Soil geochemical characteristics
Geochemical characteristics of the topsoil.The average element content of the four surface soil samples in the study area was compared with the Chinese soil background value (Wei et al., 1990)(Table 2).The content of Se, Hg, Cr, and Cu in the soil in this area were slightly higher, while the content of Cd, As, Ni, Pb and Zn and pH values were slightly lower. Compared with the average soil geochemistry of Ganzhou City (Yong et al., 2020), the contents of Se, Cr, Ni, and Cu were slightly higher, while the contents of Cd, As, Hg, Zn and pH were slightly lower. According to Table 3, the average pH value of the soil in this area was 4.81, which was strongly acidic. 50% of the soil samples were selenium-rich soils. The average selenium content in the whole area was 0.39×10− 6, and its grade was moderate. The soil environmental geochemical grade was calculated according to formula 1, and the single pollution index Pi of soil pollutant i was calculated:
![](data:image/png;base64,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)
In the formula, Ci—the measured concentration of index i in the soil; Si——the second-level standard value of pollutant i given in GB 156185-1995 "Soil Environmental Quality Standard" (Xia et al., 1995). By calculating the single pollution index of heavy metal elements in the soil in this area, Pi < 1 (Table 2), the soil environmental geochemical grade in this area was clean according to Table 3.
Table 2
The statistical table of geochemical characteristics of elements in soil (/10− 6)
Sample | Cd | As | Hg | Se | Cr | Ni | Cu | Zn | Pb | Organic matter | pH |
C1 | 0.067 | 1.79 | 0.0505 | 0.18 | 83.80 | 22.20 | 22.40 | 43.90 | 29.20 | 4.79 | 4.81 |
C2 | 0.120 | 7.07 | 0.1190 | 0.31 | 68.00 | 20.00 | 24.50 | 70.30 | 29.50 | 5.50 | 5.21 |
D1 | 0.059 | 5.51 | 0.0504 | 0.57 | 93.80 | 18.60 | 33.70 | 32.60 | 14.10 | 2.22 | 4.58 |
D2 | 0.015 | 4.82 | 0.0558 | 0.49 | 90.80 | 26.60 | 29.60 | 36.00 | 20.00 | 2.81 | 4.65 |
Average value | 0.065 | 4.80 | 0.0689 | 0.39 | 84.10 | 21.85 | 27.55 | 45.70 | 23.20 | 3.83 | 4.81 |
China´s soil(Wei et al.,1990) | 0.1 | 11.2 | 0.0650 | 0.29 | 61.00 | 26.9 | 22.6 | 74.2 | 26 | 3.1 | 6.7 |
Ganzhou soil(Yong et al.,2020) | 0.187 | 9.124 | 0.094 | 0.272 | 41.974 | 16.251 | 19.843 | 73.849 | — | — | 5.19 |
Si(Xia et al., 1995) | 0.30 | 40 | 0.30 | — | 150 | 40 | 50 | 200 | 250 | — | — |
Pi(Xia et al., 1995) | 0.22 | 0.12 | 0.23 | — | 0.56 | 0.55 | 0.55 | 0.23 | 0.09 | | |
Note: The unit of organic matter content is %, and pH is dimensionless; —means no data. |
Table 3
Boundary of soil pH, selenium(10− 6) and environmental geochemical grade [17]
Selenium | ≤ 0.125 | 0.125ཞ0.175 | 0.175ཞ 0.40 | 0.40ཞ 3.0 | > 3.0 |
Grade | Deficient | Marginal | Sufficient | Rich | Excessive |
pH | ༜ 5.0 | 5.0ཞ 6.5 | 6.5ཞ 7.5 | 7.5ཞ 8.5 | ༞ 8.5 |
Grade | Strong acidity | Acidic | Neutral | Alkalinity | Strongly alkaline |
Single pollution index | P < 1 | 1 < P ≦ 2 | 2 < P ≦ 3 | 3 < P ≦ 5 | P ≧ 5 |
Environmental geochemical grade | Clean | Slight pollution | Light pollution | Moderate pollution | Severe pollution |
Characteristics of geochemical elements in vertical profile.In order to study the vertical distribution characteristics of each element in the upland rice planting soil, a vertical profile was constructed with a profile depth of 200 cm and divided into 5 layers. The field description of the profile was shown in Table 4, and the geochemical data of the soil vertical profile was displayed in Table 5.
Table 4
The field description of soil vertical profile
Stratification | deepth/cm | Description of wild | Number of samples |
Humus layer (layer A) | 0 ~ 14 | Dark gray, sandy loam soil, deciduous, dead leaf layer, developed root system, loose soil layer, animal holes appear. | PM1-1 |
Leaching layer (layer E) | 14 ~ 45 | Light gray-yellow sandy loam with a small amount of quartz gravel, compactable, plate-like soil with a few roots and occasional animal holes. | PM1-2 |
Deposited layer(layer B) | 45 ~ 80 | Light red-light yellow, sandy soil, with a small amount of weathered parent rock and gravel, the soil is massive and compact, with only a few roots. | PM1-3 |
Parent layer (layer C) | 80 ~ 136 | Light red-yellow, mainly composed of red slate fragments and sandy soil. the soil layer is compact, fragile, and occasionally root system. | PM1-4 |
Source rock | 136 ~ 200 | Reddish-brown, mainly composed of red slate layers, the rock layers are horizontally layered, the rock layers are fragile, iron leaching occurs after local weathering, and animal holes are occasionally seen. | PM1-5 |
Through stratified sampling and experimental testing, it was found that the contents of Cd, Hg, Cr, and organic matter in the humus layer (layer A) were the highest, and their contents decreased with the increase of profile depth, indicating that these elements have a significant tendency to be enriched on the surface; while the contents of Ni, Cu, Zn and pH were the lowest. With the increase of profile depth, their values were tended to increase, indicating that these elements were susceptible to weathering. The contents of Se, Cd, Hg, Pb, and organic matter in the leaching layer (layer E) were lower than that of the humus layer and the deposition layer, while the contents of As and Ni were higher than that of the humus layer and the deposition layer, suggesting that the geochemical behavior of each element was different under strong leaching, (Fig. 3).
Table 5
The geochemistry data list of soil vertical profile (/10− 6)
Stratification | Sample | Se | Cd | As | Hg | Cr | Ni | Cu | Zn | Pb | organic matter | pH |
Humus layer (layer A) | PM1-1 | 0.53 | 0.080 | 4.39 | 75.76 | 92.37 | 30.90 | 31.57 | 40.18 | 33.49 | 5.17 | 4.71 |
Leaching layer (layer E) | PM1-2 | 0.43 | 0.034 | 6.99 | 42.71 | 92.08 | 31.49 | 33.87 | 43.00 | 30.09 | 1.45 | 4.77 |
Deposited layer(layer B) | PM1-3 | 0.55 | 0.061 | 5.26 | 63.33 | 88.34 | 31.40 | 40.52 | 49.39 | 34.45 | 1.53 | 5.11 |
Parent layer (layer C) | PM1-4 | 0.40 | 0.040 | 3.82 | 60.07 | 88.77 | 38.22 | 48.03 | 56.76 | 39.39 | 0.84 | 5.18 |
Source rock | PM1-5 | 0.39 | 0.054 | 3.66 | 66.21 | 89.25 | 43.30 | 52.91 | 61.87 | 49.42 | 0.81 | 5.15 |
Note: The unit of organic matter content was %, and pH was dimensionless. |
Depending on the analyze of the correlation between the elements in the soil of vertical profile, it was found that the content of Se was positively correlated with the contents of Cd, As, Hg, Cr, and organic matter respectively, while negatively correlated with the contents of Ni, Cu, Zn, Pb and pH respectively. Among heavy metals, Cd content was significant positively correlated with Hg content, while Ni was great positively correlated with Cu, Zn, and Pb content (Table 6). The pH was serious positively correlated with Cu, Zn content, and negatively correlated with Cr content, which indicating that Cr was easily migrated in an acidic environment.
Table 6
The correlation analysis of Se, heavy metal elements, organic matter and pH in soil vertical profile
| Se | Cd | As | Hg | Cr | Ni | Cu | Zn | Pb | Organic matter | pH |
Se | 1 | 0.715 | 0.210 | 0.410 | 0.140 | -0.776 | -0.641 | -0.649 | -0.569 | 0.639 | -0.368 |
Cd | | 1 | -0.378 | 0.888* | 0.185 | -0.266 | -0.309 | -0.323 | -0.007 | 0.801 | -0.288 |
As | | | 1 | -0.761 | 0.466 | -0.676 | -0.655 | -0.639 | -0.767 | -0.022 | -0.554 |
Hg | | | | 1 | -0.110 | 0.133 | 0.101 | 0.083 | 0.358 | 0.575 | 0.078 |
Cr | | | | | 1 | -0.505 | -0.78 | -0.778 | -0.539 | 0.680 | -0.969** |
Ni | | | | | | 1 | 0.933* | 0.933* | 0.957* | -0.571 | 0.692 |
Cu | | | | | | | 1 | 1.000** | 0.906* | -0.723 | 0.905* |
Zn | | | | | | | | 1 | 0.904* | -0.735 | 0.903* |
Pb | | | | | | | | | 1 | -0.430 | 0.687 |
Organic matter | | | | | | | | | | 1 | -0.751 |
pH | | | | | | | | | | | 1 |
**0.01significant correlation of level, *0.05 significant correlation of level, the number of samples was 5 pieces. |
4 Geochemical characteristics of mountain rice
Geochemical characteristics of rice.In accordance with GB/T 22499 − 2008 selenium-enriched rice(Xie et al.,2009), from Table 2, it could be seen that the selenium contents of mountain rice samples in the study area were 0.047×10− 6 and 0.058×10− 6, respectively, reaching the lever of selenium-rich rice (0.04×10− 6~ 0.30 × 10− 6). Taking into account the limits of contaminants from the food of GB 2762 − 2017 National Food Safety Standard Food(The Ministry of Health of the People's Republic of China,2017), as shown in Table 6, the contents of inorganic As, Hg, Cd, Cr and Pb in upland rice were all lower than the pollutant limit value of 0.20×10− 6, 0.02×10− 6, 0.20×10− 6, 1.00×10− 6 and 0.20×10− 6. Owing to no limit standard for Cu, Zn, and Ni content was set in China. Therefore, the upland rice in this area was identified as green selenium-rich upland rice.
Table 7
Analysis and test results of upland rice (/10− 6)
sample | As | Hg | Se | Cd | Cr | Cu | Pb | Zn | Ni |
SD1 | 0.006 | 0.0041 | 0.047 | 0.0768 | 0.68 | 7.29 | 0.14 | 31.24 | 1.91 |
SD2 | 0.036 | 0.0044 | 0.058 | 0.0672 | 0.30 | 6.74 | 0.14 | 30.43 | 1.85 |
The chemical characteristics of the earth globe of the root system.
The selenium contents of the root soil samples of upland rice in the study area were 0.55×10− 6 and 0.31×10− 6, respectively. The average value was calculated to be 0.43×10− 6. According to Table 3 and Table 8, the upland rice root soil was strongly acidic and rich in selenium, and the soil environmental geochemical grade was clean.
Table 8
Analysis and test results of root soil of upland rice /10− 6
sample | Inorganic As | Hg | Se | Cd | Cr | Cu | Pb | Zn | Ni | Organic matter | pH |
GXT1 | 4.41 | 0.0044 | 0.55 | 0.04 | 91.45 | 32.67 | 26.06 | 52.67 | 32.78 | 1.20 | 4.40 |
GXT2 | 2.88 | 0.0029 | 0.31 | 0.05 | 64.81 | 23.87 | 8.10 | 32.04 | 12.85 | 1.00 | 4.42 |
Note: The unit of organic matter content was %, and pH was dimensionless. |
Bio-enrichment coefficient.The biological coefficient was estimated to demonstrate the ability of crops to absorb and enrich a certain element from the soil(Jakubus et al.,2019). Biological enrichment coefficient (BCF) = C crops/C root soil, C crop was the content of the element in the crop (10− 6), C root soil represented the content of the element in the corresponding root soil of the crop (10− 6). The average enrichment coefficients of selenium and 8 heavy metal elements were displayed in Table 9. Their orders were as follows: Cd > Hg > Zn > Cu > Se > Ni > Pb > inorganic As > Cr, indicating that Cd and Hg could be easily absorbed by the upland rice in the investigative area. Moreover, Selenium-enriched mountain rice had a risk of heavy metals exceeding the standard.
Table 9
Ratio of biological concentration of Se and heavy metals
sample | Inorganic As | Hg | Se | Cd | Cr | Cu | Pb | Zn | Ni |
SD1 | 0.001 | 0.930 | 0.085 | 1.920 | 0.007 | 0.223 | 0.005 | 0.593 | 0.058 |
SD2 | 0.013 | 1.528 | 0.187 | 1.344 | 0.005 | 0.282 | 0.017 | 0.950 | 0.144 |
Average value | 0.007 | 1.229 | 0.136 | 1.632 | 0.006 | 0.2525 | 0.011 | 0.7715 | 0.101 |