**3. Results**

### *3.1. SOC, Soil TN, and Soil TP Stocks in Different Restoration Types*

Restoration age had a significant effect on SOC and soil TN stocks (*p* < 0.01) (Table S2). In orchardland, the SOC stocks of the soil at 0–30 cm depth first increased and then stabilized after 5 years; the soil TN stock at the same depth first increased and then decreased with restoration age. The soil TP stocks at 0–100 cm depth showed no significant change with restoration age (Figure 2a–c). In grassland, the SOC and soil TN stocks at 0–30 cm depth first increased and then stabilized after 18 years, and the soil TP stocks at 0–100 cm depth showed no significant change with different restoration ages (Figure 2d–f). In shrubland, the SOC stocks at 0–50 cm depth and soil TN at 0–10 cm increased at first, peaked after 5 years, and then stabilized after 30 years; the soil TP stocks at 0–100 cm depth showed no significant change (Figure 2h–j). In forestland, the SOC at 0–50 cm gradually increased, and the soil TN stocks increased before peaking after 37 years, then decreased with restoration age; the soil TP stock at 0–100 cm depth showed no significant change (Figure 2k–m). In addition, SOC and soil TN stocks from the 0–30 cm soil layer of grassland30 were significantly lower than those in natural grassland (Figure 3a,b). SOC and soil TN stocks at 0–30 cm soil depth in shrubland47, as well as SOC stocks at 0–100 cm depth and soil TN stocks at 0–20 cm depth in forestland56, were significantly lower than those in natural forest (Figure 3d,e). The soil TP stocks at all soil depths showed no significant differences between grassland30 and natural grassland, shrubland47 and natural forest, or forestland56 and natural forest (Figure 3c,f).

**Figure 2.** Changes in stocks of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) with restoration age. Note: values are mean ± standard error.

**Figure 3.** Stocks of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) at different soil depths in grassland at 30 years compared to those of natural grassland, and those of shrubland at 47 years and forestland at 56 years compared to those of natural forest. Note: values are mean ± standard error. (**<sup>a</sup>**–**<sup>c</sup>**) means the SOC stock, Soil TN stock and Soil TP stock at different soil depths in grassland30 compared to those of natural grassland. (**d**–**f**) means the SOC stock, Soil TN stock and Soil TP stock at different soil depths in shrubland47 and forestland56 compared to those of natural forest \* denotes significant differences between the natural grassland and grassland at 30 years after vegetation restoration (*p* < 0.05). \* denotes significant differences between the natural forest and shrubland at 47 years after vegetation restoration (*p* < 0.05). + denotes significant differences between the natural forest and forestland at 56 years after vegetation restoration (*p* < 0.05).

### *3.2. Changes in SOC, Soil TN, and Soil TP Stoichiometry*

In orchardland, the C:N ratio at 0–100 cm depth showed no obvious changes with restoration age, but there was an overall increasing trend (Figure 4a, Table S3). The C:P ratio at 0–30 cm increased, while the N:P ratio at 0–20 cm first increased and then decreased after 10 years of restoration (Figure 4b,c). In grassland, the C:N ratio at 0–100 cm showed an overall increasing trend with restoration age (Figure 4d, Table S3); the C:P and N:P ratios in soils of 0–30 cm depth gradually increased with restoration age (Figure 4e,f). In shrubland, the C:N ratio at 0–100 cm showed little variation with restoration age, but the overall trend was an increase (Figure 4h, Table S3); the C:P and N:P ratios at 0–10 cm depth first increased, then peaked at 30 years, before decreasing again with restoration age (Figure 4i,j). In forestland, the C:N ratio at 0–100 cm depth showed little change with restoration age, but the overall trend was an increase (Figure 4k, Table S3). The C:P ratio in soils of 0–100 cm depth gradually increased with restoration age (Figure 4l), and the N:P ratio at 0–50 cm first increased, then peaked at 37 years before decreasing again with restoration age (Figure 4m).

*Forests* **2019**, *10*, 27

**Figure 4.** Stoichiometric characteristics of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) with restoration age. Note: values are mean ± standard error.

The C:P and N:P ratios at 0–30 cm in grassland30 were significantly lower than those in natural grassland (Figure 5b,c), and the C:P and N:P ratios at 0–30 cm depth in shrubland47 and forestland56 were significantly lower than those in natural forest (Figure 5e,f). In addition, the C:N ratio decreased with soil depth in forestland (Figure 4k). Overall, the C:P and N:P ratios gradually decreased with soil depth in the three land types (Figure 4b,c,e,f,i,j,l,m).

**Figure 5.** Stoichiometry of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) at different soil depths in grassland at 30 years compared to that of natural grassland, and those of shrubland at 47 years and forestland at 56 years compared to that of natural forestland. Note: values are mean ± standard error. (**<sup>a</sup>**–**<sup>c</sup>**) means the C:N, C:P and N:P at different soil depths in grassland30 compared to those of natural grassland. (**d**–**f**) means the C:N, C:P and N:P at different soil depths in shrubland47 and forestland56 compared to those of natural forest \* denotes significant differences between the natural grassland and grassland at 30 years after vegetation restoration (*p* < 0.05). \* denotes significant differences between the natural forest and shrubland at 47 years after vegetation restoration (*p* < 0.05). + denotes significant differences between the natural forest and forestland at 56 years after vegetation restoration (*p* < 0.05).
