**4. Discussion**

### *4.1. Restoration Ages Altered SOC, Soil TN, and Soil TP Stocks and Stoichiometry*

Our results showed that the SOC stocks at 0–30 cm soil depth in orchardland, grassland, and shrubland first increased and then stabilized with restoration age, while the SOC stocks at 0–50 cm in forestland gradually increased with restoration age. SOC is mainly derived from surface litter, root secretions, and animal residues [21,22]. After vegetation restoration, a large input of litter and organic matter can enhance the accumulation of SOC [23], and with an improvement in soil structure, the levels of surface runoff, soil erosion, and soil nutrient loss may be reduced [24]. In addition, soil microbial activity is strengthened as restoration age increases [25], and soil nutrient conversion and storage are further enhanced. However, vegetation restoration involves the coordinated development of the plant community and soil environment, and the plant community structure, soil structure, and microbial diversity reach a stable level as restoration age increases [26–28]. However, in forestland, SOC stocks had not reached a steady level at 57 years, and when these stocks may stabilize needs to be evaluated. In our study, the soil TN stocks at 0–30 cm depth decreased in the later stages of orchardland, shrubland, and forestland restoration. Plants may enter a senescence phase, and the soil

nitrogen nutrients absorbed during the plant growth process are then fed back to the soil by litter; thus, the nitrogen absorption rate is lower than the release rate, resulting in the soil TN stock decreasing [29]. We also found that soil TP stocks did not significantly change with restoration age, in contrast to the results of some previous studies [30]. Soil TP is mainly influenced by parent material, land use, and biogeochemical processes in the soil [31,32]. As the parent material and climate were similar for all vegetation types, the variability and migration rate of TP were not as obvious as those of SOC and soil TN, and restoration age had little effect on soil TP. Within the range of restoration ages that we studied, the SOC and soil TN stocks in grassland, shrubland, and forestland at 0–30 cm soil depth failed to reach the stock levels found in natural grassland or natural forest. Severe soil erosion and lack of water are typical characteristics of the Loess Plateau, which may lead to the loss of soil nutrients [24,33]. Additionally, soil tillage can also cause soil nutrient loss by affecting the soil physical structure and microbial activity [34].

The C:N ratio increased with restoration age after vegetation restoration. The main factors affecting the C:N ratio are changes in the SOC and soil TN contents [35]. Both SOC and soil TN contents increased overall with restoration age (Table S3), and the rate of increase of SOC was greater than that of soil TN; consequently, the C:N ratio increased (Table S3). A previous study showed that the C:N ratio was negatively correlated with the rate of decomposition of organic matter [36], so the decomposition rate of organic matter increases with restoration age. The C:P ratio indicates the availability of soil TP in the soil [37], We found that the rate of increase of SOC was also greater than that of soil TP, resulting in an increase in the C:P ratio with restoration age (Table S3). In addition, the rate of increase of soil TN was greater than that of soil TP, resulting in an increase in the N:P ratio with restoration age (Table S3). The N:P ratio was reduced in the later stages of restoration, which may be related to the lower soil TN content (Figure S1). Soil N and soil TP are essential mineral nutrients for plant growth and common limiting elements in ecosystems, and the N:P ratio is a predictor of nutrient limitation [38]. In the grassland, shrubland, and forestland types, the N:P ratio at 0–30 cm soil depth in the later stages of recovery was lower than those of natural grassland and natural forest, which may result from the more alkaline soil and lower soil TN content in the Loess Plateau Region; the soil TP content did not differ significantly between the restoration and control sites. In the grassland, shrubland, and forestland restoration types, the C:P ratio in the 0–30 cm soil layer was lower than those of natural grassland and natural forest at the later stages of recovery, which may be related to the lower SOC content in restoration soils than in natural soils.

### *4.2. Vertical Distribution of Stocks and Stoichiometry of SOC, Soil TN, and Soil TP*

Soil depth is an important factor influencing SOC, soil TN, and soil TP distribution [39]. In our study, the SOC and soil TN stocks decreased with soil depth at all restoration ages in orchardland, grassland, shrubland, and forestland (Figure 6a,b,d,e,h,I,k,l), which is consistent with the results of previous studies [40,41]. Our study also revealed that the overall rates of SOC and soil TN content change decreased with soil depth in orchardland, grassland, shrubland, and forestland (Table S3), which indicated that the SOC and soil TN sequestration rates gradually decreased with soil depth. Meanwhile, SOC, soil TN, and soil TP were most sensitive to change in the surface soil (0–30 cm). The SOC and soil TN content at 0–30 cm represented more than 65% of the total SOC and soil TN stocks from 0–100 cm depth for all restoration types; the soil TP content at 0–30 cm represented more than 60% of the total soil TP stocks from 0–100 cm (Figure S1). Such differences in the SOC, soil TN, and soil TP profiles can be explained partly by root distribution. The surface soil is affected by external environmental factors, soil microorganisms, and the return of nutrients from surface litter, resulting in a concentration of nutrients in the surface soil [42]. With increasing soil depth, the input of organic matter is limited by the permeability of the soil, microbial decomposition activity, and root absorption [21,39]. Moreover, SOC and soil TN stocks are not only affected by soil parent material, but also by the decomposition of litter and absorption and utilization by plants [40], resulting in large spatial variability. While the soil TP content of orchardland and forestland decreased, it showed little

variation with soil depth in grassland and shrubland (Figure 6c,f,j,m). Soil TP is mainly affected by the soil parent material, which, in this case, is a sedimentary mineral with low mobility in soil; therefore, there was little vertical variation in soil TP [43].

**Figure 6.** Vertical distributions of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) contents for different vegetation types. Note: values are mean ± standard error.

Our study found that the overall C:N ratio gradually decreased with soil depth. It may be that the surface SOC and soil TN contents were higher, but as the soil depth increased, the SOC content change was larger than that of the soil TN content. When the decomposition process occurs, easily decomposed material vanishes, and soil TN is immobilized in decayed products, leaving behind more durable material with slower decomposition rates in the deeper layers [44]. This results in a relatively lower C:N ratio in the deeper soil layers. There was a significant difference in the C:P and N:P ratios at different soil depths. It may be that the soil TP content is relatively stable at different soil depths, and the C:P ratio and N:P ratio are mainly affected by SOC and soil TN content, so they showed greater variation.

### *4.3. Effect of Restoration Type on SOC, Soil TN, and Soil TP Stocks and Stoichiometry.*

Our study demonstrated that the SOC and soil TN stocks in the 0–20 cm soil layer at 5 years were highest in orchardland, which may be related to the use of fertilizer. In other years, SOC, soil TN, and soil TP stocks showed no difference between orchardland, grassland, shrubland, and forestland

(Figure S2). However, the rate of SOC and soil TN change at 0–10 cm soil depth was the highest in forestland, while the rates of SOC and soil TN change at other depths varied among different restoration types (Table S3). The rapid increases in surface SOC and soil TN are closely related to the input of litter [45,46]. Guo et al. [46] showed that forest litter was 19 times greater in mass than that of shrubs in the Loess Plateau Region. At the same time, the presence of the higher amount of organic carbon over a long period of time indicated that forestland likely had developed a higher degree of humification. The long-term development of soil organic carbon stocks suggested that the afforestation improved the humification process. It has been shown that the process of humification in the soil is critical for the ecosystem, due to its strong contributions to the improvement of fertility and, hence, the storage of both carbon and nitrogen [47,48]. This also explains why the increase in SOC and soil TN in the topsoil of forests was higher than that of shrubs. As soil depth increases, root secretions and soil microorganisms are the main sources of soil nutrition [21,22]. There are significant differences in the effects of different plant roots and litters on the community composition of microorganisms [49], which may explain the large differences in the rates of SOC and soil TN change between different restoration types.

The C:N ratios of orchardland5 and grassland5 at 0–20 cm soil depth were lower than those of shrubland5 and forestland5, and the C:N ratio of orchardland10 was significantly lower than those of grassland10, shrubland10, and forestland10 (Figure S2). The C:N ratio of orchardland was lower than those of grassland, shrubland, and forestland, which may be related to anthropogenic N deposition in orchardland (Figure S2). The lower C:N ratio in grassland5 may occur because grassland retains more organic matter content and greater nutrient absorption takes place through plant roots [8]. There were no significant differences in the C:N ratio at other soil depths of orchardland, grassland, shrubland, and forestland as there were no significant differences in the stocks of SOC and soil TN (Figure S2). This finding is related to the nutrient conditions of the soil in the study area and the feedback between plant and soil, so the soil stoichiometric changes in different restoration types showed the same characteristics in the same environmental context. In addition, the overall C:P and N:P ratios of orchardland5 and shrubland5 at 0–20 cm soil depth were higher than those of grassland5 and forestland5, which is related to the relatively higher SOC and soil TN content in orchardland5 and shrubland5 (Figure S2). However, there were no significant differences in the C:P and N:P ratios at 30–100 cm soil depth between the four restoration types because there were no significant differences in the SOC, soil TN, and soil TP stocks of orchardland, grassland, shrubland, and forestland (Figure S2).
