Journal of Geochemical Exploration 139 (2014) 97–108
Contents lists available at ScienceDirect
Journal of Geochemical Exploration
journal homepage: www.elsevier.com/locate/jgeoexp
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, Qingye Hou a, Xueqi Xia a, Haiyan Feng a, Meng Zhang a,
Lixin Jin c, Zezhong Kan c
a
b
c
China University of Geosciences, Beijing 100083, PR China
Key Laboratory of Ecological Geochemistry, Ministry of Land and Resources, Beijing 100037, PR China
Sichuan Institute of Geological Survey, Chengdu 610081, Sichuan, PR China
a r t i c l e
i n f o
Article history:
Received 29 January 2013
Accepted 30 September 2013
Available online 9 October 2013
Keywords:
Selenium
Origin
Geochemical cycle
Se-rich area
China
a b s t r a c t
Mianyang City, located in the Fujiang River Basin, Sichuan Province, is a Se-rich area of China. The distribution of
Selenium (Se) in the Mianyang area was studied based on assay data obtained from soil, irrigation water, fertilizer, and rice (grain and stem) samples. The ratio between natural and anthropogenic sources in the area was derived 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 investigated. We established a geochemical model of the Se cycle among the different media (i.e., the atmosphere,
water, soils, and plants). We then calculated the annual Se flux caused by various inputs' (such as precipitation,
fertilization, and irrigation) and outputs' (such as infiltration, crop harvest, removal of straws from cropland, and
volatilization) pathways in the topsoil. We discuss the contribution of different pathways to the Se cycle and provide evidence for exploring Se-rich land in the study area.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Selenium (Se) is an important trace element in the ecological environment. This element has been studied for more than 190 years since
Swedish chemists discovered it in 1817. Excessive exposure to Se and
lack of Se in the environment both cause health problems to humans
and animals (Wang, 1993). The excessive exposure of livestock to Se,
which caused alkaline disease and blind stagger in Europe and the
United States, had been recognized in the 1930s (Moxon, 1937). This
finding highlighted concerns regarding Se poisoning; and thus, Se
was considered as an important environmental contaminant until the
1950s (Mayland, 1994). In 1957, Schwarz and Foltz proved for the
first time that animals need Se as a nutrient (Schwarz and Foltz,
1957). Se was subsequently determined to be an important component
of glutathione peroxidase (Awasthi et al., 1975; Rotruck et al., 1973).
Moreover, Se deficiency may cause white muscle disease among livestock. Se has received universal attention in several fields, including
plant growth, human health, agricultural production, and ecology
(Combs and Combs, 1986; Fordyce et al., 2000; Huang et al., 2013;
Johnson et al., 2010; Levander and Burk, 2006; Mayland, 1994). The
World Health Organization (WHO) has also confirmed that Se is a necessary nutritional element for animals (WHO, 1987). Se is a beneficial
⁎ Corresponding author at: China University of Geosciences, No. 29, Xueyuan Road,
Haidian District, Beijing 100083, PR China. Tel./fax: +86 10 82322079.
E-mail address: [email protected] (Z. Yang).
0375-6742/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.gexplo.2013.09.006
element in plants; however, whether this element is essential for plants
remains debatable (Bañuelos et al., 1997; Lyons et al., 2009; Mateja
et al., 2007).
In the soil–plant–animal/human food system, the soils supply Se
to satisfy the requirement for plants, humans, and livestock. Human
and natural activities constantly change Se concentration in soils. Therefore, the sources, existing forms, and bioavailability of Se in soils play
decisive roles in the geochemical cycle of Se. Se concentration ranges
from 0.01 mg/kg to 2.0 mg/kg in the vast majority of soils in the world,
with an average concentration of 0.4 mg/kg and a very uneven distribution (Fordyce, 2007). To date, several countries have successively
reported soils with an excessive or deficient amount of Se (Dhillon
and Dhillon, 2003; Ermakov and Jovanovic, 2010; Fleming, 1980;
Fordyce et al., 2010; Ihnat, 1989; Jacobs, 1990; Lakin, 1972; Neal,
1995; Wang and Gao, 2001). On a worldwide scale, the areas of soils
with low Se concentration or which lack Se are relatively larger than
those with potentially harmful high concentrations of Se (Girling,
1984). China is located in a low Se area, with more than approximately
10 provinces (municipalities) exhibiting varying degrees of Se deficiency. The region with Se deficiency accounts for approximately 72% of the
national land area. Such arithmetic indicates that Se concentration of
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
behavior of Se in soils from China with relative Se deficiency is highly
significant.
98
T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108
Mianyang City is located in the Fujiang River Basin, Sichuan Province
in the southwestern of China where Se is relatively deficient in soils. A
multi-objective regional geochemical survey of the Chengdu Economic
Zone of Sichuan Province was conducted from 2002 to 2008 (Chen
et al., 2008). The survey revealed that Se-rich soils in Mianyang are
characteristically distributed along the riverbanks. An enrichment of
toxic heavy metals, such as cadmium (Cd), lead (Pb), and zinc (Zn),
was also found in the soil. We conducted a systematic study of the
sources and pathways of the geochemical cycle of Se in soils from this
area, with the aim of providing a case study of a geochemical cycle in
a Se-rich area.
2. Material and methods
(2)
2.1. Study area
The research area is located in Mianyang City in the midstream of
the Fujiang River Basin. The area measures 10,404 km2 with the geographic coordinates of 105°2′26″ to 105°43′25″ (E) and 30°0′18″ to
31°1′52″ (N). This area has a subtropical humid monsoon climate,
with neither cold winters nor hot summers. The annual average temperature is 17.3 °C, and the frost-free season is long. The annual average
precipitation is 602 mm to 1389 mm. The annual sunshine hours are
1042 h to 1665 h. The topography of the entire area is dominated by
hills. The terrain shows a discrepancy between the north and south:
high in the north and lower in the south, with an elevation of 290 m
to 650 m. Nine major soil types can be found in the study area based
on the Chinese soil taxonomy classification (CRG-CST, 2001), i.e., Calcaric
Purple-Udic Cambosols, Calcaric Purple-Orthic Primosols, Carbonatic
Udi-Orthic Primosols, Recalcaric Gleyi-Stagnic Anthrosols, Typic Feaccumuli-Stagnic Anthrosols, Typic Fe-leachi-Stagnic Anthrosols, Albic
Fe-leachi-Stagnic Anthrosols, Typic Aquic-Alluvic Primosols, and Lithic
Haplic-Perudic Ferrosols.
Two crops of rice are planted in most parts of the research area: one
crop of rice and one crop of wheat. Besides, maize is planted in a
small portion of the area. The Fujiang River is the major river running
through the study area from south to north. The main tributaries of
the river include the Anchang, Kaijiang, Furong, Zijiang, and Baoshi
Rivers. Along the upper reaches of the Fujiang River distributed varies
strata, i.e. clastic rocks and mud shales with coals in Mesozoic, and
basalts, limestones, Mackers, copper-bearing shales, phyllites, slates,
crystalline limestones, meta-sandstones, siltstones, etc. in Paleozoic.
The main mineral resources are including the Xigou Fe–Mn deposit,
the Huya Fe–Mn deposit, Moheba Fe–Mn deposit, placer mine at
Shuijing, Gucheng and Nanba, and the Wupingyinchang Hg–Au deposit
(SBGME, 2006).
(3)
2.2. Sampling media and methods
The samples collected from the research area include: (1) topsoil
and subsoil, (2) rice seeds, stems, and their corresponding cultivated
soils, (3) irrigation water, (4) fertilizers, (5) atmospheric precipitation
and infiltration, (6) soil profiles, and (7) sediments.
(1) The topsoil at a depth of 0 cm to 20 cm was collected at a density of one sample per km2. The subsoil at a depth of 150 cm to
180 cm was collected at a density of one sample per 4 km2. The
densities were according to the requirements of the DD200501 Specifications on the National Multi-objective Regional
Geochemical Survey (Li et al., 2013; Wang et al., 2007; Xi
et al., 2005). The sampling sites were located away from
areas with obvious human contaminations, such as roads, villages and garbage dumps. Five holes were dug on a grid (1 km
× 1 km for topsoil and 2 km × 2 km for subsoil) at each sampling
point, and the weight of each sample was controlled greater
than 1 kg. Composite samples were collected in cotton bags
(4)
(5)
using a bamboo spade, and each of the four sub-samples was
composited for analysis. In this study, totally 2601 composite
samples of topsoil and 650 composite samples of subsoil
were collected. After the sampling site was selected, we used
sediment sampling equipment to load equal amounts of
coarse active sediments from three to five points of the section
in plastic buckets, with the excess water drained off. The
collected samples were loaded into a clean sack and kept in a
cool, dry place. The dried samples were sieved by using a 20mesh (b0.84 mm) nylon sieve. The sieved samples were kept
in clean Teflon bags and sent to the laboratory for analysis.
The samples to be analyzed included 5% national standard
material.
We collected rice grains, stems, and the corresponding samples of the topsoil during the harvest season in the ricegrowing regions. The method of topsoil collection was the
same as the first procedure. We first analyzed the entire area
at each sampling site to determine the cropland, terrain, and
fertility status. Those plots with area about 3500 m2 and
well-growing rice were chosen as sampling sites based on
the summary investigation about plots area, landform and
rice growing conditions of study region. In each plot, 4–5 sampling units were taken. The unit scale is 50 cm × (sowing width
+ row spacing) cm. The sampling sites were more than 1 m
from the edge of the land. We collected 124 samples of rice
grains and their corresponding topsoil, and 50 samples of
stems. The rice grain and stem samples were washed with
tap water and then with deionized water to remove soil particles and dust. Subsequently, we dried the samples with tissue
paper. The rice grains were dehulled. All the plant samples
were then oven-dried at 45 °C for 72 h to a constant weight.
The dried plant samples were ground into fine powder
(b 0.074 mm) using a stainless steel mill and were kept in
clean Teflon bags prior to chemical analysis (Yang et al.,
2005).
We sited nine points based on the different irrigation water
systems in the research area, and collected the irrigation
water samples during the irrigation season. We prepared
three polyethylene containers with a capacity of 1000 mL to
collect water samples. One was used to measure heavy metal
elements, such as Pb, Cd, and arsenic (As). And as soon as a
clear water sample was collected, an amount of 10 mL HNO3
(mixed 1:1 by volume) was immediately added and mixed
with the 1000 mL water sample. Another was used to measure
mercury (Hg), and before sampling, we first added 10 mL of
K2Cr2O7 (ρK2Cr2O7 = 5%) solution into the plastic container,
then shook it well. The last one collecting water without padding any reagent was used to measure other major/minor elements. Next, we made records of the sampling and stored the
samples in a refrigerated container. After sample collection
was completed, the samples were sent to the laboratory for
analysis immediately (Ye et al., 2005).
We collected more than 90% of the local fertilizer types at each
unit using the county or district as the collection unit for fertilizer
samples. The weight of each sample was over 500 g. The application amount and the proportion for different fertilizers in each
hectare of farmland from the collection site were also recorded.
At last, we sealed the samples in clean Teflon bags to be sent to
the laboratory.
We divided the research area into seven collection units for
precipitation and infiltration water based on the local annual
precipitation distribution. Besides, soil type, soil texture,
terrain, and topography were also considered in dividing
the area. A rainfall collecting barrel with a top diameter of 0.40
m and a depth of 0.45 m was placed on a bracket 0.7 m above
the ground surface of a relatively flat area in a representative
T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108
location of each collection unit. The soil infiltration collector
was placed 20 cm beneath the topsoil at the precipitation
collection site. The collection period for precipitation and
infiltration was from July 15, 2007 to September 15, 2007.
Tremendous effort was exerted to prevent the loss and pollution of the collecting equipment for precipitation and
infiltration water. Then we accurately measured and recorded
the volume, pH, and temperature during the collection of the
samples. Protective agents were added to measure various
elements using the same method described earlier for the
collection of irrigation water.
(6) We sampled two soil profiles on the flood plains, sitting relatively wide and flat regions without human disturbance. The
first is in Anxian County, which has relatively high Se and Cd
concentrations; and the second is in Santai County, which
has relatively low Se and Cd concentrations. The depth of the
profiles and the sample density were 150 cm and 1 sample/
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).
The collection sites of the aforementioned samples in the field were
located using a Global Positioning System and were monitored with
tracks. The sampling sites are illustrated in Fig. 1.
2.3. Chemical analysis and quality control
The samples were pretreated through the following steps. The
soil samples were dried at a maximum of 40 °C to avoid the loss of
volatile elements and then ground to a grain size of less than 200
mesh (74 μm) using high-alumina agate mills. The plant samples
were washed with deionized water and dried in a low-temperature
(below 60 °C) oven before analyses. While decomposed using a mixture of HF, HNO3, HClO4 and aqua regia, samples were repeatedly
dissolved until the solution was clear. Cd, Cu, Pb, Sc and Zn elements
in all solution samples were tested with an inductively coupled
99
Table 1
Detection limits (DL) of different samples.
Sample types
Element
Unit
Detection
limit
Analytical
method
Soil
Soil
Soil
Soil
Soil
Soil
Soil
Soil
Soil
Soil, fertilizer.
Irrigation water, atmospheric
precipitation, infiltration.
Plant
As
Cd
Cu
Hg
Pb
S
Sc
Zn
SOC
Se
Se
mg/kg
mg/kg
mg/kg
μg/kg
mg/kg
mg/kg
mg/kg
mg/kg
%
mg/kg
μg/L
1
0.03
1
0.5
2
50
1
1
0.1
0.01
0.1
AFS
ICP-MS
ICP-MS
AFS
ICP-MS
XRF
ICP-MS
ICP-MS
VOL
AFS
AFS
Se
mg/kg
0.001
AFS
AFS: atomic fluorescence spectrometry; ICP-MS: inductively coupled plasma mass spectrometry; XRF: X-ray fluorescence spectrometry; VOL: volumetry.
plasma mass spectrometry (ICP-MS, Model X-SERIES); As, Se and
Hg were tested with atomic fluorescence spectrometry (AFS, Model
AFS-230E); and S was tested with X-ray fluorescence spectrometry
(XRF, Model ZSX100C). The detection limits of all the sampling
media are shown in Table 1.
The analysis precision and accuracy were controlled by inspection of standard reference materials (SRMs), recovery tests, internal and external duplicate samples and coded samples. Take
soil samples as example, the accuracy was mainly monitored by
inserting 12 primary SRMs within every 500 samples and analyzing
simultaneously. Four primary SRMs were inserted in cipher within
every batch of 50 samples to assess the precision by the logarithmic
standard deviations between the analytical values and the recommended values. The accuracy and precision of the analyses of all
samples met with the required specifications (Li et al., 2013; Xi
et al., 2005).
Fig. 1. Research area and sampling sites.
100
T. Yu et al. / Journal of Geochemical Exploration 139 (2014) 97–108
[Csubsoil,Sc] is the total concentration of Sc in the subsoil (mg/kg) in dry
matter.
Table 2
Range of Se concentration in topsoil related to health.
Soil category
Se concentration
range (mg/kg)
Effect
Proportion of the
research area (%)
Deficient
Marginal
Moderate
High
Excess
≤0.125
0.125–0.175
0.175–0.40
0.40–3
≥3
Selenium deficient
Potential selenium deficient
Sufficient selenium
Selenium-rich
Selenium poisoning
39.3
32.9
21.5
6.1
0.2
Descriptive statistics (mean, standard deviation, maximum, minimum), correlation analysis and linear regression analysis were determined using SPSS software v17.
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,
which minimize the errors of predicted values.
2.4. Enrichment factor for the soil system
The concentration ratio of the elements in topsoil (0 cm to 20 cm)
and subsoil (N150 cm) was defined as the enrichment factor (EF)
(Bergamaschi et al., 2002; Establier et al., 1985) to represent the migration of elements. Scandium (Sc), a conservative reference element, was
used for the normalization (Grousset et al., 1995). EF was calculated
from the following formula:
EF ¼
h
i h
i h
i h
i
C topsoil;i = C topsoil;Sc = C subsoil;i = C subsoil;Sc ;
2.5. Statistical analysis and GIS methodology
ð1Þ
where
[Ctopsoil,i] is the total concentration of an element in the topsoil (mg/kg)
in dry matter;
[Ctopsoil,Sc] is the total concentration of Sc in the topsoil (mg/kg) in dry
matter;
[Csubsoil,i] is the total concentration of an element in the subsoil (mg/kg)
in dry matter; and
3. Results and discussion
3.1. Concentration and source of Se element in the soil
Selenium belongs to a rare and dispersed group of elements. The
Clarke value (Taylor, 1964) of Se in the crust and its concentration
in all geological bodies are notably low. Se is a chalcophile and a
probiological element prone to secondary enrichment during the supergene surface geochemical cycling process.
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
China (0.29 mg/kg) and in the Chengdu Economic Zone (0.28 mg/kg)
(Chen et al., 2008). In this area, Se deficient and marginal categories
are depicted as the dominant ones in about 39.3% and 32.9% of the
total area respectively, according to the classification criteria of Tan
(1989) (Table 2). The section with moderate-to-high Se concentration
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
Table 3
Correlation coefficients between Se and other elements in soil.
Types
n
As
Cd
Hg
Pb
S
Cu
Zn
SOC
Topsoil
Subsoil
2601
650
0.54
0.41
0.73
0.92
0.44
0.68
0.52
0.54
0.67
0.85
0.83
0.63
0.75
0.66
0.52
0.82
Correlation is significant at P b 0.01 (two-tailed).
concentration or possible Se poisoning is 0.2% of the total (Table 2). The
spatial distribution of the soil which is relatively rich in Se is mainly
found on two banks of the northwestern rivers in the research area
(Fig. 2), with distinct distribution characteristics along the riverbanks.
However, the majority of soils in this area underscore a feature of Sedeficiency.
Other elements with spatial distributions similar to Se in soils include As, Cd, Hg, Pb, copper (Cu), and sulfur (S), which are the
main ore-forming elements associated with metallic sulfide minerals
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
on both riverbanks indicates that Se in soils is significantly correlated
to above elements and soil organic carbon (SOC) (Table 3). The correlation coefficients in subsoil are larger than those in topsoil, thus
indicating that Se is closely associated with these elements in the
soil-forming process.
The correlation analysis between Se and other elements, such as
As, Cd, Cu, Hg, Zn, and chromium (Cr), in the sediments of the river
systems (Fujiang River, Anchang River, Furong Creek, and Kaijiang
River) in the research area also indicates that Se has a significant positive correlation with the aforementioned elements (Table 4). The elemental concentrations of the sediments in the river systems can
reflect the chemical compositions of various types of rocks and deposits in the catchment of the rivers to a certain degree. The close
symbiotic relationship between Se and the other elements (particularly
Cd, As, and Cu) in the sediments of the river systems and the subsoil, as
well as the distribution characteristics along the two riverbanks, indicates that the high Se concentration of soils is probably related to
weathering, erosion, and the deposition of mountain rocks, sulfide deposits, and Fe–Mn deposits in the upper reach of the Fujiang River
(SBGME, 2006).
An analysis of Se, Cd, Cu, Zn, and Hg for the various soil layers
of the two vertical profiles near the Fujiang River in Santai County
and Anxian County in the research area (Fig. 3) reveals that the
Se, Cd, Cu, and Hg concentrations in soils fluctuate within a small
range below 20 cm (the plow pan), and that the average concentration represents the natural background level (background values).
The concentrations of these elements significantly increase in
the topsoil (0 cm to 20 cm). The soil profiles of Se and other elemental concentrations are essentially consistent with the trend in
SOC variation. Therefore, the increase of Se and other elemental
Regression equation
CSe
CSe
CSe
CSe
=
=
=
=
0.0372CCu − 0.62
1.3839CCd − 0.26
0.0569CAs − 0.14
0.0043CS − 0.68
Correlation
coefficient r
Regression equation
Correlation
coefficient r
0.90⁎⁎
0.94⁎⁎
0.81⁎⁎
0.70⁎
CSe
CSe
CSe
CSe
0.78⁎⁎
0.89⁎⁎
0.94⁎⁎
0.70⁎
=
=
=
=
0.0043CHg + 0.004
0.0095CZn − 0.43
1.554CCr + 0.357
0.0278CPb − 0.22
concentrations in topsoil is not only related to the exogenous inputs
caused by human activities but also to the bioconcentration caused
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
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 N20 cm in the soil
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 concentrations in topsoil cannot be disregarded despite of the inheritance
of Se and Cd concentrations from subsoil to topsoil. Therefore, further study of the supergene system and the geochemical cycling
pathways of elements (such as Se and Cd) in soils can provide
strong backing for guiding the development of Se-rich land resources and reducing the ecological hazards of harmful elements
(such as Cd) scientifically.
3.2. The geochemical cycle of Se
3.2.1. A model of the Se cycle in topsoil
The elemental concentrations in topsoil can be affected by the soil
parent materials and the geochemical behaviors of the elements in
soils formation process. Although a variety of human activities
have similar effects on the concentrations, they have significant
discrepancy, such as the timescale. The timescale of natural actions
that influence Se concentration in soils is generally hundreds to
tens of hundreds of years, whereas the timescale of variations concentrations in soils caused by human activities is usually several
years or even several days. Therefore, the geochemical cycling pathways of Se, the input and output of the element in soils are mainly
caused by human activities as considered in this study. These activities primarily include fertilization (F), irrigation (IR), precipitation
(P), crop planting and harvesting (CH), infiltration of precipitation
(I), and volatilization of elemental Se (VS) on the regional scale
(Fig. 4). The pathways in this cycle through which elemental Se enters the topsoil include precipitation, irrigation, and fertilization.
The pathways through which elemental Se exits the topsoil include
infiltration, volatilization, crop harvesting, and the removal of straws
(RS) from the cropland.
3.2.2. Precipitation and infiltration
Precipitation is the most important means through which Se enters
the soils. As rain falling on soils, Se is partly retained in the soils and
soil solution generates, and then it can be absorbed by plants or evaporated to return to the atmosphere. In addition, Se partly penetrates into
groundwater through the plow layer. The easily dissolved elements are
extracted and carried to the plow layer during the penetration process.
Thus, these elements become an important part of the geochemical
cycle of elements in the topsoil.
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 C P V P =100;
Table 4
Correlation equations of the elements in river sediments (n = 10).
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).
101
ð2Þ
where SP is the Se input flux in soils caused by the annual precipitation in the research area (g/ha/y), CP is the Se concentration in the
precipitation (μg/L), VP is the average amount of precipitation at
the sampling site in the research area (mm), and 100 is the unit conversion factor.
SI ¼ C I V I =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
rainfall in the research area (g/ha/y), CI is the Se concentration in the infiltrated water (μg/L), VI is the amount of infiltrated water in one year at
the sampling site in the research area (mm) derived by extrapolating
the amount of infiltrated water during the sampling period, and 100 is
the unit conversion factor.
The calculated results for the elemental concentrations (such as that
for Se) caused by precipitation input and infiltration output in soils of
the plow layer in the research area are shown in Table 6. The amount
of Se input flux from precipitation is relatively high in Anxian County,
Zhongjiang, and Santai, but relatively low in Zitong and Mianyang.
Moreover, The Se output flux is relatively high in Anxian County and
Zitong. The precipitation input and infiltration output lead to a net increase in Se concentration for all the aforementioned sites, namely,
△S N 0 (ΔS = SAD − SIPW), which indicates that precipitation is one of
the primary pathways for increasing Se concentration in soils. The increased flux of Se in soils is highest in Santai, lower in Anxian and
Zhongjian, and lowest in Zitong at only 5.4 g/ha/y.
3.2.3. Irrigation
Farmland irrigation can potentially and vitally use waste water
and improve crop yields. Irrigation in the research area is mainly divided into the Wudu Drinking Water Project and the Renmin Canal
irrigation areas. A part of the water is supplied through precipitation, but the irrigation water comes mainly from various reservoirs
that accumulate water from the Fujiang River and its tributaries.
Therefore, we used the water collected from the Fujiang River and
its major tributaries as analysis data for calculating the elemental
flux in irrigation water.
The equation for calculating the Se input flux in soils resulting from
irrigation water is:
SIR ¼ 1000 C IR V IR ;
ð4Þ
where SIR is the Se input flux in soil caused by irrigation (g/ha/y), CIR
is the Se concentration in irrigation water (μg/L), VIR is the average
of the actual irrigation water used in the farmland in the research
area (5595 m3/ha/y) (SWA, 2005), and 1000 is the unit conversion
factor.
The calculated annual Se input flux caused by irrigation water in
soils is shown in Table 7. Se concentration in river is subject to uneven
distribution among river systems and has evolved a notably diverse
array of the Se input flux. The Se input flux caused by irrigation water
is relatively high in Santai and lower in the part of the Shehong section
of the Fujiang River.
3.2.4. Fertilization
Fertilization is a significant factor in improving crop yield but it
also raises a series of environmental issues, such as soil acidification, soil compaction, and water eutrophication. The fertilizers
used in the research area are mainly produced locally, such as the
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)
Cd (mg/kg)
Cu (mg/kg)
Zn (mg/kg)
Hg (μg/kg)
Se (mg/kg)
Cd (mg/kg)
Cu (mg/kg)
Zn (mg/kg)
Hg (μg/kg)
0.57
0.65
41.00
121.70
219.69
0.32
0.26
27.60
82.80
71.48
0.12
0.23
30.33
99.02
41.78
0.13
0.19
24.10
78.14
50.59
4.73
2.83
1.35
1.23
5.26
2.46
1.37
1.15
1.06
1.41
21.1
36.0
74.0
81.4
19.0
40.3
71.5
87.3
94.4
70.8
78.9
64.0
26.0
18.6
81.0
59.7
28.5
12.7
5.6
29.2
Santai
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
XF02
XF03
XF04
XF05
XF06
XF07
XF08
XF09
Upstream of Fujiang River
Middle of Anchang River
Furong Stream
lower section of Anchang River
Midstream of Fujiang River
Santai section of Fujiang River
Kaijiang River
Upper section of Shehong, Fujiang River
Lower section of Shehong, Fujiang River
Average
7.0
3.8
1.8
3.6
3.6
10.2
4.3
2.6
10.5
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 result is 0.23 g/ha/y.
Fig. 4. Selenium input and output models in topsoil.
Co., Ltd. and the compound fertilizers produced by Deyang City
Shifang Nongdeli Tianfu Fertilizer Plant, Anxian County Chaoyang
Phosphorus Chemical Industry Co., Ltd., and Hubei Yangfeng Co.,
Ltd.
The survey indicates that the average usage of fertilizers in
Mianyang City is 630 kg/ha/y. The application ratio of phosphate fertilizer, compound fertilizer, and ammonium bicarbonate in the research area is 5:3:10. We have estimated that the applied amounts
of phosphate fertilizer, compound fertilizer, and ammonium bicarbonate in the research area based on this ratio are 175 kg/ha/y, 105
kg/ha/y, and 350 kg/ha/y, respectively. The average Se concentration
of these fertilizers ranges from 0.03 mg/kg to 1.18 mg/kg. The Se concentration is 1.18 ± 0.21 mg/kg in phosphate fertilizer (Table 8).
We use the following equation based on the elemental concentrations of fertilizers and the annual amount of fertilizer applied over a
unit area to derive the output flux:
SF ¼
n
X
c j q j =1000; j
j¼1
¼ fphosphorus fertilizer; compound fertilizer; and ammonium bicarbonateg
ð5Þ
where SF is the annual Se input flux in soils caused by using fertilizers
in the research area (g/ha/y), cj is the concentration of elemental Se
in fertilizer j (mg/kg), qj is the annual applied amount of fertilizer j
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 parameters, 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,
2001), is mainly related to SOC, which can be described as TCgrain =
32.384 e−0.2209 SOC (n = 124, r = 0.60, P b 0.05).
We estimated the Se concentration per km2 of rice grains in the
research area using the aforementioned equations, and the Se concentration and SOC data for the 1/km2 topsoil in the research area.
The measured Se concentration of the rice grains is plotted using
the estimated Se concentration of the rice grains at the corresponding site in Fig. 5. The estimated values are reliable at the P b 0.05 confidence level.
Therefore, we use the following equation to calculate the Se output
flux in the soil caused by harvesting rice grains:
Sgrain;CH ¼ 1000 C CH Y CH ; Sgrain;CH
¼ 1000 ðC CH1 Y CH1 þ C CH2 Y CH2 Þ;
ð6Þ
where Sgrain,CH is the Se output flux in soils caused by annual rice grain
harvesting (g/ha/y), CCH1 and CCH2 are the Se concentration of the rice
grains (mg/kg), and YCH1 and YCH2 are the annual rice yields per hectare
(kg/ha/y) derived from monitoring the rice yield at the sampling sites
and in situ investigations from local farmers (Table 9). The average annual rice yield in the research area is 15,000 kg/ha/y.
The linear regression analysis of the dry weight of the grains (Hgrain)
and stems (Hstem), and Se concentration of the grains (Cgrain) and stems
(Cstem) for the 50 rice samples indicates a significant positive correlation:
Hgrain =0.6814 Hstem +351.78, r=0.73, and Pb0.05; Cgrain =0.5054 Cstem
+ 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
(mm)
SP (g/ha/y)
Se
SI (g/ha/y)
Se
△S (g/ha/y)
Se
Zhongjiang
Santai
Yanting
Zitong
Jiangyou
Mianyang
Anxian
1146
913
826
913
1100
1122
1260
Average
19.8
19.1
15.6
9.7
13.9
11.9
20.1
15.8
2.2
0.2
0.1
4.3
1.6
1.7
3.8
2.0
17.6
19.0
15.5
5.4
12.3
10.3
16.4
13.8
Table 8
Average concentrations of Se in different kinds of fertilizers in Mianyang, Fujiang River
Basin, China.
Fetilizer types (number)
Annual application
amount (kg/ha/y)
Se concentration
(mg/kg)
Phosphorus fertilizer (15)
Compound fertilizer (16)
Ammonium bicarbonate (21)
175
105
350
1.18 ± 0.21
0.09 ± 0.21
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.
We estimated the annual Se output flux in soils per hectare caused
by the removal of the rice stems from the cropland using Sgrain,CH =
1000 × CCH × YCH and the aforementioned two regression equations.
Consequently, the results show that the Se output flux in soils caused
by harvesting the rice grains and stems in the research area ranges
from 0.24 g/ha/y to 19.51 g/ha/y.
3.2.6. Volatilization
Volatility is a distinct feature of Se, which has a relatively low boiling
point (684 °C) and high vapor pressure. Therefore, large volumes of
Se can be emitted into the atmosphere through high-temperature
processes such as volcanic activities, coal burning, and smelting.
Another significant biogeochemical process of Se is volatilization caused
by methylation. Animals, plants, and microorganisms in soils and sediments can also release volatile Se into the atmosphere. The volatilization
of Se in soils is related to soil microbial biomass, temperature, humidity,
and texture, and the presence of aqueous Se in soils (Wang et al., 1989).
Recent studies indicate that the annual relative volatilization of Se accounts for 0.024% of the total Se concentration in soils (Wang, 1993).
Therefore, we estimated Se volatilization in soils of each 1/km2 of the research area to be within the range of 0.014 μg/kg to 2.18 μg/kg, with an
average of 0.047 μg/kg, which was calculated using the mass of topsoil
(0 cm to 20 cm) per hectare (ha) of 2.25 × 106 kg and the average Sss
flux of 0.105 g/ha/y (Fig. 6).
3.3. Se input and output fluxes in soils and their influence on Se
concentration
3.3.1. Input and output fluxes of Se
Given the input and output flux models, the total Se input flux in
soils in the research area is Sin = SP + SIR + SF, and the Se output total
flux is Sout = SI + SCH + SSS, where SCH and SSS are the per 4 km2 flux
data of the soils in the research area. SP, SIR, SF, and SI are all assigned
values based on the sampling units and are later calculated with spatial
overlay by interpolating the values over grids to obtain flux data per 4
km2.
The input and output fluxes of soil Se in the research area are shown
in Table 10. The ratio Sin/Sout is generally 7.93, which is significantly
greater than 1 and which indicates that the net Se input flux ΔS (ΔS =
Sin − Sout) in soils is positive. The soil Se concentration in the research
area gradually increases as time passes.
Figs. 7–9 show the geochemical diagram of the Se input flux, the Se
output flux, and the net Se flux in soils, respectively. Meanwhile, three
regional distribution patterns are found. 1) The distribution area with
a relatively high Se input flux is the western hilly region of the research
area. The Se input flux near Yanting County is relatively low across the
area. Our survey reveals that Yanting County is one of the areas with a
high incidence of cancer patients in China. Further studies should be undertaken to prove the connection. 2) The distribution area with a relatively high Se element output flux is mainly the northwest of the
research area, which is adjacent to the Longmenshan area. 3) The net
Se flux is positive in most regions of the research area, which are mainly
located in the southwestern hilly areas and surrounding the Fujiang
River in a V shape.
3.3.2. Composition of input and output fluxes
The main exogenous substance input pathways that influence
the quality of the topsoil are dry and wet atmospheric deposition,
fertilization, and irrigation from the regional perspective. The percentages of different input pathways relative to the exogenous
input flux are shown in Fig. 10. The figure shows that the proportions
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)
Se concentration (μg/kg)
Yield (dry weight kg/ha/y)
Se output flux (g/ha/y)
Rice stems (n = 50)
Maximum
Minimum
Average
Maximum
Minimum
Average
150.3
10,250
7.53
15.7
6000
0.09
55.7
7500
0.29
283.9
8000
11.98
14.6
4500
0.15
94.2
6000
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
Basin being precipitation followed by irrigation water, while the exogenous input caused by fertilization is only 1%.
The element proportion diagram of the different output pathways
(Fig. 10) shows that the principal output pathway of elemental Se is
infiltration, which accounts for 69% of the total output. This pathway
is also followed by the removal of straw from the cropland, crop
harvesting, and Se volatilization.
Water, as a carrier, results in a relatively strong migration capability
during the geochemical cycling process of elements. Precipitation and
infiltration are the main input and output pathways, respectively, of
Se, particularly in the topsoil. Other pathways, such as fertilization, irrigation, and crop harvesting, have a relatively minimal influence on the
element cycle in most cases.
4. Conclusions
We established a geochemical cycle model for elemental Se
in the topsoil of the Mianyang area in the Fujiang River Basin,
Sichuan Province by studying the distribution characteristics of
Table 10
Annual input and output fluxes of Se.
Maximum
Minimum
Average
Standard deviation
Sin (g/ha/y)
Sout (g/ha/y)
Sin/Sout
46.37
5.74
20.45
7.82
22.69
0.29
2.58
1.38
17.15
2.08
7.93
3.62
topsoil Se concentration in the research area. We analyzed a variety
of input and output pathways and obtained the following significant
results:
(1) Selenium concentration in topsoil of the Fujiang River Basin is
currently categorized as deficient (39.3% of the research area)
or marginal (32.9% of the research area). The areas with medium
to high concentrations (27.8% of the research area) are mainly
distributed along the Fujiang River.
(2) The analysis of the vertical soil profile indicates that human activities and bioaccumulation significantly influence Se concentration in topsoil and may account for 59.7% to 78.9% of the total
budget.
(3) The possibility that the enrichment of elemental Se in topsoil is affected by human activities exhibits relatively high. The annual average input and output fluxes of elemental Se in the Fujiang River
Basin are 20.45 g/ha/y and 2.58 g/ha/y, respectively. Precipitation
and infiltration are the main input and output pathways, respectively, of elemental Se.
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 relationship between the geochemical environment and human
health.
Acknowledgments
This work was supported by the Major Programs of the Geological 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 Foundation of China (No. 41172326), and by the “Fundamental Research
Funds for the Central Universities” (No. 2010ZY54).
The authors would like to thank the Mineral Resources Supervision and Testing Center of Chengdu and Hefei for the analytical
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
We gratefully acknowledge Prof. Wang Dacheng for helpful suggestion. The authors also thank to the four reviewers for their critical
reviews and improvements of the manuscript. We particularly thank
Prof. Li Changjiang for his careful work and detailed suggestions.
References
Antoniadis, V., Alloway, B.J., 2001. Availability of Cd, Ni and Zn to ryegrass in sewage
sludge-treated soils at different temperatures. Water Air Soil Pollut. 132, 201–214.
Awasthi, Y.C., Beutler, E., Srivastava, S.K., 1975. Purification and properties of human
erythrocyte glutathione peroxidase. J. Biol. Chem. 250, 5144–5149.
Bañuelos, G.S., Ajwa, H.A., Wu, L., Guo, X., Akohoue, S., Zambrzuski, S., 1997. Seleniuminduced growth reduction in Brassica land races considered for phytoremediation.
Ecotoxicol. Environ. Saf. 36, 282–287.
Bergamaschi, L., Rizzion, E., Valcuvia, M.G., Verza, G., Profumo, A., Gallorini, M., 2002. Determination of trace elements and evaluation of their enrichment factors in Himalayan lichens. Environ. Pollut. 120, 137–144.
Chen, D.Y., Jin, L.X., Kan, Z.Z., Li, Z.H., Liu, Y.P., 2008. Study Report on Eco-geochemistry
Survey In the Economic Zone of Chengdu, Sichuan Province, China. (Unpublished
report in Chinese).
Combs, G.F., Combs, S.B., 1986. The Role of Selenium in Nutrition. Academic Press, New
York 15–40.
Cooperative Research Group of Chinese Soil Taxonomy (CRG-CST), 2001. Chinese Soil
Taxonomy. Science Press, Beijing. New York 1–203.
Dhillon, K.S., Dhillon, S.K., 2003. Distribution and management of seleniferous soils, In:
Sparks, D. (Ed.), Advances in Agronomy, 1st ed. Elsevier, Amsterdam, pp. 119–184.
Ermakov, V., Jovanovic, L., 2010. Selenium deficiency as a consequence of human activity
and its correction. J. Geochem. Explor. 107, 193–199.
Establier, R., Gómez-Parra, A., Blasco, J., 1985. Superficial accumulation of heavy metals in
near shore marine sediments: an objective index of environmental pollution. Environ. Contam. Toxicol. 35, 348–353.
Fleming, G.A., 1980. Essential micronutrients II: iodine and selenium, In: Davis, B.E. (Ed.),
Applied Soils Trace Elements, 1st ed. John Wiley and Sons, New York, pp. 199–234.
Fordyce, F.M., 2007. Selenium geochemistry and health. Ambio 36, 94–97.
Fordyce, F.M., Zhang, G.D., Green, K., Liu, X.P., 2000. Soil, grain and water chemistry in relation to human selenium responsive diseases in Enshi District, China. Appl. Geochem.
15, 117–132.
Fordyce, F.M., Brereton, N., Hughes, J., Luo, W., Lewis, J., 2010. An initial study to assess the
use of geological parent materials to predict the Se concentration in overlying soils
and in five staple foodstuffs produced on them in Scotland. Sci. Total Environ. 408,
5295–5305.
Gao, J., Liu, Y., Huang, Y., Lin, Z.Q., Gary, S.B., Michael, H.W.L., Yin, X.B., 2011. Daily selenium intake in a moderate selenium deficiency area of Suzhou, China. Food Chem. 126,
1088–1093.
Girling, C.A., 1984. Selenium in agriculture and the environment. Agric. Ecosyst. Environ.
11, 37–65.
Grousset, F.E., Quetel, C.R., Thomas, B., Donard, O.F.X., Lambert, C.E., Guillard, F., Monaco,
A., 1995. Anthropogenic vs. lithogenic origins of trace elements (As, Cd, Pb, Rb, Sb,
Sc, Sn, Zn) in water column particles: northwestern Mediterranean Sea. Mar. Chem.
48, 291–310.
Hu, Q.H., Zhu, J.C., Pan, G.X., 2000. Biological geochemistry and selenium in food chain.
Rural Ecoenviron. 16, 54–57 (In Chinese with English abstract).
Huang, Y., Wang, Q., Gao, J., Lin, Z.Q., Bañuelos, G.S., Yuan, L., Yin, X., 2013. Daily dietary
selenium intake in a high selenium area of Enshi, China. Nutrients 5, 700–710.
Ihnat, M., 1989. Occurrence and Distribution of Selenium. CRC Press, Boca Raton, Florida.
Jacobs, L.W., 1990. Selenium in Agriculture and the Environment. Soil Science of America
Society Special Publication, Wisconsin.
Johnson, C.C., Fordyce, F.M., Rayman, M.P., 2010. Symposium on ‘Geographical and
geological influences on nutrition’: Factors controlling the distribution of selenium
in the environment and their impact on health and nutrition. Proc. Nutr. Soc. 69,
119–132.
Lakin, H.W., 1972. Selenium accumulation in soils and its absorption by plants and animals.
Geol. Soc. Am. Bull. 83, 181–189.
Levander, O.A., Burk, R.F., 2006. Update of human dietary standards for selenium, In:
Hatfield, D.L., Berry, M.J., Gladyshev, V.N. (Eds.), Selenium its Molecular Biology and
Role in Human Health, 2nd ed. Springer-Verlag, New York, pp. 399–410.
Li, M., Xi, X.H., Xiao, G.Y., Cheng, H.X., Yang, Z.F., Zhou, G.H., Ye, J.Y., Li, Z.H., 2013. National
multi-purpose regional geochemical survey in China. J. Geochem. Explor. http://
dx.doi.org/10.1016/j.gexplo.2013.06.002.
Lv, Y.Y., Yu, T., Yang, Z.F., Zhao, W.F., Guo, W., Huang, B.M., Li, P., 2012. The regulation
mechanism of selenium distribution in Kaschin–Beck disease area: a case study
in Aba area, Sichuan Province. Environ. Chem. 31, 935–944 (In Chinese with
English abstract).
Lyons, G.H., Genc, Y., Soole, K., Stangoulis, J.C.R., Liu, F., Graham, R.D., 2009. Selenium
increases seed production in Brassica. Plant Soil 318, 73–80.
Mateja, G., Vekoslava, S., Ivan, K., 2007. Metabolic importance of selenium for plants. Eur.
J. Plant Sci. Biotechnol. y1, 91–97.
Mayland, H.F., 1994. Selenium in plant and animal nutrition. In: Frankenberger, W.T.,
Benson, S. (Eds.), Selenium in the Environment. Marcel Dekker, New York, pp. 29–45.
Moxon, A.L., 1937. Alkali disease or selenium poisoning. Bulletin, 331. South Dakota Agricultural Experimental Station, Brookings.
Neal, R.H., 1995. Selenium. In: Alloway, B.J. (Ed.), Heavy Metals in Soils. Blackie Academic
and Professional, London, pp. 260–262.
Rotruck, J.T., Pope, A.H., Ganther, H.E., Swanson, A.B., Hafeman, D.G., Hoekstra, W.G., 1973.
Selenium: biochemical role as a component of glutathione peroxidase. Science 179,
588–590.
SBGME (Sichuan Bureau of Geology and Mineral Exploration), 2006. 1:500000 Geology
and mineral resources maps in Sichuan Province. (Unpublished data in Chinese).
Schwarz, K., Foltz, C.M., 1957. Selenium as an integral part of factor 3 against dietary
necrotic liver degeneration. J. Am. Chem. Soc. 79, 3292–3293.
SWA (Sichuan Water Authority), 2005. Bulletin of water resources in Sichuan province
(In Chinese).
Tan, J.A., 1989. The Atlas of Endemic Diseases and their Environment. Science Press,
Beijing, PR China (In Chinese).
Tan, J.A., Zhu, W.Y., Wang, W.Y., Li, R.B., Hou, S.F., Wang, D.C., Yang, L.S., 2002. Selenium in
soil and endemic diseases in China. Sci. Total Environ. 28, 227–235.
Taylor, S.R., 1964. Trace element abundances and the chondritic Earth model. Geochim.
Cosmochim. Acta 28, 1989–1998.
Wang, Z.J., 1993. Some biogeochemical aspects of selenium in Chinese low-selenium belt.
Environ. Chem. 12, 237 (In Chinese with English abstract).
Wang, Z.J., Gao, Y.X., 2001. Biogeochemical cycling of selenium in Chinese environments.
Appl. Geochem. 16, 1345–1351.
Wang, Z.J., Zhao, L.H., Peng, A., 1989. Selenium evaporation from soils of Chinese Se-deficient
belt. Environ. Chem. 8, 7–11 (In Chinese with English abstract).
Wang, X.Q., Zhang, Q., Zhou, G.H., 2007. National-scale geochemical mapping projects in
China. Geostand. Geoanal. Res. 31, 311–320.
WHO (World Health Organization), 1987. Selenium. Environ. Health Criteria 58, 41.
Xi, X.H., Ren, T.X., Chen, G.G., Zhang, D.C., Ye, J.Y., Xue, S.G., Gu, J.Q., 2005. DD2005-01:
Specification for Multi-Purpose Regional Geochemical Survey (for Trial Implementation). China Geological Survey (In Chinese).
Yang, Z.F., Cheng, H.X., Zhou, G.H., Li, R.M., Qi, S.H., Feng, H.Y., 2005. DD2005-02: Technical
Requirements for Regional Ecosystem Geochemistry Assessment (for Trial Implementation). China Geological Survey (In Chinese).
Ye, J.Y., Li, X.K., Liu, Z., Xiong, C.H., Yao, L., 2005. DD2005-03: Technical Requirements for
Sample Analysis of Regional Ecosystem Geochemistry Assessment (for Trial Implementation). China Geological Survey (In Chinese).