Catena 196 (2021) 104926

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Key driving factors of selenium-enriched soil in the low-Se geological belt: A T case study in Red Beds of Sichuan Basin, China ⁎ Yonglin Liua,b, Xinglei Tianc,d,e, , Rui Liua,b, Shuling Liua,b, Andrew V. Zuzaf a The Key Laboratory of GIS Application Research, Chongqing Normal University, Chongqing 401331, China b Geography and Tourism College, Chongqing Normal University, Chongqing 401331, China c Shandong Institute of Geological Sciences, Jinan 250013, China d Key Laboratory of Gold Mineralization Processes and Resource Utilization Subordinated to the Ministry of Land and Resources, Jinan 250013, China e Key Laboratory of Metallogenic Geological Process and Resources Utilization in Shandong Province, Jinan 250013, China f Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA

ARTICLE INFO ABSTRACT

Keywords: Selenium (Se) is an essential micronutrient for humans given its varying health benefits. It is generally­ re Red Beds region cognized that China has a wide belt of low-Se soil stretching from the northeast to southwest. Nevertheless, there Geodetector are Se-enriched areas distributed in the low-Se belt of China. However, the quantificational relationships among Selenium soil properties, topographic characteristics, parent materials, land use and soil Se content in those Se-enriched Soil organic matter soils remain to be elucidated. Similarly, the key driving factors of the Se-enriched soil in the low-Se geological Spatial variation belt need to be documented. These aims could be an useful basis for evaluating the health of the soil ecosystem (in terms of Se toxicity or deficiency) and the potential intake of Se by humans from soils to food crops and animal products. To solve the above questions, Jiangjin district, Chongqing City, as an area in low Se red beds, was selected, and 156 topsoil samples were collected to explore the relationships between Se content in topsoil under various land use type and soil properties (pH, organic matter (OM), major elements content in topsoil) and topographic characteristics such as elevation (H), slope (SL), and topographic wetness index (TWI) and geolo­ gical condition (stratum). Geostatistics, principal component analysis (PCA) and Geodetector were used to analyze the controlling factors of Se distribution in topsoil. The results show that in the analyzed topsoil (1) Se contents vary from 0.039 to 1.110 mg/kg, with a mean of 0.315 mg/kg, and higher than the background value of Chinese soil (0.290 mg/kg). 82.3% are classified as having moderate Se levels (Se > 0.175 mg/kg). (2) The Se contents in northeast of Jiangjin district are higher (Se > 0.315 mg/kg) than the midwest (Se < 0.315 mg/kg). (3) The key controlling factor of the soil Se distribution is OM. In addition, the soil weathering and leaching process and pedogenic rock are the secondary factors controlling Se distribution. Together, these findings reveal that Se-rich soil of low Se belt tends to be distributed in regions with higher OM, stronger leaching, and car­ bonate parent materials. These observations are beneficial to explore the Se-rich soil resources in low Se region in China.

1. Introduction the key factor of the health of the entire ecosystem (Liu et al., 1984; Tan, 1989; Tan et al., 2002; Natasha et al., 2018; López-Bellido et al., Selenium (Se), as essential nutrient trace element for humans, has 2019; Castro et al., 2020; Ngigi et al., 2020). However, because of the antioxidant, anti-aging and anticancer properties (Rayman, 2002; complexity of the geographical environment, the soil Se distribution is Zhang et al., 2009; Dinh et al., 2018; Liu et al., 2018b; Natasha et al., uneven on the surface of the Earth (Zhang et al., 2005; Sun et al., 2008; 2018). The threshold of Se for human is narrow; deficient or excessive Yamada et al., 2009; Gabos et al., 2014; Ni et al., 2016). The soil Se intake may cause diseases (Navarro-Alarcón and Cabrera-Vique, 2008; content in Se-deficient ecological environment is lower than 0.01 mg/ Zhu et al., 2008; Li et al., 2011; Rayman, 2012; Yuan et al., 2012; Wang kg (Fordyce, 2007), and it could be up to 2018 mg/kg in high Se geo­ et al., 2017; Natasha et al., 2018). The human body’s intake of Se is logical background (Zhu et al., 2008). For these reasons, Se content and linked to soil Se through the food chain. Therefore, Se content in soil is distribution in soil and its association with controlling factors are of

⁎ Corresponding author at: No 52, Lishan Road, Lixia district, Jinan City, Shandong Province, China. E-mail address: [email protected] (X. Tian). https://doi.org/10.1016/j.catena.2020.104926 Received 19 February 2020; Received in revised form 11 September 2020; Accepted 15 September 2020 Available online 01 October 2020 0341-8162/ © 2020 Elsevier B.V. All rights reserved. Y. Liu, et al. Catena 196 (2021) 104926 growing interest for both the scientific community and government content band (Tan, 1989; Tan et al., 2002). However, there are also Se departments in the world (Dinh et al., 2018; Ullah et al., 2019; Song enrichment areas in the low-Se belt of China, such as Ziyang, Shaanxi et al., 2020; Tolu et al., 2020). So, better understanding of key con­ Province (Se ≥ 3.0 mg/kg) (Luo et al., 2004). Specifically, the red bed trolling factors of Se distribution can help more accurately evaluate Se soils of southwest China, with Mesozoic terrestrial clastic rock, is gen­ toxicity or deficiency. erally considered to be deficient (Se < 0.125 mg/kg) or marginal Numerous factors affect the Se content in the soil, including parent (0.125 mg/kg ≤ Se < 0.175 mg/kg) with regard to Se contents (Liu material, soil physicochemical properties (pH, organic matter (OM), et al., 1984; Tan, 1989; Tan et al., 2002; Dinh et al., 2018). major elements), topography and land use (Tuttle et al., 2014; Dinh The red beds of Southwest, China, with Mesozoic terrestrial facies et al., 2018; Liu et al., 2018a; Natasha et al., 2018; Shao et al., 2018; clastic rock, is mainly distributed in Sichuan Basin and Central Yunnan Chang et al., 2019; Ngigi et al., 2020; Xiao et al., 2020). Commonly, Province (Cheng et al., 2004). Sichuan Basin has commonly been excessive Se regions (Se ≥ 3.0 mg/kg) are underlain by Se-rich black identified as a deficient or marginal Se region (Tan, 1989; Tan et al., shale rocks, such as Ziyang, Shaanxi Province (Luo et al., 2004), Enshi, 2002; Dinh et al., 2018; Liu et al., 2018a; Luo et al., 2018). Huang and Hubei Province (Zhu et al., 2008), Taoyuan, Hunan Province (Ni et al., Yuan (1997) investigated the Se content in soil from Yibin 2016) and Kaiyang, Guizhou Province (Tong et al., 2013). However, Se- (Se = 0.141 mg/kg), Neijiang (Se = 0.211 mg/kg) and Suining deficient regions (Se < 0.175 mg/kg) exposed in sandstone and silt­ (Se = 0.285 mg/kg) in south-middle of Sichuan Basin and found that stone with low Se contents, such as Suining, Sichuan Province (Liu soil Se content was lower than Chinese soil background value et al., 2018b), Liangping, Chongqing City (Tong, 2016). With the pro­ (Se = 0.290 mg/kg). The soil in Bazhong (Se = 0.154 mg/kg) and cess of soil formation, the effect of soil physicochemical properties on Nanchong (Se = 0.140 mg/kg) was marginal at Se < 0.175 mg/kg (Liu the Se content tends to increase (Matos et al., 2017). Therefore, these et al., 2018a). Tong (2016) researched the spatial distribution of soil Se regions without black shales also have moderate Se soil enrichment, in Chongqing of Eastern Sichuan Basin and found that the soil Se such as Guilin, Guangxi Province (Shao et al., 2018). It is indicated that content (Se = 0.173 mg/kg) in eastern and northern of Chongqing City parent materials play critical contribution to excessive Se soil, while has was lower than 0.175 mg/kg, while that (Se = 0.215 mg/kg) of little effect on Se-rich soil. southern and western of Chongqing City was at moderate Se level Soil physical and chemical properties (pH and OM) also affect soil (0.175 ≤ Se < 0.4 mg/kg). These results showed that pedogenic rock Se distribution. Soil pH shows a negative correlation with Se content were all Mesozoic terrestrial clastic rock, but the soil Se content had (Shao et al., 2018; Xing et al., 2018), although some researchers found significant variations due to the geographical differences. In addition, that there was no significant relationship between soil pH and soil Se Tong (2016) also found that there was Se-rich soil in Red Beds of content (Zhang et al., 2009; Xu et al., 2018). Selenite (+4) and selenate Southwest, indicating that in traditional low-Se region also exist Se-rich (+6) could be adsorbed by soil OM, which affect the content of Se in soil. The spatial distribution of soil Se in the Sichuan Basin has been soil (Dinh et al., 2019). Generally, OM-rich soil had higher Se content, previously studied (Tan et al., 2002; Tong, 2016; Liu et al., 2018a), but such as dark-brown earth and black soils in northeastern China (Tan there are some problems still need to be further explored: (1) what is et al., 2002). When the soil OM content is very low, oxy-hydroxides (Al, the spatial distribution of soil Se content in Red Beds with Se-rich area? Fe, Mn) are responsible for adsorption of soluble Se (Zhang et al., 2009; (2) what are the main factors affecting the spatial distribution of soil Se Hurst et al., 2013; Shao et al., 2018). Therefore, soil physicochemical content? properties that controlled the speciation of Se had indirect effect on To provide insight into these, this study focused on the hilly Jiangjin enrichment and dispersion of Se in soil. district, Chongqing City, which is rich in Se resources (Tong, 2016; Liu Moreover, topography can indirectly affect the soil Se distribution et al., 2018b). The objectives of this study included (1) characterizing by causing the reassignment of soil mineral, moisture and energy. The the spatial distribution of Se in soils from the Jiangjin district; (2) Se content of soil in the regions with serious water and soil erosion is comparing the relationship of soil Se content to soil physicochemical very low, such as Keshan-disease area of Rangtang, Sichuan Province properties, parent material, topography, and land use; and (3) de­ (Wang et al., 2017). Conversely, the Se content in soil in the area with termining the key factors controlling soil Se distribution. This research gentle terrain shows relative enrichment (Luo et al., 2004; Zhu et al., will provide a theoretical basis for scientific management and the ef­ 2008). Land use, which is an important human factor, always strongly ficient utilization of Se-rich soil land in low-Se geological belt. influences soil properties, particularly OM(Smith, 2008; Xiao et al., 2020). Thus, land use has an important impact on the Se content and 2. Materials and methods distribution of soil (Tuttle et al., 2014; Xiao et al., 2020; Zhu et al., 2020). 2.1. Study area But at present, most studies have focused on the independent impact of single factor or several factors (Zhang et al., 2009; Dinh et al., 2018; Jiangjin district, Chongqing City, is located in southeast Sichuan Xu et al., 2018; Xiao et al., 2020), and failed to conduct a quantitative Basin, which is mainly distributed at purple soil with the low Se content analysis of the natural (such as soil pH, OM, major elements, topo­ (0.086 mg/kg) (Tan et al., 2002). Jiangjin district, covering 3200 km2, graphy, and parent material) and human (land use types) environ­ is located at 105°49′-106°38′ east longitudes and 28°28′-29°28′ north mental factors on Se content distribution. latitudes. And it belongs to the subtropical monsoon climate region The lithosphere of the Chinese continent is influenced by the with mean annual temperature of 28.5 °C and 1031 mm of precipita­ Eurasian, Indian, and Pacific plates, each of which has diverse and tion. This district spans three geomorphic zones including the east Si­ complex geologic histories and characteristics. Accordingly, the geo­ chuan parallel ridge-and-valley area, southern Sichuan hill area, and graphical distribution of soil Se in China is non uniform (Tan et al., middle-low mountain area around the basin (JJPG, 2015). According to 2002; Dinh et al., 2018). Based on the total Se concentration in soil, Tan classification system of the basic morphological types of Chinese ter­ (1989) proposed abundance and deficiency thresholds of Se in soil. The ritory (Cheng et al., 2011), geomorphologic types were classified to hill levels of Se in soil can be classified into five grades: deficient (Se < (< 200 m), low-relief mountain (200–500 m), middle-relief mountain 0.125 mg/kg), marginal (0.125 ≤ Se < 0.175 mg/kg), moderate (500–1000 m) and high-relief mountain (1000–2500 m). The terrain in (0.175 ≤ Se < 0.400 mg/kg), high (0.400 ≤ Se < 3.000 mg/kg) and Jiangjin is high in the south and low in the north, and the landforms can excessive (Se ≥ 3.000 mg/kg) (Tan, 1989; Tan et al., 2002). It has been be characterized as valley terrace, hilly, and mountain area. The valley shown that China has a wide belt of low-Se soil stretching from the terrace area occupies 3.1% of the land area, and mainly distributed northeast to southwest (Tan, 1989; Tan et al., 2002; Dinh et al., 2018), along the Yangtze River. The hilly area has an elevation of 250–500 m and the red beds region in the southwest is located within this low-Se and occupies 65.1% of the total area, mainly distributed in syncline

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Fig. 1. Geological map (A) and Land use types (B) of Jiangjin, Chongqing City. Legend description: K1w: Wotoushan Formation of Early Cretaceous (debris sand­ stone); J3p: Penglaizhen Formation of Late Jurassic (aubergine mudstone and sandstone); J3sn: Suining Formation of Late Jurassic (red mudstone and siltstone); J2s: Shaximiao Formation of Middle Jurassic (Dark purple sandstone and mudstone, Purple sandstone); J1-2zl: Ziliujing Group of Early Jurassic (variegated mudstone, sandstone); T3xj: Xujiahe Formation of Late Triassic (sandstone, shale and carbonaceous shale with coal seam); T2l: Leikoupo Formation of Middle Triassic (dolomite, limestone and variegated shale); T1j: Jialingjiang Formation of Early Triassic (dolomite, limestone and Saline breccia); T1f: Feixianguan Formation of Early Triassic (Dark purple mudstone, marlstone and limestone). valley. The low mountain region is distributed in the northern part, and the proportions of the cultivated land, woodland and construction land the inverted middle-low mountain is mainly located in southern area. are 48.6%, 45.9% and 1.4%, respectively. The Central region is the Soil types in Jiangjin district include purple soil, paddy soil, yellow soil, main center of agricultural, livestock and poultry production in the and limestone soil. The purple soil, covering about 78.5% of the culti­ Jiangjin. The Southern region, in total 743.1 km2, has an intermediate vated land in Jiangjin, is mainly distributed in central hilly region in the to low mountain landscape, of which the cultivated land, woodland and study area (JJPG, 2015, Liu et al., 2018b). construction land make up 14.4%, 83.3% and 0.8%, respectively. So, From a geological point of view, Jiangjin district is located in the Southern region has the lowest level of economic development in transitional zone of the East Sichuan Fold and the Sichuan-Guizhou SN Jiangjin. Tectonic Belt (SGMB, 1991) (Fig. 1A). The Jiangjin district was affected by Sichuan movement at late Cretaceous (the third period of Yanshan movement) (SGMB, 1991; Liu et al., 2018b). Influenced by tectonic 2.2. Sampling and analysis movement, a number of tight anticlinal and open synclinal structures are arranged sub-parallel to each other from west to east (Fig. 1A). A total of 156 topsoil samples (0–20 cm) were collected in Jiangjin Triassic strata make up the core of anticlines and Jurassic strata com­ district, Chongqing City (Fig. 1A). The sampling sites were chosen prise the core of synclines. Most of the land surface exposes at Jurassic based on the towns of Jiangjin and rock stratigraphic units. These soil (J) strata, covering about 78.7% of the total area, followed by Cretac­ samples were naturally air-dried at room temperature, and plant roots eous (K) and Triassic (T) strata (Fig. 1A). The Cenozoic stratigraphy and rubble in soil were removed. The dried soil was ground into 200 mainly includes Quaternary river sediments distributed in valley areas. mesh. The soil Se content was measured using the hydride generation The lithology of Cretaceous strata is fluviolacustrine sandstone inter­ atomic fluorescence spectrometry (HG-AFS) (Niu and Luo, 2011; Tian bedded with mudstone, which is distributed in the inverted middle-low and Luo, 2017). Using the techniques in the geological industry stan­ mountain of Jiangjin district. Jurassic strata consist of shallow lacus­ dard (MLRPRC, 2016), the concentrations of CaO, MgO, Fe2O3, Al2O3, trine and fluviolacustrine sandstone, siltstone, and mudstone, whereas Na2O, K2O and P in soil were determined by the inductively coupled Jurassic strata are mainly distributed in hills of the central part in the plasma-optical emission spectrometer (ICP-OES, PerkinElmer, Optima study area. Triassic strata with limnetic clastic rock and shallow marine 5300DV). The soil pH in ultrapure water without carbon dioxide (1:2.5, carbonate rock are distributed in anticlinal low mountain. The lithology w/v) was determined using a pH meter (Tabatabai and Bremner, 1969; in early-middle Triassic is mainly dolomite and limestone with clay Xu et al., 2018). The soil OM was determined using the potassium bi­ rocks, while that of late Triassic is clastic rocks with coal seam. chromate method (Huang et al., 2015). According to the socioeconomic level and geographical character­ To control experimental quality, soil reference materials (GBW07401, istics, Jiangjin is divided into three areas: the Northern region, the Se = 0.14 ± 0.03 mg/kg; GBW07403, Se = 0.09 ± 0.02 mg/kg; and Central region and the Southern region (Fig. 1B, JJPG, 2015). The GBW07406, Se = 1.34 ± 0.17 mg/kg) obtained from the National geomorphologic features of the northern area are anticline low moun­ Standard Sample Study Center, Beijing, China, were analyzed together tain and syncline trough valley, and the total area is 482.1 km2, of with each batch soil sample. The Se recovery rate was 95–104%. The re­ which the cultivated land, woodland and construction land are with the lative standard deviation of the measurements was better than 10% for proportion of 40.7%, 40.7% and 12.2%, respectively. The Northern both major elements and trace elements. region is the industry cluster, and the industry types mainly include The coefficient of weathering and eluviation, defined as BA, was materials, auto-motorcycle, equipment, electronics and food. The Cen­ used to represent the leaching degree of soil base ions (CaO, MgO, Na2O 2 tral region, covers a total area of 1908.6 km , its landscape is hilly, and and K2O) (Xing et al., 2018). The BA value was calculated as follows:

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Fig. 2. Topography in the Jiangjin District: (A) elevation; (B) slope; (C) aspect; and (D) topographic wetness index (TWI).

CaO+ MgO + Na O + K O BA = 2 2 2.3. Data sources Al2 O 3 Digital Elevation Model (DEM: resolution is 30 m; DEM data were The soil base ions (CaO, MgO, Na2O and K2O) are more easily lea­ sourced from the China Geospatial Data Cloud) was selected as data ched by water, whereas Al2O3 is more stable. Therefore, smaller values sources. Terrain factors including elevation (H), slope (SL), aspect (AT), of BA imply stronger degrees of leaching (Chen et al., 2016). and topographic wetness index (TWI) were extracted using ArgGIS (ESRI Inc., 2010) (Fig. 2). Slope (Fig. 2B) and aspect (Fig. 2C) were extracted via the Spatial Analyst Tool in ArcGIS 10.2. Slope values were

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Table 1 The classification results of impact factors.

Factors 1 2 3 4 5 6 7 Classification methods

Se (mg/kg) 0.043–0.124 0.124–0.175 0.176–0.300 0.301–0.400 0.401–0.600 0.601–1.110 – Natural breakpoint pH < 5.0 5.0–6.5 6.5–7.5 – – – – According to PRC (2016) OM (%) < 0.6 0.6–1.0 1.0–2.0 2.0–3.0 3.0–4.0 ≥4.0 – According to Xing et al. (2019)

Al2O3 (%) 4.06–6.20 6.21–8.34 8.35–10.48 10.48–12.62 12.63–14.76 14.77–16.90 – Equal interval CaO (%) 0.10–1.21 1.22–2.32 2.33–3.43 3.44–4.54 4.55–5.65 5.66–6.76 – Equal interval

Fe2O3 (%) 1.07–2.37 2.38–3.44 3,45–4.51 4.52–5.58 5.59–6.65 6.66–7.72 – Equal interval

K2O (%) 0.51–0.90 0.91–1.29 1.30–1.68 1.69–2.07 2.08–2.46 2.47–2.85 – Equal interval MgO (%) 0.42–0.93 0.94–1.44 1.45–1.95 1.93–2.45 2.46–2.95 2.96–3.45 – Equal interval

Na2O (%) 0.12–0.65 0.66–1.18 1.19–1.71 1.72–2.24 2.25–2.77 2.78–3.30 – Equal interval P (mg/kg) 109.2–334.5 334.6–559.8 559.9–785.1 785.2–1010.4 1010.5–1235.7 1235.8–1461.0 – Equal interval Mn (mg/kg) 65.0–192.8 192.9–320.6 320.7–448.5 448.6–576.3 576.4–704.1 704.2–832.0 – Equal interval BA 0.21–0.40 0.41–0.59 0.60–0.78 0.79–0.97 0.99–1.16 1.17–1.35 – Equal interval H (m) 184–268 269–353 354–437 438–521 522–605 605–692 – According to MWRPRC (2008) SL (°) 0–4.9 5–7.9 8–14.9 15–24.9 25–34.9 35–72.9 – According to MWRPRC (2008) TWI 4.2–5.5 5.6–6.8 6.9–8.1 8.2–9.4 9.5–10.7 10.8–12.0 – Equal interval

S K1w J3p J3sn J2s J1-2zl T3xj T1f According to SGMB (1991) Land Forestland Cropland Built-up land – – – – According to Xue and Mou (2016)

Note: BA: the coefficient of weathering and eluviation; H: elevation; SL: Slope; TWI: Topographic wetness index; S: Stratum; Land: Land use types.1 K w: Wotoushan Formation of Early Cretaceous; J3p: Penglaizhen Formation of Late Jurassic; J3sn: Suining Formation of Late Jurassic; J2s: Shaximiao Formation of Middle Jurassic;

J1-2zl: Ziliujing Group of Early Jurassic; T3xj: Xujiahe Formation of Late Triassic; T1f: Feixianguan Formation of Early Triassic classified as flat (< 5°), flat gentle (5–8°), gentle (8–15°), gentle steep more X) have an interactive influence on a response variable Y. Value (15–25°), steep (25–35°), or sharp steep (> 35°) (MWRPRC, 2008). closes to q = 1 indicates that Y is completely determined by the risk Aspect was divided into north (337.5–22.5°), northeast (22.5–67.5°), factors of X1 and X2, and value of q = 0 indicates that there is no east (67.5–112.5°), southeast (112.5–157.5°), south (157.5–202.5°), coupling between Y and more X (X1 and X2). In this study, soil Se southwest (202.5–247.5°), west (247.5–292.5°), and northwest content is the dependent variable, and the independent variable (X) (292.5–337.5°) end-members. The TWI was used to quantitatively de­ included soil pH, soil OM, and stratum (S), major elements in soil scribe the relationship between topographical factors and soil moisture (Al2O3, CaO, Fe2O3, K2O, MgO, Na2O, P, Mn), coefficient of weathering content. The higher the value of TWI was, the higher the soil moisture and eluviation (BA), elevation (H), slope (SL), TWI and land use (Land). content was (Fig. 2D) (Wang et al., 2009). The following formula was used: 2.5. Statistical analysis SCA TWI= ln tan The Kruskal-Wallis method in the non-parametric statistics methods is used to compare the Se contents in different variables (e.g., soil pH, where SCA was the contributing area per unit length of the contour OM, BA, stratum, H, SL, TWI and Land) because the data distribution 2 (m /m); β was the slope at a given point in degrees. did not follow normal distribution or logarithmic distribution. The The land use of Jiangjin district, Chongqing city was obtained from correlations between Se content and other factors are evaluated using Geospatial Data Cloud in China (http://www.gscloud.cn/) (Fig. 1B). Spearman’s rank correlation analysis. Principal component analysis (PCA) is used to explore the association between soil Se, soil physico­ 2.4. Geodetector chemical properties (pH, OM, major elements), and topographic para­ meters (H, slop, TWI). Effects are considered statistically significant The geographical detector (Geodetector) is a new statistical method with p < 0.05 based on two-tailed tests. to detect the spatial heterogeneity and reveals the driving factors be­ SPSS 18.0 and Excel 2010 were used for the statistical analysis. hind it (Wang and Xu, 2017). It includes factor detector, interaction ArcGIS 10.2 software was used to analyze spatial distribution char­ detector, ecological detector, and risk detector. In order to quantita­ acteristics of Se concentration and topographical parameters. tively analyze environmental impacts (soil pH, OM, major elements, Geodetector could be downloaded from http://www.geodetector.cn/, pedogenic rock, topography, and land use) and their interactive influ­ the first column is soil Se content (Y) and the following columns is ences on Se content distribution, factor detector and interaction de­ impact factors (X), including stratum, soil physicochemical properties tector were used in this paper. (pH, OM, major elements), topographic parameters (H, slop, TWI), and Factor detector was used to detect the influence degree of the in­ land use patterns. Geodetector can analyze the controlling factors by dependent variable (X) on the dependent variable (Y). The expression is discretizing the data (Wang and Xu, 2017), and the classification results as follows: of impact factors are listed in Table 1. L 2 Nh h SSW q= 1 h=1 = 1 N 2 SST 3. Results and discussion

L 2 2 SSW = Nh ; SST= N 3.1. The Se content in top soil h=1 h where h (h = 1, 2, ……, L) is strata of independent variable (X) or The soil Se contents in the Jiangjin district vary from 0.039 to dependent variable (Y); Nh and N are the number of units in a h strata 1.110 mg/kg (averaging 0.315 mg/kg) (Table 2), which is higher than 2 2 and study area, respectively; δh and δ are the variances of Y in a h the background soil level in China (0.290 mg/kg) (Chen et al., 1991) or strata and study area, respectively. The value of q is strictly within [0, that of the Chongqing region of Three Gorges Reservoir Region 1], and q = 0 indicates that there is no coupling between Y and (0.160 mg/kg) (Luo et al., 2018). The Se contents in the Jiangjin district X, while q = 1 indicates that Y is completely determined by X. are lower than that of American soil background values (0.390 mg/kg) Interaction detector reveals whether the risk factors X1 and X2 (and (Shacklette and Boerngen, 1984). The order of Se content in soil from

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Table 2 disease (KSD) areas in Zhangjiakou city of Hebei province (0.156 mg/ The statistical characteristic of soil Se content in Jiangjin district, Chongqing kg), Songpan county of Tibet (0.192 mg/kg) and Rangtang county of City (mg/kg). Sichuan province (0.152 mg/kg). The soil Se content of Jiangjin is Stratum N min max mean SD higher than that of Bazhong city (0.154 mg/kg), Nanchong city (0.140 mg/kg), Yibin city (0.141 mg/kg), Neijiang city (0.211 mg/kg) Total 156 0.039 1.110 0.315 0.170 and Suining city (0.285 mg/kg) in Red Beds region of Sichuan Basin K w 1 0.278 0.278 0.278 – 1 (Table 3). In summary, the topsoil Se content in Jiangjin of Red Beds J3p 8 0.039 0.316 0.234 0.086

J3sn 31 0.068 0.515 0.266 0.102 region of Sichuan Basin, where Se deficiency was considered as J2s 97 0.074 1.110 0.322 0.194 common (Tan, 1989; Huang and Yuan, 1997), is a Se-rich region. J1-2zl 15 0.109 0.531 0.367 0.119 T3xj 2 0.294 0.416 0.355 0.086 3.2. Spatial distribution of selenium in the top soil T1j 2 0.453 0.708 0.581 0.181

T1f 1 0.412 0.412 0.412 – K 1 0.278 0.278 0.278 – Using the test data of soil samples and the inverse distance weighted J 151 0.039 1.110 0.310 0.170 method (IDW), the overall geographical distribution patterns of total Se T 5 0.278 0.708 0.457 0.153 in topsoil of Jiangjin were analyzed (Fig. 3). The Se content of topsoil in northeast Jiangjin exceeds the mean Se content of topsoil in Jiangjin Note: K1w: Wotoushan Formation of Early Cretaceous; J3p: Penglaizhen district (0.315 mg/kg) (Fig. 3 and Table 4), whereas those of central, Formation of Late Jurassic; J3sn: Suining Formation of Late Jurassic; J2s: western, and southern Jiangjin are relatively low (Se < 0.315 mg/kg). Shaximiao Formation of Middle Jurassic; J1-2zl: Ziliujing Group of Early

Jurassic; T3xj: Xujiahe Formation of Late Triassic; T1j: Daye Formation and Therefore, the soil Se content is higher in northern and eastern Jiangjin,

Jialingjiang Formation of Early Triassic; T1f: Feixianguan Formation and and lower in central and western Jiangjin. Most of the soils in the Jialingjiang Formation of Early Triassic; K: Cretaceous; J: Jurassic; T: Triassic. Jiangjin district hold moderate Se contents ranging from 0.175 mg/kg to 0.40 mg/kg (∼74%), but ∼23% of the district have elevated values different strata was1 T j (0.581 mg/kg) > T1f (0.412 mg/kg) > J1-2zl higher than 0.40 mg/kg, which is the Se-rich level (Tan et al., 2002; (0.367 mg/kg) > T3xj (0.355 mg/kg) > J2s (0.322 mg/kg) > K1w CGS, 2019). (0.278 mg/kg) > J3sn (0.266 mg/kg) > J3p (0.234 mg/kg) (Kruskal- Wallis test, p = 0.042 < 0.05) (Table 2). Bedrock lithology partially 3.3. Influence factor affects Se content in soil (Tuttle et al., 2014). Ni et al. (2016) studied the spatial distribution of soil Se in Taoyuan, Hunan Province, China, To explain the spatial differentiation mechanism of Se content in and found that the mean Se content (3.18 mg/kg) in soil from lower topsoil of Jiangjin, the spearman correlations between soil Se content Cambrian black shale (rock Se with an average value of 21.59 mg/kg) and soil physicochemical properties and topography features were was the highest, whereas the soil with a mean value of 0.34 mg/kg from analyzed (Table 5). the middle and upper Cambrian limestone (rock Se with an average value of 0.68 mg/kg) was the lowest. Zhu et al. (2008) found that ex­ 3.3.1. Correlation between soil physicochemical properties and Se cessive Se in soil was distributed in Permian strata including carbo­ The results showed that Se content in topsoil is negatively corre­ naceous shale and stone coal, whereas low-Se soil was distributed in lated with soil pH (r = −0.195, p < 0.05) (Table 5). This observation early Triassic strata with limestone and mudstone. Ma et al. (2012) is in line with numerous other studies that have shown significant ne­ found that the spatial distribution of Se-rich soil was consistent with gative correlations between soil pH and soil Se content (Hofer et al., that of Se-rich sandstone of Paleogene Xining Group. 2009; Tong et al., 2013; Matos et al., 2017; Shao et al., 2018). The soil Spatially, 8.3%、8.9%、60.5% and 22.3% of soil samples in pH indirectly affects soil Se content by controlling Se adsorption onto Jiangjin were Se-deficient, Se-marginal, Se-moderate and Se-rich, re­ oxy-hydroxides, clay minerals and OM. Studies show that in alkaline spectively, indicating that Jiangjin is a Se-rich region in China. The conditions, selenate (+6) is the main form in water-soluble Se and mean Se content in soil from Jiangjin is 1.09, 0.62, 0.30, 1.05, 1.66, easily leaching. In contrast, under the acidic condition, selenite (+4) 0.95 and 0.41 times than that of Chinese Mainland, Japan, Scotland, becomes stronger than selenate (+6) in adsorption by OM and oxy- Switzerland, São Paulo of Brazil, Belgium and Hong Kong, respectively hydroxides (He et al., 2018; Chang et al., 2019; Dinh et al., 2019). (Table 3). The soil Se content in Jiangjin is 0.41 times and 0.22 times According to specification of land quality geochemical assessment of than that of Taoyuan, Hunan Province and Kaiyang, Guizhou Province China (PRC, 2016), the soil pH of Jiangjin was classified as strong acid of China with Se-excessive, respectively. Se has been proved to have (pH < 5.0), acidity (5.0 ≤ pH < 6.5) and neutral (6.5 ≤ pH < 7.5) many biological effects including antioxidant, anti-aging, and ­antic (Table 6). Soil pH ranges from 4.32 to 7.18 (mean of 5.98), which ancer properties and so on (Rayman, 2002, 2012; Natasha et al., 2018). demonstrates that the Jiangjin topsoil is acidic. Eleven soil samples If Se concentration is above or below the relatively narrow critical with an average Se content of 0.344 mg/kg (range 0.196–0.466 mg/kg) ranges, endemic diseases may occur, for example, Keshan and cardio- are belong to strong acid soil (pH < 5.0). Acidic soils (5.0 ≤ pH < vascular diseases (Tan, 1989; Rayman, 2012; Wang et al., 2017). Thus, 6.5) comprise 119 soil samples with Se mean value of 0.326 mg/kg the Se concentration of topsoil from Jiangjin district is compared with (0.039–1.110 mg/kg). Neutral pH soils (6.5 ≤ pH < 7.5) were found the Se level of topsoil from other longevous, Keshan, Kaschin-Beck, and in 27 soil samples, and their mean Se value was 0.256 mg/kg selenosis areas in China. Jiangjin district, Chongqing City, Yongfu (0.099–0.453 mg/kg). Together, these results demonstrate Se enriched county, Guangxi Province (Shao et al., 2018), Yongjia county, Zhejiang in acidic soils. The pH affects soil Se content and availability by influ­ Province (Xu et al., 2018) and Rugao county, Jiangsu Province (Sun encing Se adsorption onto oxy-hydroxides and clay minerals. Eich- et al., 2008) are longevity counties. However, the Se content of topsoil Greatorex et al. (2007) found that more Se were leached at neutral soil, in Jiangjin (0.315 mg/kg) is lower than that of Yongfu county and decreasing pH was associated with less Se leaching from pH 6.3 to (0.80 mg/kg), close to that of Yongjia county (0.382 mg/kg), and higher pH 4.9. than that of Rugao county (0.130 mg/kg). The Se soil content in There is a significant positive correlation between soil OM and soil Jiangjin is much lower than that of selenosis area in Naore village, Se content (r = 0.660, p < 0.01) (Table 5). Soil OM is an important Ziyang county of Shaanxi province (4.78 mg/kg) and Yutangba village component that impacts on the Se content (Li et al., 2017; Dinh et al., of Hubei province (4.75 mg/kg). But Se content in surface soil in 2019), but mechanisms of the OM-Se immobilization are complex (Li Jiangjin is higher than these of Kaschin-Beck disease (KBD) and Keshan et al., 2017). Soil OM immobilizes Se by both biotic and abiotic process (Li et al, 2017; Dinh et al., 2019). Based on the second nationwide

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Table 3 The Se content in surface soil from Jiangjin District and other area of China and world (mg/kg).

Region Se Source Health

Jiangjin 0.039–1.110(0.315) This study Longevity county Japan 0.05–2.80(0.51) (Yamada et al., 2009) – Scotland < 0.06–19.2(1.04) (Shand et al., 2012) – Sweden < 0.05–13.3(0.30) (Shand et al., 2012) – São Paulo state, Brazil < 0.08–1.61(0.19) (Gabos et al., 2014) – Belgium 0.14–0.70(0.33) (Temmerman et al., 2014) – Hongkong 0.07–2.26(0.76) (Zhang et al., 2005) – Taoyuan, Hunan 0.18–7.05(0.76) (Ni et al., 2016) – Kaiyang, Guizhou 0.46–2.31(1.42) (Tong et al., 2013) – Yongjia, Zhejiang 0.157–0.633(0.382) (Xu et al., 2018) Longevity county Yongfu, Guangxi 0.27–1.40(0.80) (Shao et al., 2018) Longevity county Rugao, Jiangsu 0.051–0.168(0.130) (Sun et al., 2008) Longevity county Ziyang, Shaanxi 0.50–16.96(4.78) (Du et al., 2018) Selenosis Yutangba, Hubei 0.41–42.3(4.75) (Zhu et al., 2008) Selenosis Zhangjiankou, Hebei 0.066–0.263(0.156) (Ge et al., 2000) Keshan disease Songpan, Tibet 0.059–0.305(0.192) (Wang et al., 2017) Kashin–Beck disease Zamtang, Sichuan 0.091–0.262(0.152) (Zhang et al., 2009) Kashin–Beck disease Bazhong, Sichuan 0.084–0.510(0.154) (Liu et al., 2018a) – Nanchong, Sichuan 0.033–0.349(0.140) (Liu et al., 2018a) – Yibin, Sichuan 0.080–0.225(0.141) (Huang and Yuan, 1997) – Neijiang, Sichuan 0.107–0.298(0.211) (Huang and Yuan, 1997) – Suining, Sichuan 0.223–0.345(0.285) (Huang and Yuan, 1997) –

Note: Health was defined as the special area where was longevity or endemic disease (Keshan disease, Kashin–Beck disease and Selenosis).

Table 4 Summary of the Se content for cultivated soils on township level in Jiangjin District, Chongqing City (mg/kg).

Township min max mean

Jiangjin District 0.039 1.110 0.315 Northern 0.068 1.110 0.381 Middle 0.074 0.779 0.290 Southern 0.039 1.043 0.290 Jijiang 0.068 1.110 0.415 Degan 0.093 0.801 0.411 Zhiping 0.093 0.853 0.427 Shuangfu 0.095 0.477 0.308 Youxi 0.237 0.307 0.272 Wutan 0.222 0.537 0.331 Shimen 0.085 0.314 0.226 Zhuyang 0.143 0.417 0.283 Shima 0.099 0.466 0.275 Yongxing 0.146 0.523 0.296 Tanghe 0.127 0.303 0.229 Baisha 0.140 0.327 0.252 Longhua 0.273 0.403 0.349 Lishi 0.083 0.405 0.257 Ciyun 0.196 0.449 0.273 Caijia 0.161 1.043 0.351 Zhongshan 0.039 0.365 0.238 Jiaping 0.109 0.416 0.269 Bolin 0.136 0.241 0.206 Xianfeng 0.167 0.779 0.367 Luohuang 0.214 0.736 0.402 Jiasi 0.211 0.509 0.332 Xiaba 0.182 0.607 0.356 Xihu 0.141 0.335 0.231 Dushi 0.074 0.446 0.278 Guangxing 0.186 0.531 0.354 Fig. 3. Map of Se distribution characteristics for cultivated soils in the Jiangjin Simianshan 0.331 0.383 0.357 District. general soil survey in China (Xing et al., 2019), soil OM was divided studied, but in general Se content increased strongly with increased soil into very deficient (OM < 0.6%), deficient (0.6%≤OM < 1%), mar­ OM. ginal (1%≤OM < 2%), moderate (2%≤OM < 3%), rich The BA value could represent the weathering and eluviation degree (3%≤OM < 4%), and very rich (OM ≥ 4%). Soil OM contents of of base cations in the soil (Chen et al., 2016). The lower the BA value is, Jiangjin are 0.069–4.540%, with an average value of 1.53%, which is at the more severe the base ion (CaO, MgO, Na2O, K2O) leaching will be. a moderate OM level in soil. Total soil Se is higher when soil OM is And a negative relationship between soil Se content and BA could be below 4% (Fig. 4), whereas when OM is > 4%, soil Se content suddenly observed in Jiangjin (r = −0.358, p < 0.01). There is a significant drops. The exact driving factor for this observation remains to be positive correlation between CaO, MgO and soil pH (r = 0.370, p < 0.01; r = 0.218, p < 0.01, respectively), but soil pH shows

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Table 6 Factor loading for 15 items principal component analysis. AT Parameters PC1 PC2 PC3 PC4 PC5

pH 0.376 −0.056 −0.104 −0.672 0.046 OM −0.024 −0.106 0.846 −0.045 −0.047 Se −0.210 −0.041 0.886 0.053 0.023 SL Al2O3 −0.052 0.868 −0.031 0.283 −0.022 CaO 0.883 0.085 0.046 −0.116 0.022

Fe2O3 −0.055 0.913 0.150 −0.085 −0.061

K2O 0.287 0.649 −0.117 0.254 −0.141 MgO 0.673 0.550 −0.201 0.005 −0.090

H Na2O 0.356 0.171 −0.044 0.730 0.011 P 0.399 0.314 0.627 0.146 0.133 Mn 0.415 0.668 −0.038 −0.058 0.188 BA 0.950 0.055 −0.043 0.181 −0.027 H −0.395 −0.023 0.053 0.024 0.566

BA SL 0.052 −0.094 −0.109 −0.248 0.701 TWI −0.179 −0.067 −0.151 −0.377 −0.642 Eigenvalue 4.165 2.224 1.868 1.292 1.190 Explained variance (%) 27.767 14.829 12.454 8.617 7.933 Explained variance 27.767 42.596 55.050 63.667 71.600 accumulated (%) Mn

Note: BA: the coefficient of weathering and eluviation; H: elevation; SL: Slope; TWI: Topographic wetness index.

significant negative correlation with Na2O (r = −0.175, p < 0.05). This indicates that base ions (CaO, MgO, Na2O, K2O) affected soil Se content by influencing the soil pH (Musa et al., 2017). The BA of soil O P

2 exhibits a significant positive relationship with Al and Fe in soil (Table 5), whereas there is no significant relationship between Se content and Al and Fe. The effect of Al and Fe on Se content is relatively weaker in the Jiangjin district. The total Se in soil exhibits significant positive correlation with total P in soil (r = 0.223, p < 0.01) (Table 5). There is a complex re­ lationship between Se and P in soil (Xing et al., 2018). Moreover, the application of P fertilizers should increase the Se content in soil (Exo­ genous Se) (Yu et al., 2014). The Jiangjin district is an important O MgO Na

2 agricultural area in Chongqing City. The total P in surface soil of K Jiangjin ranges from 109.20 to 1461.00 mg/kg, with a mean of 597.85 mg/kg. Total P in 64.1% of soil samples are higher than mean

3 value of P in surface soil of China (500 mg/kg) (Chen et al., 2019), O 2 indicating that fertilization may affect on the P content in surface soil Fe and bring some exogenous Se (He et al., 2018; Natasha et al., 2018).

3.3.2. Effect of topography on Se content in top soil

CaO Topography is an important factor that influences soil formation. Topography indirectly influences the soil physicochemical properties and material composition of soil by controlling the redistribution of 3 O

2 matter, energy, and biology (Chen et al., 2016; Wang et al., 2017; Shao

Al et al., 2018). Elevation (H), slope (SL), aspect (AT) and TWI are basic parameters that can quantify the effect of topography on Se content of soil. Elevation and slope are used to describe the topographic relief,

Se which affect reassignment of soil Se by causing the redistribution of minerals, moisture and energy (Wang and Zhang, 1996; Tan et al., 2002). Significant positive relationship between H and soil Se content (r = 0.165, p < 0.05) was observed, in line with Shao et al. (2018). Elevation shows significant negative correlations with soil pH OM (r = −0.242, p < 0.01), CaO (r = −0.351, p < 0.01), K2O

< 0.01; BA: the coefficient of< weathering and eluviation; H: elevation; SL: Slope; AT: aspect; TWI: Topographic wetness index. (r = −0.163, p < 0.05) and MgO (r = −0.217, p < 0.01), suggesting that elevation impacts soil Se content by influencing the reassignment of minerals and soil physicochemical properties. However, some studies

pH −0.136 −0.026 −0.060 −0.007 −0.074 0.041 0.691** 0.234** have shown that soil Se content is relatively high at elevations of

< 0.05; ** p < 200–500 m and relatively low at elevations of < 200 m (Xu et al., 3 3 2018). This difference in results may be due to other factors, such as O −0.175* −0.111 −0.165* 0.396** 0.391** −0.033 0.253** 0.286** O O 2 O −0.019 −0.064 −0.208** 0.505** 0.386** 0.426** 2 2 2 parent material, climate, soil-forming process and human activity, OM Se Al −0.121 −0.195* 0.660** CaO Fe K 0.370** −0.094 −0.281** 0.257** MgO Na 0.218** −0.230** −0.420** 0.432** 0.736** 0.445** 0.671** P Mn BA H −0.027 0.164* SL 0.214** AT 0.234** TWI −0.242** −0.249** 0.089 −0.196* −0.006 0.072 0.157 −0.241** 0.223** −0.358** −0.01 0.384** 0.032 0.219** 0.165* 0.129 −0.101 0.486** 0.496** 0.004 0.012 0.850** −0.125 0.077 0.462** 0.267** 0.268** 0.021 −0.082 −0.046 −0.351** 0.092 0.251** −0.02 −0.064 0.005 0.491** 0.452** 0.297** 0.032 0.037 −0.163* 0.251** 0.699** −0.153 0.301** 0.111 −0.217** 0.376** −0.095 −0.041 0.593** −0.035 −0.002 −0.042 0.421** 0.011 −0.091 0.074 0.392** −0.035 −0.218** −0.156 −0.026 −0.314** 0.039 −0.161* 0.067 −0.03 0.020 0.021 −0.094 0.073 −0.225** −0.041 0.062 −0.245** 0.177* 0.059 −0.117

Table 5 Correlation matrix for Se, soil environment factor and topographical parameters in Jiangjin. Note: * p which caused the regional difference of soil constituent and soil

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Fig. 4. Relationships between Se content and soil OM. Note: A: Association between Se content and OM in topsoil; B: Se content of topsoil samples at different OM value. The blue dots represent the mean values of Se at a certain OM gradient. physicochemical properties. (0.315 mg/kg), whereas that of construction land is much less than the TWI could be used to quantitatively describe the effect of topo­ average value of Se in Jiangjin district. Significant difference of soil Se graphy on the soil moisture. The higher the TWI value was, the higher among different land use types may be due to that land use changes the the soil moisture was (Wang et al., 2009; Kaiser and McGlynn, 2018). soil physicochemical properties. The total P content (Pmean Table 5 shows that there is no significant negative correlation between = 606.5 mg/kg) in the cropland is significantly higher than that of TWI and soil Se content (r = −0.125, p > 0.05). Soil moisture could forestland (Pmean = 540.0 mg/kg), and a significant correlation be­ directly affect the soil pH, soil aeration, and redox status (Sajedi et al., tween P and Se could be found in topsoil (Table 5). The application of 2012; Shao et al., 2018; Xu et al., 2018). Under higher soil moisture and phosphate fertilizers may increase the Se content in topsoil (Yu et al., bad aeration conditions, the selenite (+4) is the main present form and 2014; Xing et al., 2018), but there is a slightly lower Se concentration of it is firmly adsorbed on clay minerals, and is largely retained in the soil. soil in the cropland relative to that of forestland in the Jiangjin district. However, when under well-aerated conditions, inorganic Se is mainly One reason, the depletion of Se by continuous crop uptake and harvest present in the form of selenate (+6), which is vulnerable to loss by can result in the lower content of soil Se in the cropland relative to the leaching (Jayaweera and Biggar, 1996; Sajedi et al., 2012). forestland (Yu et al, 2014; López-Bellido et al., 2019). Another, due to competition for binding sites and increasing the plant-availability of Se, the P can decrease selenite adsorption on soil solid surface (Nakamaru 3.3.3. Effect of land use on Se content in top soil et al., 2006; Eich-Greatorex et al., 2010). Land use is accompanied by changes in plant communities and The relationship between OM and Se in soil has been studied by strongly influences soil properties, particularly OM, thus, it ­ hasim many researchers and results showed that a higher proportion of Se in portant consequences for the spatial distribution of Se content, Se soil was bound to organic matter (Li et al., 2017; Dinh et al., 2019; Xing speciation and bioavailability in soil (Luo et al., 2018; Dinh et al., 2019; et al., 2019). There is significantly higher concentration of OM accu­ Xiao et al., 2020). The order of Se content in topsoil from different land mulated in the forestland and cropland, relative to the built-up land use types is forestland (0.346 mg/kg), cropland (0.317 mg/kg), and (Fig. 5). A positive relationship between OM and Se content in soils has built-up land (0.184 mg/kg) in Jiangjin district (Kruskal-Wallis test, been found (Table 5), and widely documented in the literature (Li et al., p = 0.021 < 0.05) (Fig. 5). The Se content of topsoil from forestland 2017; Zhou et al., 2018; Dinh et al., 2019; Xiao et al., 2020; Zhu et al., and cropland is higher than the average soil Se value in Jiangjin district 2020). Additionally, due to higher human disturbance, the OM is lower and BA is higher in topsoil from built-up land (Fig. 5), and a negative relationship between Se content and BA in topsoil could be observed (Table 5). Thus, these are beneficial to loss and migration of Se (Wang et al., 2018; Xiao et al., 2020). In general, variations in natural and human factors influencing the content and distribution of OM and BA may ultimately lead to changes in Se accumulation. Rocks are the primary source of Se in the earth system. A significant difference of Se content in soil from different strata could be observed (Table 2 and Section 3.1). The soil samples of forestland are mainly distributed in Triassic and Early-Middle Jurassic strata. And the soil samples of cropland and built-up land are distributed in Middle-Late Jurassic strata (Table 2 and Fig. 1B). Thus, the difference of parent rock is one of the causes of soil Se content difference in different land use types. However, a significant positive correlation (r = 0.660, p < 0.01; Table 5) between soil Se and soil OM is found, and soil OM contents increase with the decrease of the BA (Table 5). It is indicated that the impact of soil physicochemical properties (especially OM) on the soil Se concentration gradually increase with the degree of soil development, while the role of parent material tends to decrease (Matos et al., 2017; Fig. 5. Se, OM, BA, and pH value of topsoil samples at different land use type. Shao et al., 2018).

9 Y. Liu, et al. Catena 196 (2021) 104926

Luohuang power plant with coal-fired power generation is located two factors on soil Se content is higher than that of one factor (Table 7). in the Luohuang township at the bank of the Yangtze River (Figs. 1A The interactive values (q) of OM and other factors range from 0.45 to and 3). Because of no coal mine existed in Jiangjin district, the fuel coal 0.55, with a mean of 0.50. The interactive values (q) between OM and of power plant need to be purchased from other mines (JJPG, 2015; Liu, Al2O3, K2O, MgO, Na2O, P, Mn, BA, H, SL and stratum are all higher 2015). In general, high levels of Se in soil are correspond to local than 0.5. These results suggest that soil OM may be the main driving proximity of power plants, and a decreasing trend can be observed with factor affecting on the spatial distribution of soil Se content. Thisis distance from these centers (Huang et al., 2009; Etteieb et al., 2020). consistent with the analysis results in Tables 5 and 6. This data shows However, the Se content of topsoil near Luohuang power plant is not that the combined effects of soil OM and soil-forming process may the highest (Fig. 3). Thus, power plant does not contribute to Se dis­ cause Se enrichment in soil from Jiangjin district. tribution of the topsoil in the whole Jiangjin district. In summary, the significant variations of soil physicochemical properties (i.e., OM and BA) in different land use, which are influenced 4. Conclusions by both nature and human activities (i.e., P fertilizer application), may contribute to the variation in soil Se concentration in Jiangjin district. By systematically studying the spatial variance of Se and factors affecting Se in topsoil from typical agriculture area of red bedsof 3.4. Driving factors of selenium content in soil Sichuan Basin, topsoil Se concentrations in the Jiangjin district are 0.039–1.110 mg/kg, with an average value of 0.315 mg/kg, which is The principal component analysis (PCA) and Geodetector were used higher than background soil values in China of 0.290 mg/kg. In the to reveal and explain the driving factors for the spatial distribution of analyzed topsoil samples, 82.3% are classified as containing Se at soil Se content in Jiangjin. The five principal component with values moderate levels (Se > 0.175 mg/kg), indicating that Jiangjin district is PC1, PC2, PC3, PC4 and PC5 equal to 27.76%, 14,829%, 12.454%, in a Se-rich level under low background values of soil Se in red beds 8.617% and 7.933%, respectively, were extracted (Table 6). PC1 and region of Sichuan Basin. The Se contents of topsoil in northern and

PC2 are composed of CaO, MgO, Al2O3, Fe2O3, K2O, Mn and BA, which eastern Jiangjin are relatively high (Se > 0.315 mg/kg), while those of are closely related to pedogenic processes. It indicates that leaching of central and southern Jiangjin are relatively low (Se < 0.315 mg/kg). base ion is extreme and Al, Fe and Mn are relatively enriched in soil This shows that soil Se contents decrease from northern and eastern forming process in Jiangjin, which could result in enrichment of Se in Jiangjin to central and western of Jiangjin. Soil Se concentrations are surface soil (Table 5). PC3 is composed of OM and P, which represents significantly positively correlated with soil OM in the study area. Due to biological influence, indicating that accumulation of biomass in soil lack of high Se black rock series and Mesozoic mudstone and sandstone impacted on the soil Se content. PC4 is formed by pH and Na2O, which with low-Se are widely distributed in Jiangjin, pedogenic rock is not the characterizes the acid and alkaline feature of soil. PC5 is composed of determining factor for the soil with rich Se in Jiangjin district. Based on elevation, slope and TWI, which indicates the topographical features. PCA and Geodetector analysis, we found that soil OM is the highest in The results of PCA show that the spatial distribution of Se content in top the effect and plays an important role in the enrichment and migration soil of Jiangjin is affected synthetically by the soil physical and che­ of Se in topsoil of low-Se geological belt, followed by soil-forming mical properties (pH, OM), soil-forming process (BA) and micro-topo­ process and topographic factor. In consideration of total Se content in graphical features (H, SL, TWI). soil may not reflect the Se level that enter food chain, bioavailability of Based on the theory of spatial stratified heterogeneity, Geodetector Se in soil from typical agriculture area of red beds of Sichuan Basin can be used to quantify the influence degree of soil physicochemical needs to be further elucidated. properties, major elements of soil, pedogenic rock, topography, and land use types on soil Se content (Wang and Xu, 2017) and figure out interactive effects of impact factors on Se content in soil. The analysis Declaration of Competing Interest reveals that the influence degree of soil OM on soil Se content is the highest (q = 0.41) in all factors (Table 7), followed by MgO (q = 0.17), The authors declare that they have no known competing financial Mn (q = 0.14), P (q = 0.13), BA (q = 0.11), stratum (q = 0.09), CaO interests or personal relationships that could have appeared to influ­ (q = 0.07), slope (q = 0.07), Na2O (q = 0.06), Al2O3 (q = 0.06), land ence the work reported in this paper. use (q = 0.05), H (0.04), K2O (q = 0.04), Fe2O3 (q = 0.04), TWI (q = 0.04), and pH (q = 0.02). Furthermore, interactive influence of

Table 7 Factor detection and interaction detection of soil selenium relative to affecting factor.

Factors pH OM Al2O3 CaO Fe2O3 K2O MgO Na2O P Mn BA H SL TWI S Land

pH 0.02 OM 0.46 0.41

Al2O3 0.11 0.52 0.06 CaO 0.11 0.48 0.17 0.07

Fe2O3 0.08 0.46 0.12 0.13 0.04

K2O 0.09 0.52 0.13 0.14 0.11 0.04 MgO 0.22 0.55 0.29 0.21 0.23 0.23 0.17

Na2O 0.15 0.51 0.16 0.18 0.20 0.28 0.29 0.06 P 0.17 0.53 0.24 0.26 0.24 0.28 0.39 0.34 0.13 Mn 0.20 0.52 0.23 0.20 0.23 0.28 0.28 0.32 0.41 0.14 BA 0.18 0.51 0.18 0.18 0.19 0.14 0.22 0.22 0.32 0.23 0.11 H 0.09 0.50 0.18 0.12 0.17 0.14 0.24 0.26 0.25 0.27 0.19 0.04 SL 0.13 0.51 0.17 0.20 0.22 0.22 0.28 0.29 0.26 0.30 0.23 0.21 0.07 TWI 0.09 0.47 0.17 0.13 0.19 0.17 0.28 0.28 0.24 0.28 0.24 0.18 0.16 0.04 S 0.13 0.50 0.23 0.20 0.17 0.19 0.29 0.22 0.25 0.26 0.22 0.18 0.20 0.22 0.09 Land 0.10 0.45 0.14 0.12 0.17 0.14 0.22 0.19 0.26 0.22 0.19 0.17 0.20 0.17 0.18 0.05

Note: BA: the coefficient of weathering and eluviation; H: elevation; SL: Slope; TWI: Topographic wetness index; S: Stratum; Land: Land use types.

10 Y. Liu, et al. Catena 196 (2021) 104926

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