Yuyi HUANG, Guifen CHEN, Yanfei HUANG, Bin LIU, Liumei XIONG, Xiaoqing OU, Xiuhe ZHAO, Yueyue ZHOU, Mei PANG

1. Institute of Agricultural Resource and Environment, Guangxi Academy of Agricultural Sciences, Nanning 530007, China; 2. Agricultural Research Institute of Beihai, Beihai 536005, China

Abstract [Objectives] To study the characteristics of soil heavy metal contents in facility agriculture of Guangxi and analyze its risks. [Methods] The heavy metal content of 30 surface soil samples taken from facility agriculture in Guangxi was analyzed. The status of soil heavy metal pollution in facility agriculture of Guangxi was evaluated by the single factor index method, geo-accumulation index method and Hakanson potential ecological risk index method with reference to the soil background value in Guangxi. [Results] Compared with the soil background value in Guangxi, the over-limit rates of heavy metals cadmium, lead, arsenic, chromium, mercury, copper and zinc in soil samples were 56.67%, 56.67%, 16.67%, 30.00%, 50.00%, 60.00% and 80.00%, respectively. Compared with the screening values of the Soil Environmental Quality-Risk Control Standard for Soil Contamination of Agricultural Land (for trial implementation) (GB15618-2018), the over-limit rates of the heavy metals cadmium, arsenic, copper and zinc in the surface soil of agricultural facilities in Guangxi were 56.67%, 10.00%, 16.67% and 23.33%, respectively. Except for the average content of soil cadmium exceeding the screening value, the evaluation content of the other six soil heavy metals did not exceed the screening value. According to the evaluation results of the single factor pollution index method, the surface soil arsenic and chromium of the agricultural facilities in Guangxi belonged to the clean level, lead, mercury, and copper belonged to mild pollution, and cadmium and zinc belonged to moderate pollution. According to the evaluation results of the geo-accumulation index, the lead, arsenic, chromium and mercury in the surface soil of the agricultural facilities in Guangxi belonged to the pollution-free level, while the cadmium, copper and zinc belonged to the mild-moderate pollution. According to the evaluation results of the potential ecological risk index method, the potential ecological risks of the heavy metals of lead, arsenic, chromium, copper and zinc in the soil of agricultural facilities in Guangxi belonged to low potential ecological risk level, and the potential ecological risks of cadmium and mercury belonged to moderate risk level. The potential ecological risk index (RI) of the seven heavy metals in the soil was 132.46, which belonged to moderate potential ecological risk. The largest contribution of soil heavy metals to the potential ecological risk index was cadmium and mercury. [Conclusions] Taking the soil of facility agriculture in Guangxi as the object, this paper studied the characteristics of heavy metal content in facility agricultural soil and its risk assessment, in order to provide a scientific basis for facility soil heavy metal risk prevention and control and provide safe technology for facility agriculture.

Key words Soil of facility agriculture, Heavy metal content, Risk assessment, Guangxi

1 Introduction

Facility agriculture is a modern agricultural production mode that aims to save land, water, and energy, and takes overall advantage of protection facilities, biotechnology, environmental regulation and engineering technology management. Since China is a country with large population but less land, the transformation from extensive traditional agriculture to highly intensive modern agriculture is an inevitable trend of agricultural development. In 2016, the cultivation area of protected vegetables in China had exceeded 3.701 million ha, with an annual output of 260 million t, and China has become the country with the largest area of protected vegetable cultivation in the world. Due to intensive production of facility agriculture, high multiple cropping index, relatively closed facility environment, large input of fertilizers and agrochemicals, water and heat imbalance in the closed environment,

etc

., the internal micro-ecological environment has undergone significant changes, the problems of facility soil salinization, acidification, and heavy metal pollution are becoming more and more serious, and adversely affect the sustainable use of facility soil and the safety of agricultural products and the environment. In recent years, the accumulation risk of heavy metals in protected vegetable fields has increasingly attracted the attention of the industry. Some studies have shown that the accumulation of heavy metals in facility soils is obvious year by year.

The facility agriculture has developed rapidly in Guangxi. At present, Guangxi’s vegetable, melon, grape and other greenhouses are planted with 40 000 ha. However, accumulation of heavy metals in the soil caused by long-term intensive management and exceeding the standard is unknown. In addition, there are few reports on the accumulation of soil heavy metals in facilities of Guangxi and their risk assessment research. Therefore, taking the soil of facility agriculture in Guangxi as the object, we studied the characteristics of heavy metal content in facility agricultural soil and its risk assessment, in order to provide a scientific basis for facility soil heavy metal risk prevention and control and provide safe technology for facility agriculture.

2 Materials and methods

2.1 Collection of soil samples

For this study, we selected the soil collection area in the relatively developed area of Guangxi’s facility cultivation, and collected facility agricultural soil of different planting years and the sampling depth was 0-20 cm. Each soil sample was a mixed sample of a greenhouse with an area of 667-2 000 m. The soil samples of each greenhouse were collected at multiple points, using the quincuncial sampling method, using a wooden shovel and a stainless steel soil drill tool, and randomly collecting 5-10 plow layers, and leaving 1 kg soil after fully mixing. The soil samples were put into a polyethylene plastic bag, marked and sealed, brought back to the laboratory for air drying, grinding and screening according to the requirements of the analysis items, and for soil pH and heavy metal content analysis. In total, we collected 30 soil samples.

2.2 Measurement method and data analysis

In this study, the soil pH was measured by the potentiometric method [

V

(distilled water)∶

m

(soil]=2.5∶1); the soil copper, zinc and chromium were determined with reference to

Soil

and

Sediment

—

Determination

of

Copper

,

Zinc

,

Lead

,

Nickel

and

Chromium

(HJ491-2019) —

Flame

Atomic

Absorption

Spectrophotometry

; lead and cadmium were determined in accordance with the

Soil

Quality

Determination

of

Lead

and

Cadmium

(GB/T17141-1997) —

Graphite

Furnace

Atomic

Absorption

Spectrophotometry

; the total mercury and total arsenic were determined in accordance with the

Soil

Quality

Determination

of

Total

Mercury

and

Total

Arsenic

(GB/T22105.1-2008, GB/T22105.2-2008) —

Atomic

Fluorescence

Method

.

In order to compare the effects and differences of different cultivation years on soil heavy metal pollution, we statistically analyzed the characteristics of heavy metal content in the surface soil of 0-20 cm of the greenhouse soil with the greenhouse years of 0-5, 6-10, and more than 10 years. The statistical analysis was performed with the aid of Excel 2007.

2.3 Assessment method of soil heavy metals

At present, there are many assessment methods for soil heavy metal pollution. In this study, we used single factor index evaluation method, geo-accumulation index method and potential ecological risk index method to assess and analyze the status of soil heavy metal pollution in facilities in Guangxi with reference to the soil background value in Guangxi. The background values of cadmium, mercury, arsenic, lead, chromium, copper and zinc in Guangxi soil were 0.276, 0.152, 20.50, 24.00, 82.10, 27.80 and 75.60 mg/kg, respectively.

2.3.1

Single factor pollution index method. The single factor pollution index refers to the pollution of a single heavy metal factor, and its calculation formula is as follows:

P

=

C

/

S

(1)

where

P

is the soil heavy metal single factor pollution index,

C

is the actual measured value of heavy metal element

i

in the soil,

S

is the assessment standard value of heavy metal element

i

, and Guangxi soil background value was taken as the assessment standard value. When

P

≤ 1, it indicates that the area is non-polluted; when 1<

P

≤2, it indicates that the area is mildly polluted; when 2<

P

≤3, it indicates that the area is moderately polluted; when

P

>3, it indicates that the area is highly polluted.

2.3.2

Heavy metal geo-accumulation index. The geo-accumulation index is mainly used to study the quantitative index of the pollution degree of heavy metals in sediments and the pollution assessment of different heavy metals in the environment. It can simply and intuitively judge the level of heavy metal pollution. It fully considers the impact of natural geology and human activities on heavy metal pollution. It can be expressed as follows:

I

=log[

C

/(1.5

BE

)]

(2)

where

C

is the measured concentration of the element; 1.5 is the correction index;

BE

is the background value. Classification standard:

I

≤0, no pollution; 0<

I

≤1, mild-moderate pollution; 1<

I

≤2, moderate pollution; 2<

I

≤3, moderate-high pollution; 3<

I

≤4, high pollution; 4<

I

≤5, higher pollution; 5<

I

≤10, extremely higher pollution.

2.3.3

Assessment method of potential soil ecological risk. The potential ecological risk assessment of soil heavy metals was performed with reference to the potential ecological risk index method proposed by the Swedish scholar Hakanson, and the calculation formula is as follows:

(3)

(4)

Table 1 Relationship between potential ecological risk assessment indicators and classification

3 Results and analysis

3.1 Characteristics of soil heavy metal contents

The analysis of heavy metals in the surface soil of agricultural facilities in Guangxi showed that the soil heavy metal contents have changed significantly. From Table 2, it can be seen that the coefficient of variation of cadmium, arsenic, copper, and zinc in the soil is 81.26%-106.00%, which is higher than that of lead, chromium, and mercury in the soil. Compared with the screening values of the

Soil

Environmental

Quality

-

Risk

Control

Standard

for

Soil

Contamination

of

Agricultural

Land

(

for

trial

implementation

) (GB15618-2018), the over-limit rats of the heavy metals cadmium, arsenic, copper and zinc in the surface soil of agricultural facilities in Guangxi were 56.67%, 10.00%, 16.67% and 23.33%, respectively. The content of lead, chromium and mercury in the soil are all lower than the soil heavy metal pollution risk control standard, and there is no phenomenon that exceeds the standard, but only the average content of soil cadmium exceeds the screening value, and the assessment content of the remaining six soil heavy metals does not exceed the screening value. The number of soil samples with heavy metals cadmium, lead, arsenic, chromium, mercury, copper and zinc content exceeding the soil background values in Guangxi accounted for 56.67%, 56.67%, 16.67%, 30.00%, 50.00%, 60.00% and 80.00% of the total samples.

Table 2 Characteristics of soil heavy metal contents in facilities of Guangxi

The characteristics of the surface soil content in facilities of different planting years are shown in Table 3. With the increase of planting years, the content of cadmium and zinc gradually increases, and the content of chromium and copper also has a trend of gradual increase. The over-limit rate of cadmium, copper and zinc in the surface soil for 1-5 years were 2.40, 2.08 and 2.77 times of those over 10 years, and the over-limit rate of soil arsenic did not increase with the increase of planting years. The reason needs to be further studied.

Table 3 Characteristics of soil heavy metal contents in facilities with different planting years

3.2 Assessment of soil heavy metal pollution

3.2.1

Single factor pollution index method. According to the classification standard of single pollution degree of soil, the average single factor pollution index

P

of arsenic and chromium of the surface soil in facilities of Guangxi was 0.83 and 0.81, respectively, which are both less than 1, belonging to the clean level. The average single factor pollution index

P

of lead, mercury and copper were 1.12, 1.19, and 1.72 respectively, belonging to the mild pollution. The average single factor pollution index

P

of of cadmium and zinc were 2.02 and 2.35 respectively, belonging to the moderate pollution.As shown in Table 4, the average single factor pollution index

P

of soil cadmium in facilities planted for 1-5 years was 0.97, which is less than 1, belonging to the level of no pollution. The average single factor pollution index of soil cadmium in facilities planted for more than 6 years was 2<

P

≤3, belonging to the moderate pollution. The average single factor pollution index

P

of soil lead in facilities planted for 1-10 years was 1<

P

≤2, belonging to the mild pollution, while the average single factor pollution index of soil lead in facilities planted for more than 10 years was less than 1, belonging to the level of no pollution. The average single factor pollution index of soil mercury and copper in facilities of different planting years was 1<

P

≤2, belonging to the mild pollution. The average single factor pollution index

P

of soil zinc in facilities planted for 1-5 years was 1.76, belonging to the mild pollution, while the average single factor pollution index of soil zinc in facilities planted for more than 6 years was 2<

P

≤3, belonging to the moderate pollution. The average single factor pollution index of soil arsenic and chromium in facilities of different planting years was

P

≤1, belonging to the level of no pollution.

Table 4 Single factor pollution index of soil heavy metals in facilities of different planting years

3.2.2

Geo-accumulation index. The average geo-accumulation index of heavy metals cadmium, lead, arsenic, chromium, mercury, copper and zinc in the surface soil of facilities in Guangxi was 0.38, -0.58, -0.87, -0.88, -0.34, 0.20 and 0.65, of which the average geo-accumulation index (

I

) of cadmium, copper and zinc was 0<

I

≤1, belonging to the mild-moderate pollution. The average values of the geo-accumulation index of lead, arsenic, chromium and mercury were all less than 0, and they all belong to the level of no-pollution. From Table 5, it can be seen that the geo-accumulation index of heavy metals in the surface soil of facilities with different planting years varies greatly. The average value of the geo-accumulation index (

I

) of soil zinc in facilities with 1-5 planting years and soil cadmium, copper and zinc (

I

) for in facilities with more than 6 planting years is 0<

I

≤ 1, belonging to mild-moderate pollution, and the rest is less than 0 and belongs to the level of no-pollution. The average value of the geo-accumulation index (

I

) of surface soil cadmium, copper and zinc in the facilities increased with the increase of planting years, indicating that the pollution risk of cadmium, copper and zinc in the soil gradually increased with the increase of planting years.

Table 5 Geo-accumulation index of heavy metals in the surface soil of facilities with different planting years

Table 6 Potential ecological risk coefficient and potential ecological risk index of heavy metals in surface soil of facilities with different planting years

4 Conclusions and discussion

The research on the soil heavy metal contents in facilities is mainly concentrated on elements such as cadmium, lead, arsenic, chromium, mercury, copper and zinc. Most of the soil heavy metals in facility vegetable fields have accumulatedand show different pollution degrees, while, the cadmium pollution is the most serious in facility vegetable fields in many areas in China. According to the results of this study, compared with the screening values of the

Soil

Environmental

Quality

-

Risk

Control

Standard

for

Soil

Contamination

of

Agricultural

Land

(

for

trial

implementation

) (GB15618-2018), the soil cadmium content exceeded the screening value, and the content of the other six soil heavy metals did not exceed the screening value; the content of soil heavy metals at some points exceeded the background value of soil heavy metals in Guangxi, especially the soil heavy metals cadmium, lead, mercury, copper and zinc. With the increase of planting years, the content of soil cadmium and zinc gradually increases, and the content of chromium and copper also has a trend of gradual increase, which may be caused by the application of pesticides and organic fertilizers with high levels of heavy metals in the facility soil of Guangxi.There are differences in the assessment results of three types of soil heavy metal pollution. The research results of the single factor pollution index method show that arsenic and chromium in the surface soil of facilities in Guangxi belong to the clean level, lead, mercury, and copper are mildly polluted, and cadmium and zinc are moderately polluted. The assessment of the geo-accumulation index showed that the lead, arsenic, chromium and mercury in the surface soil of the facilities in Guangxi belong to the level of no pollution, while the cadmium, copper and zinc belong to the mild-moderate pollution. Besides, research results of the potential ecological risk index method show that the potential ecological risk coefficient of lead, arsenic, chromium, copper and zinc was less than 40, belonging to low potential ecological risk; the potential ecological risk coefficient of cadmium and mercury was greater than 40, but less than 80, belong to medium risk level; the potential ecological risk index (

RI

) of the seven heavy metals in the soil was 132.46, belong to the medium potential ecological risk, and the largest contribution of soil heavy metals to the potential ecological risk index is cadmium and mercury.