*2.1. Study Area*

The study area was located in Ansai County, Shanxi Province, China in the center of the Loess Plateau (Figure 1). This region has a warm temperate and semi-arid climate with an annual average temperature of 8.8 ◦C. Annual precipitation is approximately 500 mm, 60% of which falls between July and September, and the frost-free period is 157 days. The soil is mainly composed of Huangmian soil developed on wind-deposited loessial parent material. This type of soil is characterized by weak cohesion, which has made it prone to severe soil erosion. The sand (2.00–0.02 mm grain size), silt (0.02–0.002 mm), and clay (<0.002 mm) contents are 65%, 24%, and 11%, respectively. The soil bulk density (BD) and soil pH of the tillage layer range from 1.15 to 1.35 g cm<sup>−</sup><sup>3</sup> and 8.4 to 8.6, respectively.

**Figure 1.** The locations of study sites in Ansai County, Shanxi Province, China.

### *2.2. Soil Sampling and Laboratory Analyses*

With the aim to examine changes in the stocks and stoichiometry of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) after vegetation restoration in different restoration types, this study adopted a "space for time" approach. A total of 82 sites representing four restoration types were selected based on vegetation type, topographic features, and restoration age, comprising 9 sites of orchardland (5, 10, and 20 years), 34 sites of grassland (2, 5, 8, 11, 15, 18, 26, and 30 years), 24 shrubland (5, 10, 20, 30, 36, and 47 years), and 15 forestland (5, 10, 20, 37, and 56 years). In addition, we selected three slope cropland sites, which were studied at 0 years. Because the

four restoration areas were transformed from croplands, four natural grassland sites (age > 50 years) and nine natural forest sites (age > 100 years) were selected as controls (Table S1). Three 10 × 10 m plots were chosen in each orchardland, three 2 × 2 m plots in each grassland (grassland and natural grassland), three 10 × 10 m plots in each shrubland, and three 20 × 20 m plots in each forestland type (forestland and natural forest). Each plot was at least 50 m from the other plots. A total of 15 soil samples were collected from five soil depths (0–10, 10–20, 20–30, 30–50, and 50–100 cm) in a random sampling design using a soil drilling sampler (4 cm inner diameter). Soil samples from each plot from the same soil depth were mixed to form one sample. These soil samples were brought back to the laboratory and then divided into two parts. One part of the sample was naturally air-dried, plant roots and other impurities were removed, and then the SOC, soil TN, and soil TP were measured. The other part was stored in a refrigerator at 4 ◦C until further analysis of other indicators, which are not presented in this paper.

The soil bulk density (BD) of each depth was measured using the cutting ring method. The SOC was determined using the H2SO4–K2Cr2O7 method [18]. The soil TN was measured using the Kjeldahl method [19], and the soil TP was determined colorimetrically using the ammonium molybdate method [20].

### *2.3. Calculation of SOC, Soil TN, and Soil TP Stocks*

The stocks of SOC, soil TN, and soil TP from five soil depths of 0–10, 10–20, 20–30, 30–50, and 50–100 cm in different restoration types were selected in our study to research the carbon budget of the soil and soil fertility issues after afforestation. The SOC, soil TN, and soil TP stocks (Mg ha−1) were calculated as follows:

$$\text{SOC}\_{\text{i}} \text{ stock} = \text{SOC}\_{\text{i}} \times \text{BD}\_{\text{i}} \times \text{D}\_{\text{i}} / 10 \,, \tag{1}$$

$$\text{Soil TN}\_{\text{l}} \text{ stock} = \text{soil TN}\_{\text{l}} \times \text{BD}\_{\text{l}} \times \text{D}\_{\text{l}} / 10 \,, \tag{2}$$

$$\text{Soil } \mathbf{TP\_i} \text{ stock} = \text{soil } \mathbf{TP\_i} \times \mathbf{BD\_i} \times \mathbf{D\_i}/10,\tag{3}$$

where SOCi is the soil organic carbon content of the *i*th layer of soil (g kg−1), soil TNi is the soil total N content of the *i*th layer of soil (g kg−1), soil TPi is the soil total P content of the *i*th layer of soil (g kg−1), BDi is the soil bulk density of the *i*th layer of soil (g cm<sup>−</sup>3), and Di is the soil depth of the *i*th layer of soil (cm).
