The Origin and Geochemical Cycle of Soil Selenium in a Se-Rich Area of China

Journal of Geochemical Exploration 139 (2014) 97–108

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Journal of Geochemical Exploration

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The origin and geochemical cycle of soil selenium in a Se-rich area of China

Tao Yu a, Zhongfang Yang a,b,⁎, Yaoyao Lv a,QingyeHoua, Xueqi Xia a, Haiyan Feng a,MengZhanga, Lixin Jin c, Zezhong Kan c a China University of Geosciences, Beijing 100083, PR China b Key Laboratory of Ecological Geochemistry, Ministry of Land and Resources, Beijing 100037, PR China c Sichuan Institute of Geological Survey, Chengdu 610081, Sichuan, PR China article info abstract

Article history: Mianyang City, located in the Fujiang River Basin, Sichuan Province, is a Se-rich area of China. The distribution of Received 29 January 2013 Selenium (Se) in the Mianyang area was studied based on assay data obtained from soil, irrigation water, fertil- Accepted 30 September 2013 izer, and rice (grain and stem) samples. The ratio between natural and anthropogenic sources in the area was de- Available online 9 October 2013 rived by analyzing the concentrations and spatial distributions of multiple elements (such as Se, cadmium, arsenic, and mercury) in the soil. The controlling factors affecting Se concentration in the soil were also investi- Keywords: Selenium gated. We established a geochemical model of the Se cycle among the different media (i.e., the atmosphere, ﬂ Origin water, soils, and plants). We then calculated the annual Se ux caused by various inputs' (such as precipitation, Geochemical cycle fertilization, and irrigation) and outputs' (such as inﬁltration, crop harvest, removal of straws from cropland, and Se-rich area volatilization) pathways in the topsoil. We discuss the contribution of different pathways to the Se cycle and pro- China vide evidence for exploring Se-rich land in the study area. © 2013 Elsevier B.V. All rights reserved.

1. Introduction element in plants; however, whether this element is essential for plants remains debatable (Bañuelos et al., 1997; Lyons et al., 2009; Mateja Selenium (Se) is an important trace element in the ecological envi- et al., 2007). ronment. This element has been studied for more than 190 years since In the soil–plant–animal/human food system, the soils supply Se Swedish chemists discovered it in 1817. Excessive exposure to Se and to satisfy the requirement for plants, humans, and livestock. Human lack of Se in the environment both cause health problems to humans and natural activities constantly change Se concentration in soils. There- and animals (Wang, 1993). The excessive exposure of livestock to Se, fore, the sources, existing forms, and bioavailability of Se in soils play which caused alkaline disease and blind stagger in Europe and the decisive roles in the geochemical cycle of Se. Se concentration ranges United States, had been recognized in the 1930s (Moxon, 1937). This from 0.01 mg/kg to 2.0 mg/kg in the vast majority of soils in the world, finding highlighted concerns regarding Se poisoning; and thus, Se with an average concentration of 0.4 mg/kg and a very uneven dis- was considered as an important environmental contaminant until the tribution (Fordyce, 2007). To date, several countries have successively 1950s (Mayland, 1994). In 1957, Schwarz and Foltz proved for the reported soils with an excessive or deficient amount of Se (Dhillon first time that animals need Se as a nutrient (Schwarz and Foltz, and Dhillon, 2003; Ermakov and Jovanovic, 2010; Fleming, 1980; 1957). Se was subsequently determined to be an important component Fordyce et al., 2010; Ihnat, 1989; Jacobs, 1990; Lakin, 1972; Neal, of glutathione peroxidase (Awasthi et al., 1975; Rotruck et al., 1973). 1995; Wang and Gao, 2001). On a worldwide scale, the areas of soils Moreover, Se deficiency may cause white muscle disease among live- with low Se concentration or which lack Se are relatively larger than stock. Se has received universal attention in several fields, including those with potentially harmful high concentrations of Se (Girling, plant growth, human health, agricultural production, and ecology 1984). China is located in a low Se area, with more than approximately (Combs and Combs, 1986; Fordyce et al., 2000; Huang et al., 2013; 10 provinces (municipalities) exhibiting varying degrees of Se deficien- Johnson et al., 2010; Levander and Burk, 2006; Mayland, 1994). The cy. The region with Se deficiency accounts for approximately 72% of the World Health Organization (WHO) has also confirmed that Se is a nec- national land area. Such arithmetic indicates that Se concentration of essary nutritional element for animals (WHO, 1987). Se is a beneficial soils in low-Se areas is 0.13 mg/kg (Hu et al., 2000; Tan, 1989; Tan et al., 2002). Endemic diseases, such as Kashin–Beck disease and Keshan disease, are prevalent where Se is relatively deficient in soils (Gao et al., 2011; Lv et al., 2012). These diseases seriously affect the physical health of local residents. Therefore, studying the source and geochemical ⁎ Corresponding author at: China University of Geosciences, No. 29, Xueyuan Road, fi Haidian District, Beijing 100083, PR China. Tel./fax: +86 10 82322079. behavior of Se in soils from China with relative Se de ciency is highly E-mail address: [email protected] (Z. Yang). significant.

Mianyang City is located in the Fujiang River Basin, Sichuan Province using a bamboo spade, and each of the four sub-samples was in the southwestern of China where Se is relatively deficient in soils. A composited for analysis. In this study, totally 2601 composite multi-objective regional geochemical survey of the Chengdu Economic samples of topsoil and 650 composite samples of subsoil Zone of Sichuan Province was conducted from 2002 to 2008 (Chen were collected. After the sampling site was selected, we used et al., 2008). The survey revealed that Se-rich soils in Mianyang are sediment sampling equipment to load equal amounts of characteristically distributed along the riverbanks. An enrichment of coarse active sediments from three to five points of the section toxic heavy metals, such as cadmium (Cd), lead (Pb), and zinc (Zn), in plastic buckets, with the excess water drained off. The was also found in the soil. We conducted a systematic study of the collected samples were loaded into a clean sack and kept in a sources and pathways of the geochemical cycle of Se in soils from this cool, dry place. The dried samples were sieved by using a 20- area, with the aim of providing a case study of a geochemical cycle in mesh (b0.84 mm) nylon sieve. The sieved samples were kept a Se-rich area. in clean Teflon bags and sent to the laboratory for analysis. The samples to be analyzed included 5% national standard 2. Material and methods material. (2) We collected rice grains, stems, and the corresponding sam- 2.1. Study area ples of the topsoil during the harvest season in the rice- growing regions. The method of topsoil collection was the The research area is located in Mianyang City in the midstream of same as the first procedure. We first analyzed the entire area the Fujiang River Basin. The area measures 10,404 km2 with the geo- at each sampling site to determine the cropland, terrain, and graphic coordinates of 105°2′26″ to 105°43′25″ (E) and 30°0′18″ to fertility status. Those plots with area about 3500 m2 and 31°1′52″ (N). This area has a subtropical humid monsoon climate, well-growing rice were chosen as sampling sites based on with neither cold winters nor hot summers. The annual average tem- the summary investigation about plots area, landform and perature is 17.3°C, and the frost-free season is long. The annual average rice growing conditions of study region. In each plot, 4–5sam- precipitation is 602 mm to 1389 mm. The annual sunshine hours are pling units were taken. The unit scale is 50cm×(sowing width 1042 h to 1665 h. The topography of the entire area is dominated by + row spacing) cm. The sampling sites were more than 1 m hills. The terrain shows a discrepancy between the north and south: from the edge of the land. We collected 124 samples of rice high in the north and lower in the south, with an elevation of 290 m grains and their corresponding topsoil, and 50 samples of to 650 m. Nine major soil types can be found in the study area based stems. The rice grain and stem samples were washed with on the Chinese soil taxonomy classification (CRG-CST, 2001), i.e., Calcaric tap water and then with deionized water to remove soil parti- Purple-Udic Cambosols, Calcaric Purple-Orthic Primosols, Carbonatic cles and dust. Subsequently, we dried the samples with tissue Udi-Orthic Primosols, Recalcaric Gleyi-Stagnic Anthrosols, Typic Fe- paper. The rice grains were dehulled. All the plant samples accumuli-Stagnic Anthrosols, Typic Fe-leachi-Stagnic Anthrosols, Albic were then oven-dried at 45 °C for 72 h to a constant weight. Fe-leachi-Stagnic Anthrosols, Typic Aquic-Alluvic Primosols, and Lithic The dried plant samples were ground into fine powder Haplic-Perudic Ferrosols. (b0.074 mm) using a stainless steel mill and were kept in Two crops of rice are planted in most parts of the research area: one clean Teflon bags prior to chemical analysis (Yang et al., crop of rice and one crop of wheat. Besides, maize is planted in a 2005). small portion of the area. The Fujiang River is the major river running (3) We sited nine points based on the different irrigation water through the study area from south to north. The main tributaries of systems in the research area, and collected the irrigation the river include the Anchang, Kaijiang, Furong, Zijiang, and Baoshi water samples during the irrigation season. We prepared Rivers. Along the upper reaches of the Fujiang River distributed varies three polyethylene containers with a capacity of 1000 mL to strata, i.e. clastic rocks and mud shales with coals in Mesozoic, and collect water samples. One was used to measure heavy metal basalts, limestones, Mackers, copper-bearing shales, phyllites, slates, elements, such as Pb, Cd, and arsenic (As). And as soon as a crystalline limestones, meta-sandstones, siltstones, etc. in Paleozoic. clear water sample was collected, an amount of 10 mL HNO3 The main mineral resources are including the Xigou Fe–Mn deposit, (mixed 1:1 by volume) was immediately added and mixed the Huya Fe–Mn deposit, Moheba Fe–Mn deposit, placer mine at with the 1000mL water sample. Another was used to measure Shuijing, Gucheng and Nanba, and the Wupingyinchang Hg–Au deposit mercury (Hg), and before sampling, we first added 10 mL of

(SBGME, 2006). K2Cr2O7 (ρK2Cr2O7 = 5%) solution into the plastic container, then shook it well. The last one collecting water without pad- 2.2. Sampling media and methods ding any reagent was used to measure other major/minor elements. Next, we made records of the sampling and stored the The samples collected from the research area include: (1) topsoil samples in a refrigerated container. After sample collection and subsoil, (2) rice seeds, stems, and their corresponding cultivated was completed, the samples were sent to the laboratory for soils, (3) irrigation water, (4) fertilizers, (5) atmospheric precipitation analysis immediately (Ye et al., 2005). and infiltration, (6) soil profiles, and (7) sediments. (4) We collected more than 90% of the local fertilizer types at each unit using the county or district as the collection unit for fertilizer (1) The topsoil at a depth of 0 cm to 20 cm was collected at a den- samples. The weight of each sample was over 500g. The applica- sity of one sample per km2. The subsoil at a depth of 150cm to tion amount and the proportion for different fertilizers in each 180cm was collected at a density of one sample per 4km2.The hectare of farmland from the collection site were also recorded. densities were according to the requirements of the DD2005- At last, we sealed the samples in clean Teflon bags to be sent to 01 Specifications on the National Multi-objective Regional the laboratory. Geochemical Survey (Li et al., 2013; Wang et al., 2007; Xi (5) We divided the research area into seven collection units for et al., 2005). The sampling sites were located away from precipitation and infiltration water based on the local annual areas with obvious human contaminations, such as roads, vil- precipitation distribution. Besides, soil type, soil texture, lages and garbage dumps. Five holes were dug on a grid (1 km terrain, and topography were also considered in dividing ×1km for topsoil and 2km×2km for subsoil) at each sampling the area. A rainfall collecting barrel with a top diameter of 0.40 point, and the weight of each sample was controlled greater m and a depth of 0.45 m was placed on a bracket 0.7 m above than 1 kg. Composite samples were collected in cotton bags the ground surface of a relatively flat area in a representative T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108 99

location of each collection unit. The soil infiltration collector Table 1 was placed 20 cm beneath the topsoil at the precipitation Detection limits (DL) of different samples. collection site. The collection period for precipitation and Sample types Element Unit Detection Analytical infiltration was from July 15, 2007 to September 15, 2007. limit method Tremendous effort was exerted to prevent the loss and pol- Soil As mg/kg 1 AFS lution of the collecting equipment for precipitation and Soil Cd mg/kg 0.03 ICP-MS infiltration water. Then we accurately measured and recorded Soil Cu mg/kg 1 ICP-MS μ the volume, pH, and temperature during the collection of the Soil Hg g/kg 0.5 AFS Soil Pb mg/kg 2 ICP-MS samples. Protective agents were added to measure various Soil S mg/kg 50 XRF elements using the same method described earlier for the Soil Sc mg/kg 1 ICP-MS collection of irrigation water. Soil Zn mg/kg 1 ICP-MS (6) We sampled two soil profiles on the flood plains, sitting rela- Soil SOC % 0.1 VOL Soil, fertilizer. Se mg/kg 0.01 AFS tively wide and flat regions without human disturbance. The Irrigation water, atmospheric Se μg/L 0.1 AFS fi rst is in Anxian County, which has relatively high Se and Cd precipitation, infiltration. concentrations; and the second is in Santai County, which Plant Se mg/kg 0.001 AFS

has relatively low Se and Cd concentrations. The depth of the AFS: atomic fluorescence spectrometry; ICP-MS: inductively coupled plasma mass spec- profiles and the sample density were 150 cm and 1 sample/ trometry; XRF: X-ray fluorescence spectrometry; VOL: volumetry. 10 cm, respectively. The samples over 1 kg weight were collected in sequence from bottom to top during the dry season in December 2007, which represents the process to generate the profile of soils (Yang et al., 2005). plasma mass spectrometry (ICP-MS, Model X-SERIES); As, Se and The collection sites of the aforementioned samples in the field were Hg were tested with atomic fluorescence spectrometry (AFS, Model located using a Global Positioning System and were monitored with AFS-230E); and S was tested with X-ray fluorescence spectrometry tracks. The sampling sites are illustrated in Fig. 1. (XRF, Model ZSX100C). The detection limits of all the sampling media are shown in Table 1. 2.3. Chemical analysis and quality control The analysis precision and accuracy were controlled by inspec- tion of standard reference materials (SRMs), recovery tests, in- The samples were pretreated through the following steps. The ternal and external duplicate samples and coded samples. Take soil samples were dried at a maximum of 40 °C to avoid the loss of soil samples as example, the accuracy was mainly monitored by volatile elements and then ground to a grain size of less than 200 inserting 12 primary SRMs within every 500 samples and analyzing mesh (74 μm) using high-alumina agate mills. The plant samples simultaneously. Four primary SRMs were inserted in cipher within were washed with deionized water and dried in a low-temperature every batch of 50 samples to assess the precision by the logarithmic (below 60 °C) oven before analyses. While decomposed using a mix- standard deviations between the analytical values and the recom- ture of HF, HNO3,HClO4 and aqua regia, samples were repeatedly mended values. The accuracy and precision of the analyses of all dissolved until the solution was clear. Cd, Cu, Pb, Sc and Zn elements samples met with the required specifications (Li et al., 2013; Xi in all solution samples were tested with an inductively coupled et al., 2005).

Fig. 1. Research area and sampling sites. 100 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108

Table 2 [Csubsoil,Sc] is the total concentration of Sc in the subsoil (mg/kg) in dry Range of Se concentration in topsoil related to health. matter. Soil category Se concentration Effect Proportion of the range (mg/kg) research area (%) 2.5. Statistical analysis and GIS methodology Deficient ≤0.125 Selenium deficient 39.3 Marginal 0.125–0.175 Potential selenium deficient 32.9 Descriptive statistics (mean, standard deviation, maximum, mini- Moderate 0.175–0.40 Sufficient selenium 21.5 mum), correlation analysis and linear regression analysis were deter- – High 0.40 3 Selenium-rich 6.1 mined using SPSS software v17. Excess ≥3 Selenium poisoning 0.2 The sampling points, precisely defined by GPS, were integrated to create a database in which the coordinates and the value of the analytical characteristics for each point were included by Mapgis software v6.7. To make distribution maps, Kriging was chosen to interpolate, 2.4. Enrichment factor for the soil system which minimize the errors of predicted values.

The concentration ratio of the elements in topsoil (0 cm to 20 cm) 3. Results and discussion and subsoil (N150 cm) was deﬁned as the enrichment factor (EF) (Bergamaschi et al., 2002; Establier et al., 1985) to represent the migra- 3.1. Concentration and source of Se element in the soil tion of elements. Scandium (Sc), a conservative reference element, was used for the normalization (Grousset et al., 1995). EF was calculated Selenium belongs to a rare and dispersed group of elements. The from the following formula: Clarke value (Taylor, 1964) of Se in the crust and its concentration hihihihi in all geological bodies are notably low. Se is a chalcophile and a = = = ; EF ¼ Ctopsoil;i Ctopsoil;Sc Csubsoil;i Csubsoil;Sc ð1Þ probiological element prone to secondary enrichment during the supergene surface geochemical cycling process. where The average Se concentration in topsoil of the research area is 0.20 mg/kg, which is lower than the average Se concentration for topsoil in

[Ctopsoil,i] is the total concentration of an element in the topsoil (mg/kg) China (0.29 mg/kg) and in the Chengdu Economic Zone (0.28 mg/kg) in dry matter; (Chen et al., 2008). In this area, Se deﬁcient and marginal categories

[Ctopsoil,Sc] is the total concentration of Sc in the topsoil (mg/kg) in dry are depicted as the dominant ones in about 39.3% and 32.9% of the matter; total area respectively, according to the classiﬁcation criteria of Tan

[Csubsoil,i] is the total concentration of an element in the subsoil (mg/kg) (1989) (Table 2). The section with moderate-to-high Se concentration in dry matter; and is only 27.6% of the total area, and the section with excessive Se

Fig. 2. Selenium concentration in topsoil (mg/kg). T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108 101

Table 3 concentrations in topsoil is not only related to the exogenous inputs Correlation coefficients between Se and other elements in soil. caused by human activities but also to the bioconcentration caused Types n As Cd Hg Pb S Cu Zn SOC by the increase of organic matter concentration in soils. Fig. 3 shows that the ranges of the EF of Se and Cd in topsoil (0 cm to 10 cm) are Topsoil 2601 0.54 0.73 0.44 0.52 0.67 0.83 0.75 0.52 Subsoil 650 0.41 0.92 0.68 0.54 0.85 0.63 0.66 0.82 2.46 to 4.73 and 1.37 to 2.83, respectively, compared with the background values of the soil (the average of all layers N20cm in the soil Correlation is significant at P b 0.01 (two-tailed). profile). The contributions of human activities and bioconcentration to Se and Cd concentrations in topsoil account for 59.7% to 78.9% and 28.5% to 64%, respectively (Table 5). Therefore, the influence of human activities and bioconcentration on Se and Cd soil concen- concentration or possible Se poisoning is 0.2% of the total (Table 2). The trations in topsoil cannot be disregarded despite of the inheritance spatial distribution of the soil which is relatively rich in Se is mainly of Se and Cd concentrations from subsoil to topsoil. Therefore, fur- found on two banks of the northwestern rivers in the research area ther study of the supergene system and the geochemical cycling (Fig. 2), with distinct distribution characteristics along the riverbanks. pathways of elements (such as Se and Cd) in soils can provide However, the majority of soils in this area underscore a feature of Se- strong backing for guiding the development of Se-rich land re- deficiency. sources and reducing the ecological hazards of harmful elements Other elements with spatial distributions similar to Se in soils in- (such as Cd) scientifically. clude As, Cd, Hg, Pb, copper (Cu), and sulfur (S), which are the main ore-forming elements associated with metallic sulfide minerals 3.2. The geochemical cycle of Se in the upper stream catchment area of the Fujiang River. The correlation analysis of Se and other elements (such as As and Cd) in soils 3.2.1. A model of the Se cycle in topsoil on both riverbanks indicates that Se in soils is significantly correlated The elemental concentrations in topsoil can be affected by the soil to above elements and soil organic carbon (SOC) (Table 3). The cor- parent materials and the geochemical behaviors of the elements in relation coefficients in subsoil are larger than those in topsoil, thus soils formation process. Although a variety of human activities indicating that Se is closely associated with these elements in the have similar effects on the concentrations, they have significant soil-forming process. discrepancy, such as the timescale. The timescale of natural actions The correlation analysis between Se and other elements, such as that influence Se concentration in soils is generally hundreds to As, Cd, Cu, Hg, Zn, and chromium (Cr), in the sediments of the river tens of hundreds of years, whereas the timescale of variations con- systems (Fujiang River, Anchang River, Furong Creek, and Kaijiang centrations in soils caused by human activities is usually several River) in the research area also indicates that Se has a significant pos- years or even several days. Therefore, the geochemical cycling path- itive correlation with the aforementioned elements (Table 4). The el- ways of Se, the input and output of the element in soils are mainly emental concentrations of the sediments in the river systems can caused by human activities as considered in this study. These activi- reflect the chemical compositions of various types of rocks and de- ties primarily include fertilization (F), irrigation (IR), precipitation posits in the catchment of the rivers to a certain degree. The close (P), crop planting and harvesting (CH), infiltration of precipitation symbiotic relationship between Se and the other elements (particularly (I), and volatilization of elemental Se (VS) on the regional scale Cd, As, and Cu) in the sediments of the river systems and the subsoil, as (Fig. 4). The pathways in this cycle through which elemental Se en- well as the distribution characteristics along the two riverbanks, indi- ters the topsoil include precipitation, irrigation, and fertilization. cates that the high Se concentration of soils is probably related to The pathways through which elemental Se exits the topsoil include weathering, erosion, and the deposition of mountain rocks, sulfide de- infiltration, volatilization, crop harvesting, and the removal of straws posits, and Fe–Mn deposits in the upper reach of the Fujiang River (RS) from the cropland. (SBGME, 2006). An analysis of Se, Cd, Cu, Zn, and Hg for the various soil layers 3.2.2. Precipitation and infiltration of the two vertical profiles near the Fujiang River in Santai County Precipitation is the most important means through which Se enters and Anxian County in the research area (Fig. 3) reveals that the the soils. As rain falling on soils, Se is partly retained in the soils and Se, Cd, Cu, and Hg concentrations in soils fluctuate within a small soil solution generates, and then it can be absorbed by plants or evapo- range below 20 cm (the plow pan), and that the average concentra- rated to return to the atmosphere. In addition, Se partly penetrates into tion represents the natural background level (background values). groundwater through the plow layer. The easily dissolved elements are The concentrations of these elements significantly increase in extracted and carried to the plow layer during the penetration process. the topsoil (0cm to 20cm). The soil profiles of Se and other elemen- Thus, these elements become an important part of the geochemical tal concentrations are essentially consistent with the trend in cycle of elements in the topsoil. SOC variation. Therefore, the increase of Se and other elemental The equations for calculating the input and output fluxes of the elements in soils of the plow layer through precipitation or infiltration are as follows:

= ; SP ¼ CP V P 100 ð2Þ Table 4 Correlation equations of the elements in river sediments (n = 10).

Regression equation Correlation Regression equation Correlation where SP istheSeinputflux in soils caused by the annual precipita- coefficient r coefficient r tion in the research area (g/ha/y), CP is the Se concentration in the ⁎⁎ ⁎⁎ μ CSe = 0.0372CCu − 0.62 0.90 CSe = 0.0043CHg + 0.004 0.78 precipitation ( g/L), VP is the average amount of precipitation at − ⁎⁎ − ⁎⁎ CSe = 1.3839CCd 0.26 0.94 CSe = 0.0095CZn 0.43 0.89 the sampling site in the research area (mm), and 100 is the unit con- ⁎⁎ ⁎⁎ CSe = 0.0569CAs − 0.14 0.81 CSe =1.554CCr + 0.357 0.94 ⁎ ⁎ version factor. CSe = 0.0043CS − 0.68 0.70 CSe = 0.0278CPb − 0.22 0.70

Ci means the concentration of element i;Hg:μg/kg; Other elements: mg/kg. ⁎⁎ Correlation is signiﬁcant at P b 0.01 (two-tailed). = ; ⁎ Correlation is signiﬁcant at P b 0.05 (two-tailed). SI ¼ CI VI 100 ð3Þ 102 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108

Fig. 3. Element concentrations in soil proﬁles in Santai and Anxian County.

where SI is the Se output flux in soils caused by the annual infiltration of Therefore, we used the water collected from the Fujiang River and rainfall in the research area (g/ha/y), CI is the Se concentration in the in- its major tributaries as analysis data for calculating the elemental filtrated water (μg/L), VI is the amount of infiltrated water in one year at flux in irrigation water. the sampling site in the research area (mm) derived by extrapolating The equation for calculating the Se input flux in soils resulting from the amount of infiltrated water during the sampling period, and 100 is irrigation water is: the unit conversion factor. ; The calculated results for the elemental concentrations (such as that SIR ¼ 1000 CIR V IR ð4Þ for Se) caused by precipitation input and infiltration output in soils of the plow layer in the research area are shown in Table 6.Theamount where SIR istheSeinputflux in soil caused by irrigation (g/ha/y), CIR fl of Se input ux from precipitation is relatively high in Anxian County, is the Se concentration in irrigation water (μg/L), VIR is the average Zhongjiang, and Santai, but relatively low in Zitong and Mianyang. of the actual irrigation water used in the farmland in the research Moreover, The Se output flux is relatively high in Anxian County and area (5595 m3/ha/y) (SWA, 2005), and 1000 is the unit conversion Zitong. The precipitation input and infiltration output lead to a net in- factor. crease in Se concentration for all the aforementioned sites, namely, The calculated annual Se input flux caused by irrigation water in △S N 0(ΔS=SAD − SIPW), which indicates that precipitation is one of soils is shown in Table 7. Se concentration in river is subject to uneven the primary pathways for increasing Se concentration in soils. The in- distribution among river systems and has evolved a notably diverse creased flux of Se in soils is highest in Santai, lower in Anxian and array of the Se input flux. The Se input flux caused by irrigation water Zhongjian, and lowest in Zitong at only 5.4 g/ha/y. is relatively high in Santai and lower in the part of the Shehong section of the Fujiang River. 3.2.3. Irrigation Farmland irrigation can potentially and vitally use waste water 3.2.4. Fertilization and improve crop yields. Irrigation in the research area is mainly di- Fertilization is a significant factor in improving crop yield but it vided into the Wudu Drinking Water Project and the Renmin Canal also raises a series of environmental issues, such as soil acidifica- irrigation areas. A part of the water is supplied through precipitation, soil compaction, and water eutrophication. The fertilizers tion, but the irrigation water comes mainly from various reservoirs used in the research area are mainly produced locally, such as the that accumulate water from the Fujiang River and its tributaries. phosphate fertilizers produced by Anxian County Hongda Chemical

Table 5 Enrichment factor of Se and Cd in topsoil from natural and anthropogenic sources.

Sampling location Elements Depth 0–10 cm Depth N 20 cm Enrichment factor Natural source (%) Anthropogenic sources (%)

Anxian Se (mg/kg) 0.57 0.12 4.73 21.1 78.9 Cd (mg/kg) 0.65 0.23 2.83 36.0 64.0 Cu (mg/kg) 41.00 30.33 1.35 74.0 26.0 Zn (mg/kg) 121.70 99.02 1.23 81.4 18.6 Hg (μg/kg) 219.69 41.78 5.26 19.0 81.0 Santai Se (mg/kg) 0.32 0.13 2.46 40.3 59.7 Cd (mg/kg) 0.26 0.19 1.37 71.5 28.5 Cu (mg/kg) 27.60 24.10 1.15 87.3 12.7 Zn (mg/kg) 82.80 78.14 1.06 94.4 5.6 Hg (μg/kg) 71.48 50.59 1.41 70.8 29.2 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108 103

Table 7 Annual input ﬂux of Se from irrigation.

Samples River system SIR (g/ha/y) XF01 Upstream of Fujiang River 7.0 XF02 Middle of Anchang River 3.8 XF03 Furong Stream 1.8 XF04 lower section of Anchang River 3.6 XF05 Midstream of Fujiang River 3.6 XF06 Santai section of Fujiang River 10.2 XF07 Kaijiang River 4.3 XF08 Upper section of Shehong, Fujiang River 2.6 XF09 Lower section of Shehong, Fujiang River 10.5 Average 5.27

per hectare in the research area (kg/ha/y), and 1000 is the unit conversion factor. The annual Se input ﬂux in soils from the application of fertilizers in the research area is relatively small. The calculated re- sult is 0.23 g/ha/y.

3.2.5. Crop harvest Harvesting crops (including grains and stems) is the main pathway for the biogeochemical cycle of elements in the plow layer. The analysis of the relationship between Se concentration in the grains and the pa- Fig. 4. Selenium input and output models in topsoil. rameters, such as Se concentration, pH, and SOC in the corresponding root soil for 124 rice samples, indicates that the Se transfer coefﬁcient

of rice grains, TCgrain (TCgrain = Segrain/Sesoil)(Antoniadis and Alloway, Co., Ltd. and the compound fertilizers produced by Deyang City 2001), is mainly related to SOC, which can be described as TCgrain = Shifang Nongdeli Tianfu Fertilizer Plant, Anxian County Chaoyang 32.384 e−0.2209 SOC (n =124,r = 0.60, P b 0.05). Phosphorus Chemical Industry Co., Ltd., and Hubei Yangfeng Co., We estimated the Se concentration per km2 of rice grains in the Ltd. research area using the aforementioned equations, and the Se con- The survey indicates that the average usage of fertilizers in centration and SOC data for the 1/km2 topsoil in the research area. Mianyang City is 630 kg/ha/y. The application ratio of phosphate fer- The measured Se concentration of the rice grains is plotted using tilizer, compound fertilizer, and ammonium bicarbonate in the re- the estimated Se concentration of the rice grains at the correspond- search area is 5:3:10. We have estimated that the applied amounts ing site in Fig. 5. The estimated values are reliable at the P b0.05 con- of phosphate fertilizer, compound fertilizer, and ammonium bicar- fidence level. bonate in the research area based on this ratio are 175 kg/ha/y, 105 Therefore, we use the following equation to calculate the Se output kg/ha/y, and 350kg/ha/y, respectively. The average Se concentration flux in the soil caused by harvesting rice grains: of these fertilizers ranges from 0.03mg/kg to 1.18mg/kg. The Se concentration is 1.18 ± 0.21 mg/kg in phosphate fertilizer (Table 8). ; Sgrain;CH ¼ 1000 CCH YCH Sgrain;CH We use the following equation based on the elemental concentra- ; ¼ 1000 ðÞCCH1 YCH1 þ CCH2 YCH2 ð6Þ tions of fertilizers and the annual amount of fertilizer applied over a unit area to derive the output flux: where Sgrain,CH is the Se output flux in soils caused by annual rice grain Xn harvesting (g/ha/y), CCH1 and CCH2 are the Se concentration of the rice S ¼ c q =1000; j F j j grains (mg/kg), and YCH1 and YCH2 are the annual rice yields per hectare j¼1 (kg/ha/y) derived from monitoring the rice yield at the sampling sites ¼ fphosphorus fertilizer; compound fertilizer; and ammonium bicarbonateg and in situ investigations from local farmers (Table 9). The average an- ð5Þ nual rice yield in the research area is 15,000 kg/ha/y.

The linear regression analysis of the dry weight of the grains (Hgrain) where SF is the annual Se input ﬂux in soils caused by using fertilizers and stems (Hstem), and Se concentration of the grains (Cgrain)andstems in the research area (g/ha/y), cj is the concentration of elemental Se (Cstem) for the 50 rice samples indicates a signiﬁcant positive correlation: in fertilizer j (mg/kg), qj is the annual applied amount of fertilizer j Hgrain=0.6814 Hstem+351.78, r=0.73, and Pb0.05; Cgrain=0.5054Cstem + 8.1033, r = 0.99, P b 0.05.

Table 6 Annual input and output ﬂuxes of Se in topsoil with precipitation and inﬁltration.

Region The average annual rainfall SP (g/ha/y) SI (g/ha/y) △S (g/ha/y) (mm) Se Se Se Table 8 Zhongjiang 1146 19.8 2.2 17.6 Average concentrations of Se in different kinds of fertilizers in Mianyang, Fujiang River Santai 913 19.1 0.2 19.0 Basin, China. Yanting 826 15.6 0.1 15.5 Fetilizer types (number) Annual application Se concentration Zitong 913 9.7 4.3 5.4 amount (kg/ha/y) (mg/kg) Jiangyou 1100 13.9 1.6 12.3 Mianyang 1122 11.9 1.7 10.3 Phosphorus fertilizer (15) 175 1.18 ± 0.21 Anxian 1260 20.1 3.8 16.4 Compound fertilizer (16) 105 0.09 ± 0.21 Average 15.8 2.0 13.8 Ammonium bicarbonate (21) 350 0.03 ± 0.21 104 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108

Fig. 5. Measured and regression analyzed Se concentration of rice.

2 We estimated the annual Se output flux in soils per hectare caused flux is Sout = SI + SCH + SSS,whereSCH and SSS are the per 4 km flux by the removal of the rice stems from the cropland using Sgrain,CH = data of the soils in the research area. SP, SIR, SF,andSI are all assigned 1000 × CCH × YCH and the aforementioned two regression equations. values based on the sampling units and are later calculated with spatial Consequently, the results show that the Se output flux in soils caused overlay by interpolating the values over grids to obtain flux data per 4 by harvesting the rice grains and stems in the research area ranges km2. from 0.24 g/ha/y to 19.51 g/ha/y. The input and output fluxes of soil Se in the research area are shown

in Table 10.TheratioSin/Sout is generally 7.93, which is signiﬁcantly 3.2.6. Volatilization greater than 1 and which indicates that the net Se input ﬂux ΔS (ΔS =

Volatility is a distinct feature of Se, which has a relatively low boiling Sin − Sout) in soils is positive. The soil Se concentration in the research point (684 °C) and high vapor pressure. Therefore, large volumes of area gradually increases as time passes. Se can be emitted into the atmosphere through high-temperature Figs. 7–9 show the geochemical diagram of the Se input flux, the Se processes such as volcanic activities, coal burning, and smelting. output flux, and the net Se flux in soils, respectively. Meanwhile, three Another significant biogeochemical process of Se is volatilization caused regional distribution patterns are found. 1) The distribution area with by methylation. Animals, plants, and microorganisms in soils and sedi- a relatively high Se input flux is the western hilly region of the research ments can also release volatile Se into the atmosphere. The volatilization area. The Se input flux near Yanting County is relatively low across the of Se in soils is related to soil microbial biomass, temperature, humidity, area. Our survey reveals that Yanting County is one of the areas with a and texture, and the presence of aqueous Se in soils (Wang et al., 1989). high incidence of cancer patients in China. Further studies should be un- Recent studies indicate that the annual relative volatilization of Se ac- dertaken to prove the connection. 2) The distribution area with a rela- counts for 0.024% of the total Se concentration in soils (Wang, 1993). tively high Se element output flux is mainly the northwest of the Therefore, we estimated Se volatilization in soils of each 1/km2 of the re- research area, which is adjacent to the Longmenshan area. 3) The net search area to be within the range of 0.014 μg/kg to 2.18 μg/kg, with an Se flux is positive in most regions of the research area, which are mainly average of 0.047 μg/kg, which was calculated using the mass of topsoil located in the southwestern hilly areas and surrounding the Fujiang 6 (0 cm to 20 cm) per hectare (ha) of 2.25 × 10 kg and the average Sss River in a V shape. flux of 0.105 g/ha/y (Fig. 6). 3.3.2. Composition of input and output fluxes 3.3. Se input and output fluxes in soils and their influence on Se The main exogenous substance input pathways that influence concentration the quality of the topsoil are dry and wet atmospheric deposition, fertilization, and irrigation from the regional perspective. The per- 3.3.1. Input and output fluxes of Se centages of different input pathways relative to the exogenous Given the input and output flux models, the total Se input flux in input flux are shown in Fig. 10.Thefigure shows that the proportions soils in the research area is Sin = SP + SIR + SF,andtheSeoutputtotal of different input pathways of elemental Se into the soils are

Table 9 Comparison of the measured and surveyed rice yields.

Rice grains (n = 124) Rice stems (n = 50)

Maximum Minimum Average Maximum Minimum Average

Se concentration (μg/kg) 150.3 15.7 55.7 283.9 14.6 94.2 Yield (dry weight kg/ha/y) 10,250 6000 7500 8000 4500 6000 Se output ﬂux (g/ha/y) 7.53 0.09 0.29 11.98 0.15 0.45 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108 105

Fig. 6. Volatilization output ﬂuxes of selenium from topsoil (g/ha/y).

discrepant, with 89% of the exogenous Se input in the Fujiang River topsoil Se concentration in the research area. We analyzed a variety Basin being precipitation followed by irrigation water, while the ex- of input and output pathways and obtained the following significant ogenous input caused by fertilization is only 1%. results: The element proportion diagram of the different output pathways (Fig. 10) shows that the principal output pathway of elemental Se is (1) Selenium concentration in topsoil of the Fujiang River Basin is infiltration, which accounts for 69% of the total output. This pathway currently categorized as deficient (39.3% of the research area) is also followed by the removal of straw from the cropland, crop or marginal (32.9% of the research area). The areas with medium harvesting, and Se volatilization. to high concentrations (27.8% of the research area) are mainly Water, as a carrier, results in a relatively strong migration capability distributed along the Fujiang River. during the geochemical cycling process of elements. Precipitation and (2) The analysis of the vertical soil profile indicates that human activ- infiltration are the main input and output pathways, respectively, of ities and bioaccumulation significantly influence Se concentra- Se, particularly in the topsoil. Other pathways, such as fertilization, irri- tion in topsoil and may account for 59.7% to 78.9% of the total gation, and crop harvesting, have a relatively minimal influence on the budget. element cycle in most cases. (3) The possibility that the enrichment of elemental Se in topsoil is affected by human activities exhibits relatively high. The annual av- fl 4. Conclusions erage input and output uxes of elemental Se in the Fujiang River Basin are 20.45 g/ha/y and 2.58 g/ha/y, respectively. Precipitation fi We established a geochemical cycle model for elemental Se and in ltration are the main input and output pathways, respec- in the topsoil of the Mianyang area in the Fujiang River Basin, tively, of elemental Se. Sichuan Province by studying the distribution characteristics of In future research, combining the research results on elemental Se cycle in the Fujiang River Basin with an assessment of the safety of dietary Se for residents in the Fujiang River Basin is necessary, which will boost the prospect for revealing the re- Table 10 lationship between the geochemical environment and human Annual input and output fluxes of Se. health.

Sin (g/ha/y) Sout (g/ha/y) Sin/Sout Maximum 46.37 22.69 17.15 Acknowledgments Minimum 5.74 0.29 2.08 Average 20.45 2.58 7.93 This work was supported by the Major Programs of the Geolog- Standard deviation 7.82 1.38 3.62 ical Survey of Land Resources, China Geological Survey (Nos. 106 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108

Fig. 7. Spatial distribution of selenium input ﬂuxes (g/ha/y).

Fig. 8. Spatial distribution of selenium output ﬂuxes (g/ha/y). T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108 107

Fig. 9. Net ﬂuxes of selenium in topsoil (g/ha/y).

1212010560101 and 1212010511218), National Natural Science Foun- The authors would like to thank the Mineral Resources Super- dation of China (No. 41172326), and by the “Fundamental Research vision and Testing Center of Chengdu and Hefei for the analytical Funds for the Central Universities” (No. 2010ZY54). support.

Fig. 10. Average composition (%) of inputs and outputs of selenium in topsoil. 108 T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108

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