Overview of the study area
Geographically, Ningxia Hui Autonomous Region is located in northwest China (35°14ʹN-39°23ʹ N, 104°17ʹ E-107° 39ʹ E). In Ningxia, the average altitude exceeds 1,000 m, and mountains are interspersed with plains. Also, Ningxia has a terrain that is is high in the south and low in the north, featuring a temperate continental arid and semi-arid climate14. In the study area, solar irradiation is abundant, with an annual sunshine duration ranging from 1997.1 to 3041.4 h. On average, the annual temperature is 5.6-9.9 ℃, and the precipitation concentrates in summer, reaching the range of 175.9-644.8 mm on average. The annual evapotranspiration ranges from 1214.3-2482 mm, and the frost-free period lasts 105-164 days15.
Sample collection and testing methods
The vegetable heart test soil used in this study was collected from five typical planting areas in Ningxia, namely Pingluo, Helan, Yongning, Wuzhong and Guyuan. The sampling area basically covered the areas of vegetable hearts planting in Ningxia. The number of years of vegetable hearts planting was 1-5, 6-9, and 10-13 years, respectively. Five vegetable fields were selected to collect soil samples for each cropping year in each area. Figure 1 shows the distribution of sampling points. The diagonal five-point sampling method was used to collect the soil samples at a depth of 0-20 cm with the assistance of a soil auger. After the removal of stones, plant roots, stems, leaves and other impurities, the soil samples were air-dried, with some of them ground to pass through a 2 mm nylon sieve for determining soil TN, AN, TC, and TOC. While the remaining ones were ground to pass through a 200-mesh nylon sieve for determining soil heavy metals.
The content of nitrogen dissolved by soil alkaline was determined by alkaline dissolved diffusion method, and the total organic carbon content was determined using TOC-L organic carbon analyzer (Shimadzu, Japan). The content of As, Cr, Cd, Mn, Tl, Zn, and Cu in the soil samples was determined by inductively coupled plasma mass spectrometry (ICP-MS, Perkin-Elmer, USA), respectively, the results of which were restricted to the relative standard deviation of ≤ 5%. Also, all spiked recoveries fell within the upper limit of ≤ 1%. The relative standard deviations (RSDs) of the results were restricted to be ≤ 5%, and the recoveries were all around 90%.
Methods of analysis
The ground accumulation index (Igeo) is a quantitative index proposed by German scientist Muller in the late 1960s to evaluate the severity of heavy metal contamination in the sediments. It can be used to reflect the natural characteristics of change in heavy metal distribution16. This index is expressed as follows:
(1)
Where Igeo represents the ground accumulation index, Cn denotes the tested concentration of element n in soil samples (mg·kg-1), and Bn indicates the geochemical background value of element n in the soil samples (mg·kg-1), which is taken in this study as the background value of the Ningxia soil17 (Table 1). In order to correct the variations in background value as caused by rock differences, a correction factor of 1.5 was introduced into this study18. The classification criteria of Igeo were divided into seven classes19 (Table 2).
Enterprise
|
As
|
Cr
|
Cd
|
Mn
|
Tl
|
Zn
|
Cu
|
Background value
|
11.9
|
60
|
0.112
|
524
|
0.4
|
58.8
|
22.1
|
Table 1 Background reference values of soil heavy metals in Ningxia (mg·kg-1)
Pollution level
|
Non-polluting
|
Mild pollution
|
Medium pollution
|
Medium to heavy pollution
|
Heavy pollution
|
Extremely heavy pollution
|
Extreme pollution
|
Igeo
|
≤0
|
(0,1]
|
(1,2]
|
(2,3]
|
(3,4]
|
(4,5]
|
>5
|
Table 2 Classification standard of soil geo-accumulation index
Data processing and analysis
SPSS 17.0 statistical software was used to conduct significance and correlation analyses. Origin 2021 was used for plotting curves. The fitting equations used for soil AN and heavy metals include quadratic equation, exponential equation, power function and linear equation.
Analysis of results
Heavy metal accumulation and pollution degree of soil in different cropping years
As shown in Table 3, the continuous cropping soil was divided into three classes by the number of years of planting: 1-5, 6-9 and 10-13. After the detection of seven heavy metals in the five different planting areas, it was found out that there was an increase in the content of As, Cr, Cd, Mn, TL, Zn and Cu in the soil with an increase in the number of years of planting. As the content increased from 5.89-12.44 mg·kg-1 to 9.9-22.10 mg·kg-1, Cr content surged from 44.89-61.97 mg·kg-1 to 69.14-87.74 mg·kg-1; Cd content increased from 0.05-0.08 mg·kg-1 to 0.10-0.16 mg·kg-1; Mn content rose from 374.92-512.92 mg·kg-1 to 547.63-778.22 mg·kg-1; Tl content increased from 0.37-0.49 mg·kg-1 to 0.47-0.64 mg·kg-1; Zn content rose from 32.91-68.08 mg·kg-1 to 78.20-78.73 mg·kg-1. 89.73 mg·kg-1; and Cu content surged from 18.97-22.67 mg·kg-1 to 27.54-34.01 mg·kg-1. Overall, the content of soil heavy metals increased significantly with a rise in the number of years during which the continuous cropping of vegetable hearts was practiced in the five planting areas.
As indicated by the results obtained from the Igeo calculation of 7 heavy metals detected in the soil after different years of continuous cropping (Table 4), the Igeo of all seven heavy metals in 25 fields with less than five years of continuous cropping was negative, except for Zn in the soil after 1-5 years of continuous cropping, with mild pollution found in 1 soil sample. It was indicated that there was almost no pollution of heavy metals occurring to the soil used to grow vegetable hearts after less than five years of continuous cropping. The highest Igeo value of As, Cd, Tl and Zn was 0.234, 0.063, 0.015 and 0.101, respectively, with 24%, 4%, 4% and 4% of the soils being insignificantly contaminated. In comparison, the highest value of As, Cr, Cd, Mn, Tl, Zn and Cu in the soils after planting for 10-13 years was 0.4%, 0.4%, 0.4%, 0.4%, 0.4% and 0.4%, respectively. The highest value of As, Cr, Cd, Mn, Tl, Zn and Cu in the soils after 10-13 years of continuous cropping reached 0.480, 0.093, 0.219, 0.054, 0.133, 0.121 and 0.054, respectively. The sample soils with mild contamination accounted for 56%, 11.54%, 22.22%, 14.29%, 34.48%, 16.67% and 29.03%, respectively.
Region
|
Years of continuous planting
|
As content
(mg·kg-1 )
|
Cr content
(mg·kg-1 )
|
Cd content
(mg·kg-1 )
|
Mn content
(mg·kg-1 )
|
Tl content
(mg·kg-1 )
|
Zn content
(mg·kg-1 )
|
Cu content
(mg·kg-1 )
|
Pingluo
|
1-5
|
5.89±0.14 b
|
44.89±2.11c
|
0.07±0.01b
|
420.52±27.70c
|
0.37±0.01b
|
32.91±1.91c
|
18.97±1.27c
|
6-9
|
9.53±0.41 a
|
59.37±1.54b
|
0.10±0.01a
|
474.96±52.89b
|
0.39±0.02b
|
45.06±4.42b
|
24.08±1.86b
|
10-13
|
9.99±0.31 a
|
69.14±4.05a
|
0.10±0.01a
|
547.63±4.55a
|
0.47±0.01a
|
78.20±3.46a
|
27.54±2.87a
|
Helan
|
1-5
|
6.71±0.27 c
|
50.14±1.32b
|
0.07±0.01c
|
467.84±15.30b
|
0.43±0.01c
|
43.45±1.05c
|
19.22±1.48c
|
6-9
|
11.50±1.27b
|
63.39±4.88a
|
0.10±0.01b
|
633.41±16.38a
|
0.48±0.01b
|
65.60±1.44b
|
24.89±3.10b
|
10-13
|
14.40±1.34a
|
69.74±5.48a
|
0.11±0.01a
|
634.34±8.57 a
|
0.50±0.01a
|
83.45±1.85a
|
30.02±1.76a
|
Yongning
|
1-5
|
8.54±1.20 b
|
53.22±0.50c
|
0.06±0.01c
|
484.25±13.37c
|
0.45±0.02c
|
47.74±1.87c
|
22.67±0.75c
|
6-9
|
17.68±1.21a
|
69.33±5.45b
|
0.11±0.01b
|
551.05±11.00b
|
0.52±0.02b
|
64.03±3.71b
|
28.78±1.55b
|
10-13
|
20.02±2.73a
|
77.68±10.18a
|
0.16±0.02a
|
743.50±53.78a
|
0.57±0.04a
|
84.61±0.54a
|
31.91±1.71a
|
Wuzhong
|
1-5
|
12.44±0.58c
|
61.97±1.16c
|
0.08±0.01b
|
512.92±19.35c
|
0.49±0.01c
|
68.08±3.80b
|
22.51±1.00c
|
6-9
|
19.61±1.24b
|
79.13±6.31b
|
0.13±0.03a
|
698.09±18.46b
|
0.57±0.02b
|
78.82±7.45a
|
28.85±2.70b
|
10-13
|
22.10±1.89a
|
87.74±5.85a
|
0.16±0.03a
|
778.22±29.91a
|
0.62±0.03a
|
82.18±7.88a
|
33.82±2.42a
|
Guyuan
|
1-5
|
10.32±0.60c
|
45.29±1.04c
|
0.05±0.01c
|
374.92±6.79c
|
0.42±0.01c
|
58.31±2.44b
|
21.31±0.83c
|
6-9
|
17.29±0.76b
|
62.25±2.13b
|
0.10±0.01b
|
502.61±15.73b
|
0.56±0.03b
|
81.56±8.18a
|
28.32±0.93b
|
10-13
|
20.69±1.61a
|
75.21±5.65a
|
0.14±0.03a
|
646.11±26.88a
|
0.64±0.01a
|
89.73±5.82a
|
34.01±1.58a
|
Table 3 Changes in soil heavy metal content in five different planting areas of Ningxia cauliflower heart in three years
Note: Lowercase letters indicate significance
Igeo
|
Years of continuous planting
|
Heavy metal
|
Minimum value
|
Maximum values
|
Average value
|
Non-polluting
|
Mild pollution
|
Medium pollution
|
1-5
|
As
|
-1.337
|
-0.153
|
-0.818
|
25
|
0
|
0
|
Cr
|
-1.086
|
-0.329
|
-0.678
|
25
|
0
|
0
|
Cd
|
-1.675
|
-0.677
|
-1.143
|
25
|
0
|
0
|
Mn
|
-1.007
|
-0.429
|
-0.688
|
25
|
0
|
0
|
Tl
|
-0.731
|
-0.063
|
-0.367
|
25
|
0
|
0
|
Zn
|
-1.283
|
0.021
|
-0.575
|
24
|
1
|
0
|
Cu
|
-0.878
|
-0.302
|
-0.533
|
25
|
0
|
0
|
6-9
|
As
|
-1.004
|
0.243
|
-0.296
|
16
|
9
|
0
|
Cr
|
-0.698
|
-0.047
|
-0.443
|
25
|
0
|
0
|
Cd
|
-0.984
|
0.063
|
-0.674
|
24
|
1
|
0
|
Mn
|
-0.992
|
-0.127
|
-0.476
|
25
|
0
|
0
|
Tl
|
-0.687
|
0.015
|
-0.262
|
24
|
1
|
0
|
Zn
|
-1.214
|
0.101
|
-0.431
|
24
|
1
|
0
|
Cu
|
-0.725
|
-0.005
|
-0.306
|
25
|
0
|
0
|
10-13
|
As
|
-0.907
|
0.480
|
-0.098
|
11
|
14
|
0
|
Cr
|
-0.535
|
0.093
|
-0.257
|
22
|
3
|
0
|
Cd
|
-0.867
|
0.219
|
-0.353
|
19
|
6
|
0
|
Mn
|
-0.536
|
0.054
|
-0.243
|
21
|
4
|
0
|
Tl
|
-0.426
|
0.133
|
-0.112
|
15
|
10
|
0
|
Zn
|
-0.304
|
0.121
|
-0.081
|
20
|
5
|
0
|
Cu
|
-0.459
|
0.154
|
-0.084
|
16
|
9
|
0
|
Table 4 Heavy metal pollution degree of flowering Chinese cabbage soil with different continuous cropping years
Dynamic changes of soil C and N contents in different years of continuous cropping
Fig. 1 shows a comparison of soil AN, TN, TC and TOC contents in the five study areas, revealing that the content of AN, TN, TC, and TOC in the soil decreased with an increase in the number of years of continuous cropping in Pingluo, Helan, Yongning, Wuzhong and Guyuan as the area of cauliflower heart planting. Specifically, the soil AN content decreased from 89.97 g·kg-1 after 1-5 years of continuous cropping to 42.51 g·kg-1 after 10-13 years in Pingluo, from 96.90 g·kg-1 after 1-5 years of continuous cropping to 51.33 g·kg-1 after 10-13 years in Helan, from 79.68 g·kg-1 after 1-5 years of continuous cropping to 51.33 g·kg-1 after 10-13 years in Yongning, and from 79.68 g·kg-1 after 1-5 years of continuous cropping to 46.29 g·kg-1 after 10-13 years in Yongning. Besides, soil AN content decreased from 91.09 g·kg-1 after 1-5 years of continuous cropping to 52.45 g·kg-1 after 10-13 years in Wuzhong, and from 80.24 g·kg-1 after 1-5 years of continuous cropping to 49.65 g·kg-1 after 10-13 years in Guyuan decreased. Overall, the decrease ranged from 38.1% to 52.8%, with that of AN reaching the most significant extent in Helan. During the period between 1-5 years of continuous cropping and 10-13 years of continuous cropping, soil TN content decreased by 39.5%, 38.5%, 42.0%, 37.7%, and 18.2% in Pingluo, Helan, Yongning, Wuzhong, and Guyuan, respectively. During the period between 1-5 years of continuous cropping and 10-13 years of continuous cropping, soil TC content decreased by 29.5%, 47.3%, 34.9%, 10.5%, and 45.5% in Pingluo, Helan, Yongning, Wuzhong, and Guyuan Caishin planting areas, respectively. During the period between 1-5 years of continuous cropping and 10-13 years of continuous cropping, soil TOC content decreased by 54.2%, 55.8%, 50.7%, 24.3% and 51.9% in Pingluo, Helan, Yongning, Wuzhong and Guyuan, respectively. Overall, Helan is the area in which the decrease in soil content of TN, AN, TC, and TOC reached the most significant extent.
Figure 1. Changes in soil TN, AN, TC, and TOC contents with years of continuous cropping in different vegetable heart planting areas
Correlation between soil heavy metals, growth of vegetable hearts and C and N contents in different years of continuous cropping
Fig. 2b shows the correlation between the content of soil heavy metals and the indicators of C and N contents. As shown in Fig. 2, the content of seven heavy metals, As, Cr, Cd, Mn, TL, Zn, and Cu in the soil exhibited a significant negative correlation with the soil content of AN. The correlation coefficient of AN with As, Cr, Cd, Mn, TL, Zn, and Cu reached -0.60, -0.69, -0.71, -0.60, -0.56, -0.73, and -0.83, respectively. Differently, there was no significant correlation exhibited by the seven heavy metals with TC and TOC. Therefore, it can be inferred that the soil content of AN was most affected by heavy metal accumulation.
For the plant, AN exhibited a significant correlation shown with soil TN, TC, TOC, plant DWG, FWG, RFW, RDW, Yeild, and NO3- content, as shown in Fig. 2a. Therefore, soil AN exerts a significant effect on soil fertility transformation and plant growth. Thus, soil change is considered a crucial factor for the study on fertility.
Figure 2. Correlation between soil heavy metal content, growth of vegetable hearts and soil C and N content (p ≤0.05)
Effects of different cropping years on dry matter, yield and nitrate of Chinese flowering cabbage
Table 5 shows the effects of different numbers of years of cropping on the growth of vegetable hearts and their NO3- content. Apparently, there was a decreasing trend shown by the dry matter and yield of vegetable hearts in all regions with an increase in the number of years of cropping. In the meantime, the yield of vegetable hearts declined significantly in all regions. However, after the planting of 1-5 years or 6-9 years, there was no significant difference found in fresh weight of the above-ground part of the vegetable heart in Pingluo, Wuzhong, and Guyuan. Also, there was no significant difference observed in the dry weight of the above-ground part of the root in Guyuan, and the difference in the dry weight of the root was insignificant in Wuzhong. In the three areas including Pingluo, Wuzhong, and Guyuan, there was no significant difference found in the dry weight of the root. Similarly, there was no significant difference found in above-ground dry weight in Guyuan. In addition, nitrate content increased with a rise in the number of years of cropping in all regions. However, nitrate content remained at a relatively high levels after the planting of 1-5 years or 6-9 years in Pingluo, Helan and Guyuan. In addition, the yield and nitrate content of vegetable hearts varied by regions, with the highest yield and nitrate content found in Pingluo.
Region
|
Years of continuous planting
|
Fesh weight on the ground/g
|
Dry weight of the ground/g
|
Root fresh weight/g
|
Root dry weight/g
|
Yields/
(kg·hm-2)
|
Nitrate salt/(mg·kg-1)
|
Pingluo
|
1-5
|
212.28±7.47a
|
12.41±0.38a
|
6.00±0.13a
|
1.19±0.03a
|
720.50±7.44a
|
2.21±0.08b
|
6-9
|
201.70±7.71a
|
10.93±0.72b
|
5.60±0.24b
|
1.04±0.07b
|
653.60±19.02b
|
2.22±0.04b
|
10-13
|
124.66±5.50b
|
7.69±0.59c
|
3.46±0.15c
|
0.80±0.02c
|
602.20±11.60c
|
2.88±0.38a
|
Helan
|
1-5
|
158.62±4.30a
|
9.52±0.26a
|
5.39±0.16a
|
1.11±0.01a
|
660.40±3.07a
|
1.92±0.10b
|
6-9
|
140.68±8.78b
|
6.98±0.10b
|
4.87±0.14b
|
1.03±0.03b
|
590.00±8.32b
|
2.05±0.16b
|
10-13
|
108.69±12.19c
|
5.01±0.08c
|
2.34±0.21c
|
0.30±0.01c
|
526.60±14.88c
|
2.27±0.09a
|
Yongning
|
1-5
|
146.62±6.50a
|
9.61±0.22a
|
4.98±0.13a
|
1.26±0.05a
|
652.40±10.14a
|
1.79±0.06c
|
6-9
|
126.42±5.11b
|
7.16±0.17b
|
4.27±0.20b
|
1.00±0.06b
|
588.60±10.09b
|
1.91±0.05b
|
10-13
|
104.50±7.80c
|
5.04±0.08c
|
2.21±0.09c
|
0.23±0.04c
|
526.80±13.79c
|
2.14±0.08a
|
Wuzhong
|
1-5
|
124.99±7.02a
|
9.63±0.23a
|
5.60±0.26a
|
1.48±0.03a
|
657.80±10.65a
|
1.88±0.05b
|
6-9
|
116.49±7.46a
|
8.75±0.72b
|
4.92±0.22b
|
1.41±0.06a
|
622.80±33.80b
|
2.14±0.12a
|
10-13
|
100.30±7.61b
|
7.52±0.51c
|
3.43±0.58c
|
1.08±0.14b
|
519.00±7.48c
|
2.17±0.17a
|
Guyuan
|
1-5
|
165.39±4.06a
|
8.95±0.14a
|
4.63±0.23a
|
1.02±0.06a
|
675.70±5.15a
|
1.82±0.06b
|
6-9
|
155.65±4.28a
|
8.56±0.38a
|
3.70±0.32b
|
0.84±0.06b
|
624.60±9.77b
|
1.89±0.07b
|
10-13
|
101.18±11.06b
|
4.57±0.51b
|
2.50±0.19c
|
0.23±0.01c
|
588.00±9.10c
|
2.11±0.16a
|
Table 5 Effects of different years of continuous cropping on the growth and NO3- content of cauliflower hearts
Note: Lowercase letters indicate significance
Modeling of different heavy metals and AN in vegetable heart soil
According to the correlation analysis shown in Fig. 2, the content of all seven heavy metals in the soil was significantly correlated with AN. By averaging the heavy metal contents and AN contents in each planting area, the linear, exponential, quadratic and power function equations between different heavy metals and AN contents were constructed, as listed in Table 6. The R2 of the four models ranged from 0.95 to 1, indicating the high accuracy of fitting and their applicability to data analysis and prediction.
Heavy metal types
|
Models
|
Regression equation
|
R2
|
AS
|
Multiplication by a power equation (math)
|
y=499.32x-0.794
|
0.95
|
Exponential equation (math)
|
y=157.57e-0.065x
|
0.97
|
Quadratic equation (math)
|
y=-0.26x2+2.21x +87.96
|
1
|
Linear equation (math)
|
y=-4.39x+126.75
|
0.95
|
Cr
|
Multiplication by a power equation (math)
|
y=27770x-1.46
|
0.98
|
Exponential equation (math)
|
y=295.51e-0.024x
|
0.99
|
Quadratic equation (math)
|
y=0.002x2-1.82x+175.67
|
1
|
Linear equation (math)
|
y=-1.58x+168.25
|
0.99
|
Cd
|
Multiplication by a power equation (math)
|
y = 9.8131x-0.811
|
0.98
|
Exponential equation (math)
|
y=154.93e-8.566x
|
0.99
|
Quadratic equation (math)
|
y=1681.1x2-905.98x+140.17
|
1
|
Linear equation (math)
|
y=-570.76x+124.82
|
0.99
|
Mn
|
Multiplication by a power equation (math)
|
y=838905x-1.499
|
0.99
|
Exponential equation (math)
|
y=298.78e-0.003x
|
0.99
|
Quadratic equation (math)
|
y=0.0003x2-0.50x+254.84
|
1
|
Linear equation (math)
|
y=-0.18x+168.24
|
0.99
|
Tl
|
Multiplication by a power equation (math)
|
y=13.17x-2.262
|
0.99
|
Exponential equation (math)
|
y=641.91e-4.612x
|
0.99
|
Quadratic equation (math)
|
y=586.37x2-886.37x+360.86
|
1
|
Linear equation (math)
|
y=-307.02x + 219.39
|
0.99
|
Zn
|
Multiplication by a power equation (math)
|
y=8051.9x-1.155
|
1
|
Exponential equation (math)
|
y=209.68e-0.018x
|
0.99
|
Quadratic equation (math)
|
y=0.018x2-3.6x+222.64
|
1
|
Linear equation (math)
|
y=-1.17x+144.39
|
0.97
|
Cu
|
Multiplication by a power equation (math)
|
y=7045.5x-1.44
|
0.99
|
Exponential equation (math)
|
y=284.5e-0.056x
|
0.99
|
Quadratic equation (math)
|
y=0.09x2-8.32x+223.22
|
1
|
Linear equation (math)
|
y=-3.74x+165.22
|
0.99
|
Table 6 Correlation model between heavy metals and AN in vegetable heart crop soil
Prediction of soil AN extreme deficiency warning value
According to the correlation between soil heavy metals and AN in vegetable hearts, the soil AN content was significantly correlated with the soil content of As, Cr, Cd, Mn, Tl, Zn and Cu (Fig 2). It is indicated that the accumulation of heavy metals in the soil could reflect the severity of AN deficiency in the area of vegetable hearts planting in Ningxia. Therefore, the prediction model between heavy metals As, Cr, Cd, Mn, Tl, Zn, Cu and AN in the soil of vegetable hearts planting and the AN content grading of the China Soil Census were taken as the criterion. When AN ≤ 30 g·kg-1, the AN in the soil is considered too deficient to support crop growth. With AN ≤ 30 g·kg-1 as the threshold, the accumulation value of each metal was predicted as the alarming level of AN deficiency. When the content of As, Cr, Cd, Mn, Tl, Zn, and Cu reached the alarming level, AN may be severely deficient. In this circumstance, crop rotation should be considered, and more AN chemical fertilizers should be applied as soon as possible to maintain the fertility of the soil at the normal level.
By using the four function models of As, Cr, Cd, Mn, Tl, Zn, Cu and AN (Table 7), it was deduced that the As content in the soil was 34.5 mg·kg-1, 25.5 mg·kg-1, 19.8 mg·kg-1, and 22.0 mg·kg-1, respectively; the Cr content in the soil was 107.6 mg·kg-1, 598.76 mg·kg-1, and 768 mg·kg-1, respectively; the Tl content in the soil was 0.74 mg·kg-1, 0.67 mg·kg-1, 0.67 mg·kg-1, and 0.62 mg·kg-1, respectively; the Zn content was 126.72 mg·kg-1, 108.02 mg·kg-1, 73.5 mg·kg-1, and 97.77 mg·kg-1, respectively; and the Cu content was 44.30 mg·kg-1, 40.17 mg·kg-1, 42.99 mg·kg-1, and 36.15 mg·kg-1, respectively (Table 7). In order to maintain the soil AN content at a level required for sufficient productivity, the minimum content of the above seven heavy metals should be taken as the best choice for early warning of severe soil AN deficiency. Therefore, in the areas of vegetable hearts planting, the content of As, Cr, Cd, Mn, Tl, Zn, and Cu in the soil for vegetable hearts planting reached 19.80 mg·kg-1, 87.50 mg·kg-1, 0.17 mg·kg-1, 598.76 mg·kg-1, 0.62 mg·kg-1, 73.50 mg·kg-1, and 36.15 mg·kg-1, respectively. In this case, there may be a severe deficiency of AN in the soil, which is worthy of attention. Meanwhile, necessary measures should be taken to replenish AN and maintain the production capacity at the normal level.
Heavy metal types
|
Models
|
Predicting heavy metal early warning values for extreme soil AN deficiencies(mg·kg-1)
|
As
|
Multiplication by a power equation (math)
|
34.5
|
Exponential equation (math)
|
25.5
|
Quadratic equation (math)
|
19.8
|
Linear equation (math)
|
22.0
|
Cr
|
Multiplication by a power equation (math)
|
107.6
|
Exponential equation (math)
|
95.3
|
Quadratic equation (math)
|
88.68
|
Linear equation (math)
|
87.5
|
Cd
|
Multiplication by a power equation (math)
|
0.25
|
Exponential equation (math)
|
0.19
|
Quadratic equation (math)
|
0.18
|
Linear equation (math)
|
0.17
|
Mn
|
Multiplication by a power equation (math)
|
925.49
|
Exponential equation (math)
|
766.17
|
Quadratic equation (math)
|
598.76
|
Linear equation (math)
|
768
|
Tl
|
Multiplication by a power equation (math)
|
0.74
|
Exponential equation (math)
|
0.67
|
Quadratic equation (math)
|
0.67
|
Linear equation (math)
|
0.62
|
Zn
|
Multiplication by a power equation (math)
|
126.72
|
Exponential equation (math)
|
108.02
|
Quadratic equation (math)
|
73.50
|
Linear equation (math)
|
97.77
|
Cu
|
Multiplication by a power equation (math)
|
44.30
|
Exponential equation (math)
|
40.17
|
Quadratic equation (math)
|
42.99
|
Linear equation (math)
|
36.15
|
Table. 7 Predicting heavy metal early warning values for extreme soil AN deficiencies
Early warning thresholds and cumulative prediction of heavy metals causing extreme soil AN deficiency
As shown in Table 4, mild contamination had already occurred at some sampling sites after 6-9 years of continuous cropping. Also from Table 8, the maximum threshold of heavy metal contamination was determined and the number of years during which continuous cropping was practiced under the context of severe AN deficiency in the soil. It is worth noting that the mean value of soil Mn content in Helan and Wuzhong after 6-9 years of continuous cropping fell within the range of 633.41 ± 16.38 mg·kg-1 and 698.09 ± 18.46 mg·kg-1, respectively, while the minimum value was 617.03 mg·kg-1 and 679.63 mg·kg-1, respectively (Table 1), exceeding the alarming level of 598.76 mg·kg-1 for AN by Mn (Fig. 2). In addition, the soil content of Zn after 6-9 years of continuous cropping in Wuzhong and Guyuan areas was also above the alarming level of 73.50 mg·kg-1 for Zn (Table 1). Therefore, the upper limit is 6 years for the continuous cropping of vegetable hearts in Helan, Wuzhong and Guyuan. The cumulative increase of soil metal contents in Yongning and Pingluo was relatively limited. Considering that both Mn and Zn were below the alarming level, the duration of continuous cropping in these two areas should be controlled to less than 9 years.
Years of continuous planting
|
Region
|
Heavy metal type
|
As
|
Cr
|
Cd
|
Mn
|
Tl
|
Zn
|
Cu
|
Prediction threshold
|
19.80
|
87.50
|
0.17
|
598.76
|
0.62
|
73.50
|
36.15
|
6-9
|
Pingluo
|
|
/
|
/
|
/
|
/
|
/
|
/
|
/
|
Yongning
|
|
/
|
/
|
/
|
/
|
/
|
/
|
/
|
Helan
|
|
/
|
/
|
/
|
*
|
/
|
/
|
/
|
Wuzhong
|
|
/
|
/
|
/
|
*
|
/
|
*
|
/
|
Guyuan
|
|
/
|
/
|
/
|
/
|
/
|
*
|
/
|
10-13
|
Pingluo
|
|
/
|
/
|
/
|
/
|
/
|
*
|
/
|
Yongning
|
|
*
|
/
|
/
|
*
|
/
|
*
|
/
|
Helan
|
|
/
|
/
|
/
|
*
|
/
|
*
|
/
|
Wuzhong
|
|
*
|
*
|
/
|
*
|
*
|
*
|
/
|
Guyuan
|
|
*
|
/
|
/
|
*
|
*
|
*
|
/
|
Predicted number of years of continuous cropping
|
<9a
|
<9a
|
<9a
|
<6a
|
<9a
|
<6a
|
<9a
|
Table 8 Different heavy metal accumulation thresholds and predicted years of continuous cropping
Note: / indicates less than the warning value, * indicates more than the warning value.