3 Results and discussion
3.1 Soil HMs contents
The respective HM (Cu, Zn, Pb, Cd, As, Hg, Cr, and Ni) concentration in
PC topsoil (0-20 cm) is presented in Table 1. Compared with the soil
environmental background values in various provinces and the national
standard, the amounts of 8 HMs were increased to varying degrees; the
increase was more prominent for Cu, Zn, Pb, Cd, and Hg. In the whole
area, soil amounts of Cu, Zn, Pb, and Cd were 2.09, 1.56, 1.21, 4.70,
and 1.06 times higher than the national background value, respectively.
Cr and Ni were 1.13 and 1.15 times higher, respectively. The
corresponding soil samples in provinces showed a similar trend. In
Liaoning province, the greenhouse soil concentration of Cu, Zn, Pb, Cd,
and Hg reached up to 1.87, 1.77, 1.10, 1.82, and 1.65 times the
background value, respectively. Likewise, compared to the background
value, in Hebei province, Cu, Zn, Pb, Cd, and Hg were higher by 1.07,
1.40, 1.03, 2.44, and 0.89 times; in Shandong province, the increase was
1.18, 1.95, 0.76, 3.38, and 2.10 times; In Henan province, the increase
was 3.33, 2.78, 1.26, 8.28, and 1.82 times; in Shaanxi province, the
increase was 1.24, 1.26, 1.04, 1.69, and 6.92 times; in Ningxia
Autonomous Region, the increase was 1.14, 1.39, 0.84, 1.55, and 1.57
times; in Jiangsu province, the increase was 0.77, 1.26, 0.76, 0.92, and
1.85 times, and in Yunnan Province, the increase was 2.30, 1.57, 1.73,
5.82, 2.16 times, respectively.
Based on GB15618-2018 standard and the distribution point exceeding the
rate of soil HMs, Cd (27.3%) was critically above the over-standard
rate, followed by Cu (15.2%), Zn (4.5%), Pb (3.0%), and As (2.3%),
while Hg and Cr were within the over-standard rates. The over-standard
rate of the 8 HMs in greenhouse soil from respective provinces also
showed higher levels of Cd. Yunnan province was most significantly
affected (84.0%) followed by Henan (50.0%), Liaoning (16.7%),
Shandong (11.8%), and Hebei (6.7%) provinces, respectively. Shaanxi
province, Ningxia Autonomous Region, and Jiangsu province had Cd within
the standard range. In general, Yunnan province was most significantly
affected; apart from Cd, the over-standard rates of Cu, Zn, Pb, As and
Ni reached 64.0, 4.0, 16.0, 12.0, and 8.0%, respectively. Next was
Henan province with 25.0% over-standard rates for Cu and Zn. Studies
showed that HMs enrichment occurs under low pH conditions, mobility, and
biological activity (Lu et al., 2020). Notably, PC soil pH and in
general soil pH of the whole area was 0.8 units higher than the average
background value in China. This can be attributed to soil conditioners
or remediation agents used in PC (Hamid et al., 2019; Lebrun et al.,
2020; Wu et al., 2016). However, some areas, such as Liaoning, Henan,
and Jiangsu, showed acidification characteristics, which could promote
soil HMs exposure to the food chain causing health risks.
3.2 Risk assessment of soil HMs
Next, we calculated the SFPI, NCPI (Pnc ), and
soil comprehensive quality index (IICQ) to evaluate the cumulative risk
of HMs in PC soil in the whole area. The results are shown in Figure 1.
We found that the whole area average Pnc and IICQ
reached up to 1.03 and 1.71, suggesting a slightly contaminated level.
Cd contributed the largest; the average PCd was
1.25 at a slightly contaminated level. The remaining 7 HMs were within
the safe limits.
Among the different test regions, the pollution risks are more serious
in Henan and Yunnan provinces. Yunnan province is at the highest
cumulative risk with the NCPI (Pnc average 2.92)
reaching to moderately contaminated level andIICQs (average 6.04) reaching the extremely
contaminated level. Cd (PCd average 3.71) and Cu
(PCu average 1.52) contributed most with the
average SFPI (Pi ) at the heavily and slightly
contaminated levels, respectively. The average NCPI
(Pnc 1.23) and IICQs (2.10)
respectively suggested that Henan Province is at slightly and moderately
contaminated levels. Here too, Cd contributed the highest with the SFPI
(PCd 1.60) at a slightly contaminated level.
Meanwhile, the PC soils in the remaining 6 provinces are at the
uncontaminated or safe level.
Excessive soil HMs accumulation, with concerns of phytotoxicity and risk
to human health via the food chain, has become one of the prime
environmental issues worldwide (Ye et al., 2020; Corguinha et al., 2015;
Zhang et al., 2018). Cd is considered redundant for metabolism and other
biological function and has been listed in the top 20 hazardous
substances by the United States Environmental Protection Agency and the
Agency for Toxic Substances and Disease Registry (ATSDR) (Rai et al.,
2019; Khalid et al., 2017). On the contrary, although Cu is relatively
low-toxic, excessive Cu concentrations may pose a serious health risk,
such as liver and kidney damage.
3.3 Sources of soil HMs
Principal component analysis (PCA), a classic multivariate statistical
method is popularly used to identify the natural and anthropogenic
sources of soil HMs (Chen et al., 2019). Kaiser-Meyer-Olkin (KMO) and
Bartlett tests were performed to analyze the concentration data of HMs.
The KMO value at 0.76 >0.5 (P <0.05)
suggested that the data was suitable for PCA analysis. Based on the
principal component analysis, Kaiser standardized orthogonal rotation
method, and the maximum variance method, the factor loading matrix was
orthogonally rotated. The cumulative contribution rate (76.78%) of the
variance of the soil HMs concentration eigenvalues, principal factor 1
(PF1), and principal factor 2 (PF2) with eigenvalues >1
explain the sources of HMs in the PC soil (Table 2). The variance
contribution rate of PF1 was 44.35%, and the PF1 factor loadings for
Pb, Cd, and As were 0.894, 0.840, and 0.924, respectively. The variance
contribution rate of PF2 was 32.43%, and the factor loadings for Cu and
Zn were 0.930 and 0.713, respectively. The other three elements, Hg, Cr,
and Ni were mainly affected by PF1 and PF2, with factor loadings for Hg
reaching 0.756 and 0.433, Cr 0.417 and 0.625, and Ni 0.626 and 0.656,
respectively.
Next, we analyzed the Pearson correlation (Table 3) between the HMs
concentration and physiochemical indicators (the proportion of sand,
powder, clay, soil pH, SOM, and CEC). We found that Cu and Zn showed a
significant correlation (R =0.547, P <0.05) with 6 soil
physiochemical indices (P <0.01). The correlation coefficient
(R -value) between Cu concentration and soil organic matter (SOM)
reached 0.482, and between Zn concentration and SOM reached 0.453,
respectively. These indicated that soil Cu and Zn concentrations were
strongly correlated to SOM. Notably, SOM in the PC soil mainly
originates from the return of crop residues and organic fertilizer.
Furthermore, PF1 can be attributed to chemical fertilizer, organic
fertilizer, and other anthropogenic activities (Geng W et al., 2013;
Sager, 2007; Liu et al., 2018; Huang et al., 2017). Soil Pb, Cd, and As
exhibited a very significant correlation with soil physical indicators
(the proportion of sand, powder, and clay) (P <0.01),
but not with soil chemical indicators (pH, SOM, CEC) (P>0.05). This suggests that Pb, Cd, and As are mainly from
soil geological backgrounds, such as soil parent material and the PF2
can be attributed to soil parent material and other natural factors
(Yang et al., 2020). Some studies showed that
livestock
manure and chemical fertilizers (especially phosphate fertilizers) can
be a source of Pb, Cd, and As (Luo et al., 2009); however, compared with
the geological background source factor, the contribution of
fertilization was found minimal for Pb, Cd, and As soil contamination in
PC. Moreover, Hg, Cr, and Ni contents in PC soils were found affected by
anthropogenic agricultural activities and natural factors such as soil
parent material.
3.4 Relationship between the soil HMs contents and planting year
HMs have an obvious cumulative effect in the PC soil; the over-standard
GB15618-2018 rates of Cu, Zn, Pb, Cd, and As exceeded 2%. Specifically,
in the study area, the accumulation of Cu and Cd is at serious
environmental risk. Several studies suggest that the available soil
concentration is the bioavailable part of the total HMs concentration
(Fang et al., 2018; Xia et al., 2016; Wang et al., 2020). Excessive
bioavailable HMs in the soil get easily absorbed into the food chain
(crops) or leached by irrigation water as
agricultural
non-point source pollution (Natasha et al., 2021). To further understand
the relationship between the available concentration of five HMs (Cu,
Zn, Pb, Cd, and As) and the planting years in PC, we did a correlation
analysis (Fig. 2). We found that the available Cu (Fig. 2 a), Zn (Fig. 2
b), and As (Fig. 2 e) exhibited a significant (P <0.05)
linear correlation with the planting years, while the available Cd (Fig.
2 c) and Pb (Fig. 2 d) did not (P >0.05). This
further indicates that Cu and Zn are mainly derived from frequent
anthropogenic agricultural activities, and the cumulative effect and
potential cumulative risk increase with the extension of planting years
in PC. Likewise, available As showed a significant linear relationship
(Fig. 2 e) with the planting years; however, the main source of As is
the soil parent material. This suggests that frequent anthropogenic
agricultural activities in the PC can increase total As conversion into
available As (Antoniadis et al., 2019). Available Pb and Cd did not
correlate with the extension of planting years, indicating a minor
contribution of anthropogenic agricultural activities compared to soil
geological background value.