2 Materials and method
2.1 Soil sample collection and preparation
In China, the main PC area includes the northeast temperate zone
(~4.0%), northwest temperate arid and Qinghai-Tibet
alpine zones (~10.7%), Huanghuaihai and Bohai Rim warm
temperature zones (~50.7%), and subtropical rainy area
in Yangtze River Basin (~23.6%). Covering the four
major PC areas in China, from May to November 2019, we collected 132
soil samples from Liaoning province (including Tieling city, Shenyang
city, and Jinzhou city), Shaanxi province (including Yanˊan city and
Xianyang city), Ningxia Autonomous Region (including Yinchuan city and
Wuzhong city), Hebei province (including Cangzhou city and Baoding
city), Shandong province (including Liaocheng city and Weifang city),
Henan province (including Xinxiang city), Jiangsu province (including
Nantong city), and Yunnan province (including Kunming city, Honghe
prefecture and Chuxiong prefecture), respectively. The specific sampling
area information is shown in Fig. 1.
The soil samples were collected from a depth of 0-20 cm in the
greenhouses of 0 (open-air cultivation), 1-5, 6-10, and >10
years of PC using the S-shaped 5-point sampling method. The soil samples
were evenly mixed and then packed in marked kraft bags to transport to
the Yunnan Soil Fertilization and Pollution Remediation Engineering
Laboratory in Yunnan Agricultural University. The samples were freed
from impurities, naturally air-dried, ground, and passed through 2.0,
1.0, and 0.149-mm nylon sieves. The final samples were tested following
the 4-point method.
2.2 Soil analysis
To estimate soil HMs concentration, the soil samples were digested with
concentrated nitric acid-hydrochloric acid (1:1, v/v) according to the
National Standards GB/T 22105.1-2008 and GB/T 2205.2-2008 (General
Administration of Quality Inspection and Quarantine Supervision of the
People’s Republic of China, et al., 2008a, 2008b) and Industry Standards
NY/T 1613-2008 (Ministry of Agriculture of the People’s Republic of
China, 2008) of the People’s Republic of China. Cu, Zn, Ni, and Cr
amounts were determined by flame atomic absorption spectroscopy (FAAS,
PerkinElmer 900H, PerkinElmer, Akron, OH, U.S.A.); Pb and Cd amounts
were estimated by graphite furnace atomic absorption spectroscopy
(GFAAS, PerkinElmer 900H, PerkinElmer, Akron, OH, U.S.A.); As and Hg
amounts were determined by
atomic
fluorescence spectroscopy (AFS-933, Titan Instrument, Beijing, China).
The bioavailable Cu, Zn, Pb, and Cd, extracted with
Diethylenetriaminepentaacetic acid (DTPA) buffered solution, were
determined by Inductively coupled plasma optical emission spectrometry
(ICP-OES iCAP7400) according to National Environmental Protection
Standards of the People’s Republic of China (State Environmental
Protection Administration, 2016). The bioavailable As, extracted with
sodium dihydrogen phosphate-buffered solution, was determined by atomic
fluorescence spectroscopy. Two certified reference soils (GBW07413a and
GBW07451) from the National Research Center for Certified Reference
Materials of China were used for quality control. Concentrations of
reference materials were within the certified ranges, and the average
recoveries for soil Cu, Zn, Pb, Cd, As, Hg, Cr, and Ni were 94.6, 103,
98.2, 93.5, 90.5, 95.7, 108, and 93.5%, respectively.
The main soil physicochemical indicators such as texture (including
sand, clay, powder), pH, organic matter (SOM), and cation exchange
capacity (CEC) were determined. The particle size distribution of the
soil was determined by a laser particle size analyzer (BT-9300HT,
Dandong Bettersize Instruments Ltd., China) using water as the medium.
Soil pH was measured by a glass electrode (PHSJ-3F, Shanghai Instrument
Instruments Co. Ltd., China) in a 1:2.5 soil/water (w/v) suspension. SOM
was determined using the
K2Cr2O7-H2SO4oxidation method (Bao, 2000). Extracted CEC was determined using the
ammonium acetate method (Lu, 2000).
2.3 Risk assessment of soil HMs accumulation
1) Single-factor pollution index (SFPI)
The SFPI signifies the cumulative risk characteristics of soil elements
(Cheng et al., 2007; Yu et al., 2016). It is the ratio of the determined
concentration of soil HMs to the corresponding limit in the standard GB
15618-2018 (Ministry of Ecology and Environment of China, 2019). This
can be calculated as follows: