*2.1. Experimental Design*

This study was carried out in 4 sample stands of Masson pine (*P. massoniana* Lamb.), located in Dushan County in Guizhou Province within state-owned forest farms, covering an area of more than 18,860 km<sup>2</sup> and extending at least 50 km from north to south (107◦27–107◦30 E, 25◦41–25◦41 N) (Figure 1). The mean annual precipitation in this area is approximately 1346 mm, the mean annual temperature is 15 ◦C, and the altitude is 830–1479 m. The soil in the region is classified as yellow soil. Masson pine mostly forms uniform stands with a small admixture of other tree species, usually fir. The understorey vegetation and some basic information of the sites are shown in Table 1.

**Figure 1.** Location of the state-owned forest farms in Dushan County, Guizhou Province.


**Table 1.** Basic characteristics of the four Masson pine plantations.

We established three 30 × 30 m standard quadrats in each plantation, covering four stand age classes (7-, 14-, 25-, and 30-year-old secondary forest stands) in the forest farms. Three points were randomly selected in each Masson pine plantation quadrat, and soil samples were collected from 0−20 and 20−40 cm soil layers after removal of plant residues, gravel, or other debris. We collected a total of five random samples from each plot as per the S-shaped sampling method and then placed them into an aluminum specimen box to ensure that the main structure was maintained during transport to the laboratory. Three ring-knife samples from each soil layer were collected at the same time.

#### *2.2. Soil Analyses and Calculations*

Soil samples were divided into two groups related to research indicators. One part of the soil samples was broken into blocks with a diameter approximately 10 mm according to the natural structure, and litter stones and roots were removed. When air-dried, these samples were used to determine soil aggregation characteristics. The other soil samples were air-dried and sieved to 2 mm for chemical analyses. The wet-sieving method was applied to determine the composition of water-stable aggregates in different Masson pine plantations. Aggregated soils successively passed through a column of sieves with 5, 3, 2, 1, 0.5, and 0.25 mm diameters to quantify the losses of sediment of different sizes.

Soil bulk density (SBD), saturated hydraulic conductivity (Ks), and soil porosity were measured using the ring-knife method [33]. The method of Walkley and Black [34] was employed to measure the soil organic carbon and organic matter contents. Soil pH in water 1:2.5 (soil:water) was measured after shaking for 5 min [35]. The ammonium acetate saturation (AMAS) method was used to study the cation-exchange capacity (CEC) [36]. Soil total nitrogen (N) and the available N (AN) were measured by Kjeldahl digestion and alkaline hydrolysis diffusion method, respectively; phosphorus (P) and the available P (AP) were measured using molybdenum blue colorimetric analysis; potassium (K) and the available K (AK) were determined using a flame photometer [14].

The soil aggregate stability was characterized by mean weight diameter (MWD), fractal dimension (FD), geometric mean diameter (GMD), and proportion of >0.25 mm waterstable aggregates (WSA > 0.25 mm). Stability parameters of aggregates were calculated using the following formulae [37]:

$$\text{MWD} = \sum\_{i=1}^{n} \mathbf{x\_i} \times \mathbf{w\_i} \tag{1}$$

$$\text{GMD} = \exp\left[\frac{\sum\_{i=1}^{n} (\text{w}\_{i} \times \ln \text{x}\_{i})}{\sum\_{i=1}^{n} \text{w}\_{i}}\right] \tag{2}$$

$$\mathcal{R}\_{0.25} = \frac{\mathcal{M}\_{\text{t} > 0.25}}{\mathcal{M}\_{\text{t}}} \times 100\% \tag{3}$$

$$\frac{\mathbf{M}\_{\left(\mathbf{r}<\mathbf{x}\_{\mathrm{i}}\right)}}{\mathbf{M}\_{\mathbf{t}}} = \left(\frac{\mathbf{x}\_{\mathrm{i}}}{\mathbf{d}\_{\max}}\right)^{\mathbf{3}-\mathbf{D}} \tag{4}$$

where xi is the mean diameter (mm) of the soil aggregate size fractions, wi is the proportion of all soil in the ith size fraction (%), Mt is the total mass of aggregates (g), Mt>0.25 is the mass of aggregates larger than 0.25 mm (g), M(r<xi) is the mass of aggregates smaller than xi (g), and dmax is the maximum diameter of the soil aggregate size fractions (mm).

The calculation formula of soil macronutrient density (Mg <sup>C</sup>·ha−1) in a soil layer is as follows [38]:

$$\text{SMD}\_{\text{i}} = \sum\_{\text{i}=1}^{n} \text{C}\_{\text{i}} \times \text{D}\_{\text{i}} \times \text{T}\_{\text{i}} \times (1 - \text{G}\_{\text{i}}) \times 10^{-1} \tag{5}$$

$$\text{SMS}\_{\text{i}} = \text{SMD}\_{\text{i}} \times \text{S} \tag{6}$$

where SMDi is the soil macronutrient density in the i layer of soil (Mg <sup>C</sup>·ha−1), Ci is the soil macronutrient concentration in the i layer of soil (g·kg−1), Ti, Di, and Gi are the soil thickness (cm), soil bulk density (g·cm<sup>−</sup>3), and volume percentage of gravel that is larger than 2 mm in soil, respectively. 10−<sup>1</sup> is the unit conversion factor. SMSi is the soil macronutrient stock in the i layer of soil (Mg C) and S is the soil acreage of the calculation grid.
