3. Carbon stock characteristics of natural vegetation in the mining area
3.1 Carbon content, biomass and carbon stock characteristics of the tree layer
According to Table 3, the carbon content in the tree layer shows the following order: stem > leaf > bark > root > branch, ranging from 48.05–58.76%. The biomass of the tree layer is highest in the stem (198.76 t∙hm-2), followed by the branch (143.04 t∙hm-2) and the root (73.12 t∙hm-2), while the leaf (51.21 t∙hm-2) and the bark (28.28 t∙hm-2) have the lowest biomass. Carbon storage in the stem accounts for the majority of carbon storage in the tree layer, representing 45% of the total, followed by the branch (25%) and root (12%), while the leaf (10%) and bark (5%) have the smallest contributions. The order of carbon storage is stem > branch > root > leaf > bark.
Table 3
Tree layer carbon content, biomass and carbon loss reserves
Components
|
ω (C)/%
|
Biomass/(t/hm2)
|
VCS/(t/hm2)
|
Leaf
|
55.46 ± 3.78a
|
51.21 ± 16.24b
|
27.63 ± 4.07c
|
Branch
|
48.05 ± 1.10a
|
143.04 ± 37.68a
|
70.67 ± 7.10b
|
Stem
|
58.76 ± 3.17a
|
198.76 ± 51.05a
|
123.84 ± 25.03a
|
Bark
|
53.28 ± 3.53a
|
28.28 ± 7.28c
|
15.76 ± 3.17c
|
Root
|
51.21 ± 7.94a
|
73.12 ± 23.63b
|
35.20 ± 4.64c
|
Average/Total
|
53.29 ± 3.91
|
494.42 ± 135.89
|
273.12 ± 44.03
|
Note: In the "Mean/total" column for components, carbon content is the mean, and biomass and carbon increment are total. Different lowercase letters in the same column indicate significant differences (P < 0.05); mean ± standard deviation, n = 3. Same below |
3.2 Carbon content, biomass and carbon stock characteristics of shrub layer
Based on the carbon content of the shrub layer (Table 4), the order from highest to lowest is branch > root > leaf. Among the organs of the shrub layer, there is no significant difference between the root and the branch (P > 0.05), while the leaf shows a significant difference from the root and the branch (P < 0.05). The biomass distribution of the different organs in the shrub layer is consistent with the carbon content. The branch has the highest biomass in the shrub layer with 44.7% of the total biomass, followed by the root (34.7%) and the leaf has the lowest biomass (20.6%). The order is branch > root > leaf.
When comparing different vegetation types within the same shrub layer, there are differences in biomass and significant differences between organs (P < 0.05). The main organ contributing to the carbon storage in the shrub layer is the branch, which accounts for 54.0% of the carbon storage in the shrub layer, followed by the root (30.7%), and the leaf has the smallest contribution (16.3%). The order is: branch > root > leaf.
Table 4
Carbon content, biomass and carbon loss of shrub layer
Components
|
ω (C)/%
|
Biomass/(t/hm2)
|
VCS/(t/hm2)
|
Leaf
|
29.03 ± 4.47b
|
11.34 ± 0.80b
|
1.41 ± 0.18b
|
Root
|
41.60 ± 2.64a
|
19.27 ± 1.78a
|
2.84 ± 1.03b
|
Branch
|
48.14 ± 4.84a
|
24.77 ± 2.35a
|
4.99 ± 0.53a
|
Average/Total
|
39.59 ± 9.13
|
55.38 ± 6.04
|
9.24 ± 1.66
|
3.3 Carbon content, biomass and carbon stock characteristics of the herbaceous layer
The below ground part of the herbaceous layer accounts for 55.1% of the carbon content, followed by the above ground part (44.9%). The carbon content is higher in the belowground part than in the aboveground part (Table 5). The biomass of the aboveground part of the herbaceous layer (10.15 t∙hm-2) is higher than that of the belowground part (7.10 t∙hm-2). Similarly, the above-ground portion contributes to 57.6% of the carbon storage in the herbaceous layer, followed by the below-ground portion (42.4%).
Tab.5 Carbon content, biomass and carbon storage of herbaceous layer
Components
|
ω (C)/%
|
Biomass/(t/hm2)
|
VCS/(t/hm2)
|
Aboveground
|
21.99 ± 5.08a
|
10.15 ± 2.03a
|
1.65 ± 0.38a
|
Underground
|
27.00 ± 6.64a
|
7.10 ± 4.15a
|
1.21 ± 0.23a
|
Average/Total
|
24.45 ± 5.86
|
17.25 ± 6.18
|
2.86 ± 0.61
|
3.4 Carbon content, biomass and carbon stock characteristics of the apoplankton layer
The carbon content in the litter layer shows a significant difference (P < 0.05) between the different decomposition stages, with the undecomposed layer having the highest carbon content, followed by the partially decomposed layer and the fully decomposed layer. The carbon content decreases with increasing degree of decomposition (Table 6). The biomass distribution in the litter layer is highest in the fully decomposed layer (6.96 t∙hm-2), followed by the partially decomposed layer (3.59 t∙hm-2), and lowest in the undecomposed layer (1.24 t∙hm-2), indicating a pattern of fully decomposed layer > partially decomposed layer > undecomposed layer, which shows significant differences (P < 0.05). In terms of carbon storage in the litter layer, the partially decomposed layer has the highest carbon storage, accounting for 63.9% of the carbon storage in the litter layer, followed by the partially decomposed layer (24.5%), and the fully decomposed layer has the lowest carbon storage (11.6%).
Table 6
Carbon content, biomass and carbon loss reserves of litter layer
Components
|
ω (C)/%
|
Biomass/(t/hm2)
|
VCS/(t/hm2)
|
Fresh litter layer
|
39.18 ± 3.73a
|
1.24 ± 0.61b
|
0.79 ± 0.23b
|
Fragmented litter layer
|
19.84 ± 4.57b
|
3.59 ± 1.36b
|
2.06 ± 0.55a
|
Humified litter layer
|
9.90 ± 1.84b
|
6.96 ± 1.50a
|
0.36 ± 0.13b
|
Average/Total
|
22.97 ± 5.07
|
11.80 ± 3.49
|
3.22 ± 0.93
|
3.5 Carbon content, biomass and carbon stock characteristics of soil layers
The soil carbon content at the three sampling sites decreases significantly with increasing soil depth (P < 0.05). The distribution of soil carbon content varies between different soil layers, with the topsoil layer (0–10 cm) having the highest carbon content and the 40–60 cm soil layer having the lowest carbon content (Table 7). The factors influencing soil carbon storage are soil carbon content and bulk density. As soil carbon storage increases, soil depth decreases and the magnitude of the change is generally consistent with soil carbon content. Soil carbon storage in the 0–30 cm soil layer accounts for 64.2% of total soil carbon storage, indicating that the majority of soil carbon storage is concentrated in the surface soil layer.
Table 7
Carbon content, biomass and carbon loss characteristics of soil layer
Soil layer/cm
|
ω (C)/%
|
Biomass/(t/hm2)
|
VCS/(t/hm2)
|
0–10
|
24.12 ± 5.15a
|
0.67 ± 0.11b
|
44.1 ± 6.3a
|
10–20
|
18.04 ± 5.49b
|
1.55 ± 0.31b
|
36.6 ± 8.0a
|
20–30
|
14.36 ± 1.23b
|
2.52 ± 0.16a
|
28.5 ± 9.3b
|
30–40
|
9.39 ± 1.06b
|
2.89 ± 0.17a
|
21.5 ± 5.0b
|
40–60
|
3.81 ± 1.10c
|
3.24 ± 0.38a
|
39.0 ± 21.0a
|
Average/Total
|
13.94 ± 2.80
|
10.88 ± 1.15
|
169.9 ± 12.7
|
3.6 Carbon content, biomass and carbon stock characteristics of natural vegetation
The carbon content of the different components in natural vegetation follows the order: tree layer > soil layer > litter layer > herb layer > shrub layer (Table 8). In natural vegetation, the main carbon storage component is the tree layer, which accounts for 60% of the carbon storage in natural vegetation. The soil layer contributes 37% of the carbon storage, while the shrub layer (0.6%), the herbaceous layer (0.6%) and the litter layer (0.7%) have relatively small shares in carbon storage.
Table 8
Carbon content, biomass and carbon loss reserves of natural vegetation
Components
|
ω (C)/%
|
Biomass/(t/hm2) /Bulk density/(g/cm3)
|
VCS/(t/hm2)
|
Arbor layer
|
53.29 ± 3.91a
|
494.42 ± 135.89a
|
273.12 ± 44.03a
|
Shrub layer
|
39.59 ± 9.13b
|
55.38 ± 6.04b
|
9.24 ± 1.66b
|
Herb layer
|
24.45 ± 5.86b
|
17.25 ± 6.18c
|
2.86 ± 0.61b
|
Litter layer
|
22.97 ± 5.07b
|
11.80 ± 3.49c
|
3.22 ± 0.93b
|
Soil layer
|
13.94 ± 2.80c
|
10.88 ± 1.15c
|
169.9 ± 12.7a
|
4. Characteristics of the carbon increment of vegetation restoration in mining areas
4.1 Carbon content, biomass and carbon increment characteristics of the tree layer
Table 9 shows that within the tree layer the carbon content followed a descending order: leaf > root > branch > bark > stem. The highest biomass in the tree layer was found in roots (45.00 t∙hm-2), followed by leaves (40.91 t∙hm-2) and stems (31.33 t∙hm-2), while the lowest biomass was found in peels (29.66 t∙hm-2) and branches (26.36 t∙hm-2). Leaves were responsible for the largest carbon increment in the tree layer, contributing 47.3% of the total, followed by roots (25.1%) and branches (11.6%). The smallest carbon increment came from the stem (8.8%) and bark (7.2%). In summary, the order of carbon accumulation in the tree layer was: leaves > roots > branches > stem > bark.
Table 9
Carbon content, biomass and carbon increment in the tree layer
Grouping
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
Leaf
|
18.97 ± 16.72c
|
40.91 ± 35.61b
|
7.76 ± 5.95c
|
Branch
|
7.25 ± 7.65b
|
26.36 ± 27.75a
|
1.91 ± 0.42a
|
Dry Stem
|
4.63 ± 4.01a
|
31.33 ± 29.20a
|
1.45 ± 0.50a
|
Pi Bark
|
4.89 ± 4.35a
|
29.66 ± 27.06a
|
1.13 ± 0.23a
|
Root Root
|
9.19 ± 9.12b
|
45.00 ± 15.00b
|
4.13 ± 0.71b
|
Average/Total
|
8.99 ± 8.37
|
161.58 ± 149.71
|
16.39 ± 2.64
|
Note: In the "Mean/total" column for components, carbon content is the mean, and biomass and carbon increment are total. Different lowercase letters in the same column indicate significant differences (P < 0.05); mean ± standard deviation, n = 3. Same below |
4.2 Carbon content, biomass and carbon increment characteristics of the shrub layer
As for the shrub layer (Table 10), branches had the highest carbon content, followed by roots and leaves, which differed significantly from each other (P < 0.05). Branches also accounted for the largest part of the shrub layer biomass (43.3%), followed by leaves (29.5%) and roots (29.2%). The biomass distribution of the shrub layer organs differed significantly (P < 0.05), in the order: branches > leaves > roots. Branches were also responsible for the highest carbon increment in the shrub layer, contributing 59.1% of the total, followed by roots (24.3%) and leaves (16.6%). Therefore the order of carbon increment in the shrub layer was: branches > roots > leaves.
Table 10
Carbon content, biomass and carbon increment in the shrub layer
Grouping
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
Leaf
|
9.26 ± 4.19a
|
2.06 ± 1.85a
|
0.19 ± 0.07a
|
Root Root
|
15.80 ± 3.37b
|
1.80 ± 0.62a
|
0.28 ± 0.09a
|
Branch
|
22.49 ± 3.00b
|
3.03 ± 0.51b
|
0.68 ± 0.04b
|
Average/Total
|
15.85 ± 3.52
|
6.89 ± 2.99
|
1.15 ± 0.20
|
4.3 Carbon content, biomass and carbon increment characteristics of the herbaceous layer
In the herbaceous layer (Table 11), the carbon content was higher in the above-ground part than in the below-ground part, with shares of 55.3% and 44.7%, respectively. However, biomass was higher in the below-ground part (0.54 t∙hm-2) than in the above-ground part (0.26 t∙hm-2). Similarly, the carbon increment in the below-ground part accounted for 66.7% of the herbaceous layer, while the above-ground part contributed 33.3%.
Table 11 Carbon content, biomass and carbon gain in the herbaceous layer
Grouping
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
Above ground section
|
5.97 ± 2.70a
|
0.26 ± 0.55a
|
0.01 ± 0.03 a
|
Ground floor section
|
4.83 ± 2.55a
|
0.54 ± 0.11a
|
0.02 ± 0.02a
|
Average/Total
|
5.40 ± 2.55
|
0.80 ± 0.66
|
0.03 ± 0.05
|
4.4 Characteristics of carbon content, biomass and carbon increment in the apoplankton layer
Regarding the apoplastic layer (Table 12), the carbon content decreased with the decomposition of the material in the following order: undecomposed layer > semi-decomposed layer > decomposed layer. The carbon content of the different organs also differed significantly (P < 0.05). The biomass distribution of the apoplastic layer was opposite to the carbon content, with the highest biomass found in the decomposed layer (1.76 t∙hm-2), followed by the semi-decomposed layer (1.22 t∙hm-2) and the lowest biomass in the undecomposed layer (0.98 t∙hm-2). In terms of carbon increment, the highest contribution came from the decomposed layer (40.0%), followed by the semi-decomposed layer (31.4%), and the lowest from the undecomposed layer (28.6%). Therefore, the order of carbon increment in the apoplastic layer was: decomposed layer > semi-decomposed layer > undecomposed layer.
Table 12
Carbon content, biomass and carbon increment in the apoplankton layer
Grouping
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
Undecomposed layer
|
10.94 ± 1.99a
|
0.98 ± 0.30a
|
0.10 ± 0.06a
|
Semi-decomposed layer
|
9.51 ± 1.78a
|
1.22 ± 0.35a
|
0.11 ± 0.07a
|
Decomposed layers
|
8.13 ± 1.27a
|
1.76 ± 0.80b
|
0.14 ± 0.02a
|
Average/Total
|
9.52 ± 1.53
|
3.96 ± 1.45
|
0.35 ± 0.15
|
4.5 Soil layer carbon content, biomass and carbon increment characteristics
Soil carbon content was found to be strongly influenced by soil depth, with a significant decrease (P < 0.05) observed with increasing depth. Carbon content was unevenly distributed among the soil layers, with the highest concentration found in the topsoil layer (0–10 cm) and the lowest in the 40–60 cm layer (Table 13). In contrast, soil bulk density increased with depth, and the bulk density of the 40–60 cm layer was 3.8 times higher than that of the 0–10 cm layer.
Both soil carbon content and capacity affect soil carbon increment, which also decreases with increasing soil depth, consistent with the trend observed for soil carbon content. The top 30 cm of soil accounted for 69.5% of total soil carbon increment, indicating that most of the carbon increment in the soil layer was concentrated in the topsoil.
Table 13
Soil layer carbon content, biomass and carbon increment
Soil layer/cm
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
0–10
|
13.71 ± 3.26a
|
0.23 ± 0.09b
|
13.9 ± 2.0a
|
10–20
|
7.04 ± 2.65b
|
0.42 ± 0.23b
|
11.2 ± 1.2a
|
20–30
|
3.39 ± 2.15b
|
0.61 ± 0.32a
|
8.5 ± 0.8b
|
30–40
|
6.13 ± 5.83b
|
0.81 ± 0.50a
|
6.3 ± 2.5b
|
40–60
|
1.19 ± 0.48c
|
0.88 ± 0.36c
|
8.4 ± 3.8a
|
Average/Total
|
6.29 ± 2.88
|
2.95 ± 1.50
|
3.61 ± 3.3
|
4.6 Ecosystem carbon content, biomass and carbon increment characteristics
In the vegetation layer, carbon increment is determined by both the carbon content and biomass of each component, with higher carbon content and biomass leading to higher carbon increment. In the forest ecosystem, the carbon content of each component followed the order tree layer > soil layer > apoplankton layer > herb layer > shrub layer (Table 14). Biomass (capacity) was highest in the tree layer (161.58 t∙hm-2), followed by the shrub layer (6.89 t∙hm-2), coppice layer (0.35 t∙hm-2), soil layer (2.95 t∙hm-2) and herb layer (0.03 t∙hm-2). The lowest carbon accumulation in the forest ecosystem was observed in the soil layer (2.95 t∙hm-2) and herb layer (0.03 t∙hm-2), indicating that the order of carbon accumulation was tree layer > shrub layer > apoplankton layer > soil layer > herb layer. The tree layer was found to be the main contributor to carbon increment in the forest ecosystem, accounting for 75.1% of the total, followed by the soil layer (16.5%), shrub layer (5.2%) and apoplankton layer (1.6%), with the herbaceous layer contributing the least (0.01%).
Table 14
Ecosystem carbon content, biomass and carbon increment
Grouping
|
Carbon content/%
|
Biomass/(t/hm )2
|
Carbon gain/(t/hm )2
|
Tree layer
|
8.99 ± 8.37a
|
161.58 ± 149.71d
|
16.39 ± 2.64d
|
Shrub layer
|
15.85 ± 3.52c
|
6.89 ± 2.99c
|
1.15 ± 0.20b
|
Herbaceous layer
|
5.40 ± 2.55a
|
0.80 ± 0.66a
|
0.03 ± 0.01a
|
Apoplankton layer
|
9.52 ± 1.53b
|
3.96 ± 1.45b
|
0.35 ± 0.15b
|
Soil layer
|
6.29 ± 2.88a
|
2.95 ± 1.50b
|
3.61 ± 3.3c
|