lower concentrations in the western and northern regions. By combining Figure 6 with the spatial interpolation shown in Figure 7, it can be concluded that the Zn content in paddy soil is relatively higher in the eastern region.

Heavy metals in soils can be transported to rivers during precipitation events and subsequently migrate downstream, resulting in long-range pollution and becomes a significant source of heavy metal contamination in downstream areas(Bird et al., 2010). The Xiangjiang River basin, with its large population and high contribution to the national GDP, has abundant lead-zinc mineral resources in its upstream region. The presence of numerous industrial enterprises in the area can lead to the discharge of pollutants, and the lack of proper management of many historical pollution sources has made it one of the more severely polluted basins in terms of heavy metal contamination(Zhang et al., 2015). Heavy metals in rivers are transported with water and can affect the soil within the downstream basin via surface and subsurface runoff. By combining Fig. 5, Fig. 6, and Fig. 7, it can be observed that there are similarities in the horizontal distribution of Pb and Zn. Correlation analysis showed that the correlation coefficient between Pb and Zn was 0.501 (p<0.01), indicating a positive correlation between the two metals. This suggests that Pb and Zn may have similar sources or coexist naturally, possibly due to their similar physicochemical properties as chalcophile and lithophile elements. The sampling points with high contents are mainly located in the middle and lower reaches of the Xiangjiang River. On the one hand, this may be due to the application of pesticides and fertilizers, and on the other hand, it may be because Pb and Zn migrate over long distances with the river and enter the soil through irrigation with river water(Zhang et al., 2015). In addition, the pH of the soil environment directly affects the migration and diffusion of Pb and Zn. The average pH of paddy soils in Changsha, Zhuzhou, and Xiangtan, which are located in the middle and lower reaches of the Xiangjiang River basin, is 5.49, lower than the average pH of paddy soils in the entire province. The acidic soil environment is conducive to the diffusion and migration of Pb and Zn. Consistent with the research results of Guo Chaohui et al.(2008), the spatial distribution characteristics of contaminant concentrations in the middle and lower reaches of the Xiangjiang River basin indicate a severe pollution problem, with relatively high levels of Pb and Zn contamination in agricultural soils located in Changsha, Zhuzhou, and Xiangtan. Overall, the Pb content in paddy soils falls within the normal range, which is likely attributed to the careful selection of sampling points, less affected by industrial activities. Spatial interpolation results reveal that the Zn content in many areas is below the soil background value of Hunan province. This can be attributed to the continuous uptake of Zn by rice plants from the soil during their growth, leading to a gradual reduction in the Zn content of the soil surface over time. To ensure adequate rice growth in regions with a zinc deficiency, appropriate application of zinc fertilizer is necessary.

Vertical Distribution Characteristics of Pb and Zn Content in Paddy Soil.

Vertical Distribution of Pb and Zn in Paddy Soil.

Studies have shown that the distribution of heavy metals in soil profiles is influenced by both natural and human factors, resulting in complex vertical distribution patterns(Zu et al., 2013). Fig. 8 and Fig. 9 illustrate the vertical distribution characteristics of Pb and Zn in paddy soils, indicating that their content displays complex vertical migration trends, such as surface enrichment, sub-surface enrichment, enrichment in both surface and bottom layers, and fluctuating changes. Despite the complex vertical migration characteristics of Pb and Zn, they still exhibit some similarities. Seven profiles showed an increasing then decreasing trend, accounting for 77.78% of Pb profiles and 46.67% of Zn profiles; four profiles showed a gradual decrease trend, accounting for 26.67% of Pb profiles and 80% of Zn profiles; nine profiles showed a decreasing then increasing trend, accounting for 64.29% of Pb profiles and 52.94% of Zn profiles; and fifteen profiles showed fluctuating trends, accounting for 62.5% of Pb profiles and 57.69% of Zn profiles. These findings suggest that Pb and Zn exhibit similar enrichment and migration characteristics during soil formation and human activities. This conclusion is consistent with research on the distribution of Pb and Zn in soil profiles in Xiamen and Zhangzhou(Zhang, 2013).

In the first type, which is characterized by surface enrichment and a gradually decreasing trend, the higher susceptibility of surface soil to external factors is suggested. The Pb content in these profiles was mainly divided into two trends. The first trend showed small fluctuations, with a maximum difference of 10.56mg/kg among different soil layers in the same section. The second trend showed larger variations, with a maximum difference of 64.79mg/kg among different soil layers in the same section. Compared to the variation in Pb content, the trend in Zn content was more stable, with a maximum difference of 38.55mg/kg among different depths in the same profile. The highest concentrations of Pb and Zn in this type mainly appeared in the surface soils, indicating that the input rate was faster than the downward migration rate. Additionally, the interception function of surface soils occurred in the process of vertical migration(Zhang et al., 1982), which explains why high concentrations appear in surface layers and low concentrations appear in the bottom layers.

The second type is characterized by sub-surface enrichment, which is evident in profiles where the maximum concentration of Pb is primarily found within the range of 10-30 cm. These profiles were mainly divided into two trends. The first trend showed a rapid increase and then decrease, which may be related to the tillage system. During tillage, the soil in the plow layer can be incorporated into the plow pan, leading to a rapid increase in concentration with depth. The second trend showed a slow decrease after the highest concentration appeared. The highest concentration of the first trend was higher than that of the second trend, and the variation range in the second profile was small, indicating that the first trend was more affected by human activities and the second trend was a natural enrichment affected by natural factors. The highest content of Zn appeared at different depths in different profiles, ranging from 10-20cm and 40-80cm. The environment of paddy soils was generally acidic in the study area. Zn was easily dissolved in the acidic environment, so it was prone to dissolution in the research area, which might be one of the reasons for the highest concentration appearing at different depths. The greater number of profiles exhibiting surface and sub-surface accumulation of Pb compared to Zn suggests that the mobility of Pb in profiles is lower than that of Zn, and that Pb is more strongly influenced by human activities than Zn.

The third type is characterized by enrichment in both surface and bottom layers. The minimum Pb concentration appeared at different depths in different sections, and the trend mainly presented a rapid decrease at first and then a slow increase. The average concentration in topsoil was 32.13 mg/kg, which was higher than the soil background value in Hunan Province, and decreased rapidly with depth, indicating that the concentration of Pb in topsoil was greatly affected by human activities. The variation of Zn content was divided into two categories. The first one showed drastic changes, with a maximum difference of 77.45mg/kg at different depths. The vertical distribution of heavy metals showed a gradually increasing trend under the influence of soil parent materials(Zinn et al., 2020), while it showed a gradually decreasing trend under the influence of human activities(Shao et al., 2014). This type indicates that surface soils are greatly affected by human activities, while subsoils are greatly affected by soil parent materials and soil-forming processes. The second one showed stable changes, with a maximum difference of 21.87mg/kg at different depths. Except for YY18 in subsoil, the Zn concentration in all profiles did not exceed the soil background value of Hunan Province and gradually increased with depth, indicating that these profiles are greatly influenced by parent materials.

The fourth type is characterized by fluctuating changes, which manifest as cyclic variations of decreasing and increasing magnitudes. Profiles exhibiting this type of change have large fluctuations in both Pb and Zn concentrations, and the maximum concentrations of these elements are also found in these profiles. Among them, the highest content of Pb appeared at a depth of 0-20 cm, which was 114.07 mg/kg, and the highest content of Zn appeared at a depth of 40-60 cm, which was 233.68 mg/kg. The soil type in the study area is paddy soil, and the long-term cultivation of rice has led to periodic oxidation-reduction in the soil. Oxidation-reduction changes the valence and morphology of Pb and Zn, and the migration ability of Pb and Zn with different valence and morphology is different in the soil(Fundamentals of soil science and soil geography, 1980). As a result, the profiles exhibit fluctuating distribution characteristics, and the amplitude of fluctuation is affected by the soil's physical and chemical properties and the intensity of the redox reaction.

Accumulation and migration of Pb and Zn

To assess the migration of Pb and Zn in shallow paddy soil and elucidate their enrichment characteristics, the RAF (relative accumulation coefficient) and W (enrichment coefficient) were calculated and presented in Table 4. The RAF values for Pb and Zn in the plow layer and plow pan were greater than 1, indicating an upper-accumulation pattern. The RAF value for Pb was higher than that of Zn, suggesting that the migration ability of Pb was weaker than that of Zn. Previous studies on heavy metals in soil have also reported that the migration capacity of Zn in soil was stronger than that of Pb(Sun et al., 2022; Wong et al., 2002; Zhang et al., 2018), a finding that has been confirmed by indoor simulation experiments(Zu et al., 2013). The migration capacity of heavy metals in the soil profile was closely related to the adsorption capacity of soil colloids for heavy metal ions. The adsorption capacity of soil colloids for Pb²⁺ was greater than that for Zn²⁺, and the colloidal compounds of Pb were more stable than those of Zn(Liu et al., 2010), which explains why the migration capacity of Pb was weaker than that of Zn. The W values for Pb and Zn in the plow layer and plow pan were greater than 0, indicating their enrichment in shallow soil. In the same soil layer, the W value for Pb was greater than that for Zn. Previous studies have shown that heavy metals imported from outside were mainly accumulated in the soil surface(Liu et al., 2006). The analysis indicates that the impact of human activities on the Pb content in paddy soils in the research area is greater than that of Zn, and the migration ability of Pb is weaker than that of Zn, resulting in a stronger enrichment ability of Pb in different soil layers. In different soil layers, the W value of Pb was highest in the plow pan, followed by the plow layer, indicating a stronger enrichment capacity of the plow pan. As a compact soil layer under the plow layer, the plow pan was not conducive to the transfer of elements. The W value for Zn in the plow layer and plow pan was small, indicating a relatively weak enrichment ability of shallow soils. This phenomenon was related to the lesser influence of human activities and the strong vertical migration ability of Zn.

Table 4 RAF values and W values of Pb and Zn in paddy soils in shallow layers

soil layer	RAF (the relative accumulation coefficient)		W (enrichment coefficient)
soil layer	Pb	Zn	Pb	Zn
plow layer	1.888	1.428	0.554	0.104
plow pan	1.556	1.267	0.908	0.094

Pb and Zn content and differentiation analysis in paddy soils developed from different parent materials

Pb and Zn content in paddy soils developed from different parent materials

The soil parent material represents the primary source of heavy metals and is an inherent and integral component that cannot be overlooked in soil analysis. Fig. 10 shows the content characteristics of Pb and Zn in paddy soils formed by different parent materials. The highest concentration of Pb was observed in paddy soil developed from fluvial sediment in the plow layer, while the lowest concentration was observed in subsoil formed by the combination of purple sand weathering and quaternary red clay. The biggest concentration difference was observed between the soil developed from fluvial sediment in the plow layer and subsoil, and in the plow pan, it appeared in soil developed by plate shale weathering. Similarly, the highest concentration of Zn was observed in soil developed from plate shale weathering in the plow layer, and the lowest concentration was observed in soil developed from purple sand shale weathering in subsoil. The biggest concentration difference was observed between the soil developed from plate shale weathering in the plow layer and plow pan, while in subsoil, the biggest concentration difference was observed in soil developed from quaternary red clay. Generally, soil developed from weathered sandy shale had relatively low elemental contents, which is consistent with the findings of previous studies such as Qu et al.(2020). The weathering of parent rock can be divided into in-situ weathering and non in-situ weathering. The abundance of elements in soil formed by in-situ weathering is mainly affected by the concentrations of elements in the bedrock. On the other hand, the element concentrations in soil formed by non in-situ weathering are affected by many factors, which leads to great uncertainty and wide fluctuation range. For example, fluvial sediment belongs to the non in-situ weathering, which explains the wide changes in element concentrations(Luo et al., 2021).

Differences in Pb and Zn contents among paddy soils from different parent materials

The coefficient of variation, as proposed by He et al.(2008), was used to measure the differences in Pb and Zn contents among different soil parent materials. The results are presented in Table 5. In plow layer and plow pan, the greatest difference in Pb concentration was observed between soils developed from granite weathering and lake sediments of purple sand shale, and the greatest difference in Zn concentration was observed between soils developed from granite weathering and the combination of purple sand weathering and quaternary red clay. In subsoil, the greatest differences in Pb and Zn concentrations were observed between soils developed from fluvial sediment and the combination of purple sand weathering and quaternary red clay. The K values of Pb and Zn were shown to be highest in subsoil, followed by plow pan and plow layer. The coefficient of variation (CV) followed the same trend as the K values, with the greatest fluctuation observed in subsoil. These results indicate that the concentrations of Pb and Zn in subsoil are most strongly influenced by the parent materials. To further clarify the influence of parent materials, this study treated it as the influencing factor. The content of Pb in different parent materials was found to have a non-normal distribution in different soil layers, so a nonparametric test was used. The results showed that there was no significant difference in Pb concentration with different parent materials in plow layer and plow pan (P>0.05), while there was a significant difference in Pb concentration with different parent materials in subsoil (P<0.05), indicating that the parent materials had a remarkable effect on the concentration of Pb in subsoil. The content of Zn with different parent materials had a normal distribution in different layers and passed the homogeneity test, so a one-way ANOVA was used. The results showed that in plow layer, Zn concentration developed by limestone weathering had a significant difference compared to that developed by granite weathering and plate shale weathering. In plow pan, Zn concentration developed by purple sand shale weathering had a significant difference compared to that developed by granite weathering, plate shale weathering, and fluvial sediment. In subsoil, Zn concentration developed by fluvial sediment had a significant difference compared to that developed by granite weathering, quaternary red clay, plate weathering, purple sand shale weathering, and purple shale weathering. A significant difference in Zn content was also observed between soils developed from limestone weathering and purple sand shale weathering, while there was no significant difference in Zn content among soils developed from other parent materials. Overall, the soil parent material had an impact on the Zn concentration in each soil layer, with the greatest effect observed in the subsoil. Paddy soils in the subsoil exhibited the strongest inheritance from the parent material. These findings suggest that the parent material plays a significant role in shaping the Zn content of soil, particularly in the subsoil layer.

Table 5 K values of Pb and Zn formed by different parent materials

heavy metal	plow layer	plow pan	subsoil
Pb	2.52	3.28	6.32
Zn	1.85	3.01	4.68

(1)The Pb and Zn contents in surface paddy soils ranged from 17.62 mg/kg to 114.07 mg/kg and 44.98 mg/kg to 146.84 mg/kg, respectively, with mean contents of 36.91 mg/kg and 76.90 mg/kg, respectively. The concentrations of Pb and Zn in surface paddy soils did not exceed the risk screening values, indicating no potential threat to rice production safety. A positive correlation between the concentrations of Pb and Zn was observed. High content sampling points were located in the middle and lower reaches of the Xiang River Basin, and the comprehensive soil quality was considered to be in a safe state.

(2) The distribution trends of Pb and Zn in soil profiles can be classified into four types: surface enrichment, sub-surface enrichment, surface and bottom enrichment, and fluctuation. Although the distribution trends are complex, some similarities can be observed. Specifically, four profiles exhibit surface enrichment, seven profiles exhibit sub-surface enrichment, nine profiles exhibit surface and bottom enrichment, and fifteen profiles exhibit fluctuation. These results suggest that Pb and Zn elements have similar patterns of enrichment and migration during soil formation, development, and later human cultivation processes.

(3) In the plow layer and plow pan, the largest difference in Pb concentration was observed between granite weathering and lake sediments of purple sand shale, and the largest difference in Zn concentration was observed between granite weathering and the combination of purple sand weathering and quaternary red clay. In the subsoil, the largest difference in Pb and Zn concentrations was observed between fluvial sediment and the combination of purple sand weathering and quaternary red clay. The K values of Pb and Zn were highest in the subsoil, followed by the plow pan and plow layer. The non-parametric test and one-way ANOVA results indicated that parent material has the greatest influence on the Pb and Zn content in the subsoil layer.

Acknowledgments

The authors would like to acknowledge all the help we received in the process of completing the thesis and the laboratory facility for analytical work provided by Hunan Agricultural University.

Author contributions

Yi-fan XIANG, Chen-ran WANG, Jia-qi CHEN, Ning-xiang OUYANG determined the concentrations of Zn and Pb and the physical and chemical properties of paddy soils. Yi-fan XIANG, Chen-ran WANG, Da-juan WAN, Jun lin HUANG, Xue-ying CAO, Jia-qi CHEN, analyzed the results of this experiment, Yi-fan XIANG composed the manuscript, Da-juan WAN, Yi-fan XIANG discussed and revised the manuscript.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Funding

This research was supported by Hunan Innovative Province Construction Special Fund(2020NK2001) and the Natural Science Foundation of Changsha (kq2007082).

Declarations

Ethical statements

This research was a basic research on Pb and Zn concentrations in paddy soil in Hunan province, and it did not require ethics approval.

Consent to participate

The authors consent to participate.

Consent for publication

The authors consent for publication.

Conflict of interest

The authors declare no competing interests.

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No competing interests reported.

Spatial distribution and migration of lead and zinc and the influence of parent materials in typical paddy soils of Hunan Province, China

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Abstract

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Introduction

Materials And Methods

Results and analysis

Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

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