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: