3.1. Geochemical Characteristics of Soil Profile in Mining Area
The horizon differentiation of the layers in the soil profile is the result of natural soil formation and long-term human activities. The soil profile in the Shewushan gold deposit area records the migration and accumulation process and vertical distribution characteristics of gold and other metallogenic elements. With the influence of long-term weathering, tectonic activity, and mining activity, the types and contents of soil metal elements at different depths in the mining area were significantly different from the soil profile in the background area. As can be seen from the
Figure 3, the background profile of both of two mine profiles had similar stratified soil layers. The top soil with humus layer is on the surface at the depth of 2.0 m–2.4 m, the middle part is laterite, and the bottom part is brown clay where gold mainly concentrated in. The soils geochemical characteristics of the three profiles were shown in
Table 1 and
Table 2. The content of ore elements in the profile and the background profile are shown in
Figure 3 and
Table 1 and
Table 2. The average content of Ca, Mg, and K in the soil of the mining area were 1.4%, 1.24%, and 2.5%, while the average content of Fe and Al were 4.87% and 8.05% respectively.
From the
Table 1 and
Table 2, the content of Fe in the laterite and brown clay layer was greater than in the topsoil, and gradually decreased with the increasing depth in the profile. The average content of Ca, Mg, and K in the soil in the background area is 0.18%, 0.135%, and 0.475%, and the content of Fe and Al were 4.37% and 7.66%, respectively. The contents of ore elements at depth were lower than those at top. The contents of Ca, K, and Mg in the soil in the mining area were significantly lower than that in the background profile, while the contents of Fe and Al were slightly higher than that in the background profile, respectively.
It can be seen from
Figure 4 that the gold was mainly located at a depth of 7–11 m in the soil profile of the mining area. Most of the gold was concentrated in the lateritic layer and clay layer, and the contents of Fe in the 1–3 m topsoil were significantly higher than those in the background profile. In the process of soil-forming, potassium, sodium, calcium, magnesium, and iron were released under oxidizing stress. The average content of Ca, Mg, K and Na in the surface soil in background profile is 12.86%, 10.89%, 18.96%, and 6.67% compared to the mine area. These data indicate the weathering process. Gold can be enriched by clay mineral and limonite in the oxidizing environment in topsoil and may be absorbed by some dominant plant species in mine. The contents of Mg, Fe, Au, Pb, Cu, Mg, Mn, and Ag varied greatly in each layer of the profile, showing the enrichment and differentiation characteristics of trace elements from the deep layer of the soil to the topsoil layer. The contents of the heavy metal elements Ag, Sn, Au, Hg, Pb, Cu, and As in the topsoil of mine profiles were 7.0, 5.2, 4.7, 4.5, 4.5, 4.2, 2.6, 2.3, and 2.1 times of the background values of Chinese soil, respectively.
The contents of metals in
Figure 3 were unified by normalization processing. Compared to the background, gold could be detected in all layers with high levels of Pb. Except for silver, the concentrations of gold (Au), plumbum (Pb), arsenic (As), antimony (Sb), iron (Fe), and cobalt (Co) in mine profiles were much higher than that in background profile, especially in the laterite and the brown clay layer (which may be the effect of gold mineralization and weathering). From
Table 2 and
Table 3, it can be seen that the contents of As and Sb in the brown clay soils were as much as 2.7 times and 30 times larger in the mine profiles than in the background profile, while the content of Co in the topsoil was 7 times larger than background profiles.
3.2. Enrichment Characteristics of Trace Elements in Soil by Plants
Compared to the topsoil of the background profile, Au, Co, As, and Pb with high contents should be focused upon, as dominant plants may accumulate these elements from the topsoil.
The contents of heavy metals in plants are not only related to the mobility of metal elements but are also closely related to plants’ tolerance to metals stresses [
8,
20]. Elements in the soil are the main source of elements in plants, and therefore the elements related to mineralization will be enriched in the dominant plants. Compared with the plants growing in background profile, dominant species in the mine area are better adapted to the mine soil conditions [
21,
22,
23]. The dominant plants have the capacity to enrich ore-forming elements enrichment and some may be sensitive to the mine environment. The parameters and indexes of these species could be indicators for locating mineral resources. The biological absorption coefficient (BAC) can indicate the extent to which elements are transferred from the soil to the plant. The BAC can be calculated as follows [
24]:
where
Px represents the content of the element in the plant and
Sx represents the content of the element in the root soil.
As can be seen in
Table 3, the biological absorption coefficients of trace elements in different plants under the same growth environment were different. The Mn, Zn, Ba, Cd, and Pb biological absorption coefficient of the
Populus canadensis and camphor were greater than 0.1. Plants enrichment of trace elements from the soil does not completely depend on certain element content in the soil; plants can exist and adjust the physiological mechanism of trace element levels in their parts. Therefore, it is necessary to evaluate the ability of plant leaves to indicate soil geochemical anomalies by integrating the contrast coefficient of each element.
The contrast coefficient (CM) is calculated by the equation:
where
Cmine is the average value of an element in the species in the mine area.
Cbackground is the average element content values of plants in background region.
This coefficient can assess the concentrating ability and sensitivity of plants. According to the coefficient, the contrast of elements in plants can be divided into four levels: low (CM < 3), medium (3 ≤ CM < 5), high (5 ≤ CM < 10), and very high (10 ≤ CM). The larger the CM of an element, the more obvious the intensity indicator of target elements [
25].
Table 4 proved that Au, Ag, Pb, K, Cu, Co, As in the mine area are all higher than the background area. The elements with high contrast coefficient in
Cinnamomun camphora were Au (74.6) and Co (7.78). The contrast coefficients of Au, Ag, Co and As in
Populus canadensis leaves were 34.5, 2.53, 4.06 and 6.03, respectively. The contrast coefficient shows that the content of Au, Co and As can be indicated by
Populus canadensis and
Cinnamomun camphora.
3.3. Effects of Trace Elements on Biochemical Characteristics of Indicator Plants
The absorption of heavy metal elements in plants will cause a series of biological reactions, which can be observed as changes in the enzyme activity and malondialdehyde (MDA) content [
26,
27]. Toxic elements can produce free radicals that lead to membrane lipid peroxides and cell membrane damage [
28]. Therefore, MDA content can reflect the degrees of the toxic elements stress. The content of MDA in plant leaves in mine profile was greater than that growing in background profiles (
Table 5 and
Table S1). With the increase of content of Au, Cu, Zn, Hg, the MDA content in
Populus canadensis leaves increase gradually.
The chlorophyll contents of Populus canadensis and Cinnamomun camphora with various concentrations of Au, Cu, and Zn were shown in
Table 6. Compared to the background profile, the chlorophyll content of poplar and camphor growing in the mine profiles was reduced by 19.2% and 21.6%, respectively, which means that the chlorophyll content of the two species was sensitive to these metals. Plants have an anti-oxidation enzyme system to mop up free radicals [
18]. Superoxide dismutase (SOD) and peroxidase (POD) are clearance agents for free radicals which can clear lipid peroxides and have roles to play in protecting cells and tissue; they can work against the oxidative damage caused by free radicals on the plant cell membrane structure [
25,
29].
Figure 5 proved that the POD and SOD activities in the canadensis grown in background profiles were 2.6 times and 2.1 times lower than those in mine profiles, respectively. The SOD and POD contents in camphor leaves were 1.5 and 1.8 times greater than those in the background profiles, respectively, which reflected that the protective enzyme system in leaves of the two plants was activated. In general, SOD and POD showed an increasing trend with the increase of concentrations of Cu, Au, and Pb. Therefore, the
Cinnamomun camphora and
Populus canadensis can grow normally under the stress of Au, As, Pb, and Co.