**4. Discussion**

#### *4.1. Changes in Soil Macronutrient Stocks*

Significant spatial and temporal variability in soil macronutrient stocks was observed in this study. Deep root distribution, rhizosphere effects, and higher organic inputs may increase SOC [39]. Although the SOC concentration increased gradually with stand age, there was no statistically significant difference in C stocks of the 0 −20 cm soil layer between different stands. As revealed by Wellock et al. [40], there was a significant decline in the C density of the surface soil after 27 years of afforestation. In our study, the stocks of N, P, and K increased first and then decreased with stand age in both the 0 −20 and 20 −40 cm soil layers. These results are different from other studies, which have shown that not only the N stock but also the C stock in mineral soil increased with increasing stand age, while the soil P stock exhibited a trend of increasing first and then decreasing [41], or that soil P stocks tended to increase but N stocks slightly decreased with stand age [42].

The increasing trend of macronutrient stocks in the 14-year-old plantation may be illustrated by the increase in litter quantity and decomposition, which returns nutrients to the soil [43]. This was the main reason why the C, N, and P stocks declined with increasing soil depth across all sites. The decreasing trend in macronutrient stocks could be because of the nutrient requirements of plantations during the vigorous growing stage [44]. Carbon, nitrogen, and phosphorus are essential elements for soil organisms. With increasing stand age, the microenvironment becomes more hospitable for soil organisms. A large number of soil organisms and strong biological activity can increase macronutrient stocks by accelerating the decomposition of litter and can decrease macronutrient stocks by immobilizing elements for growth or transformation [45,46]. The pH plays an important role in nitrogen biogeochemical cycling through autotrophic nitrification [47], and the mechanisms of P sorption are also affected in different ways by pH [48]. Lower pH is beneficial for phosphate in binding to Fe and Al precipitates [43], and organic acids could form strong bonds with metal ions through metal chelation, which might affect the P stock [49]. Therefore, bacterial biomass and mineral concentration should be considered together with pH in the study of the circulation of soil phosphorus [50]. This is the reason for the highest P stock in the 14-year-old plantation.

The C stocks were highest in each soil layer for all plantations, followed by the K, N, and P stocks; therefore, K plays an important role in terrestrial ecosystem. Soil acidification also promotes the release of mineral K, and potassium deficiency occurs when the available K is absorbed by plants; moreover, cation exchange capacity is another important controlling factor for soil K availability [51]. Increased pH and cation exchange capacity facilitated the accumulation of K in the 25-year-old plantation. The lower K stock was observed in the 30-year-old plantation, probably because a large amount of organic matter and root exudates increased the release of mineral K in the soil [52]. The other reason was that more K-solubilizing bacteria facilitate the conversion of potassium from mineral K into available K [8]. Additionally, the K stock was higher in the 20 −40 cm soil layer than in the 0 −20 cm soil layer, which could be because of the leaching process. The soil structure improved as the vegetation grew, which was conducive to leaching K from the soil surface into the deeper soil layers.

#### *4.2. Factors Controlling Soil Macronutrient Stocks*

The spatial patterns of soil macronutrients were significantly different at various soil depths across a chronosequence of Masson pine plantations. Soil macronutrients and their stoichiometric ratios are important parameters of soil quality. Climate, topography, vegetation, and soil properties interact to influence the characteristics of soil nutrients, especially climate and soil properties [9,10]. The results of the present study showed that soil macronutrient stocks were mainly affected by soil physicochemical properties under similar climatic conditions, but the dominant controlling factors of soil macronutrient stocks varied at various soil depths and stand ages.

Effects of soil physicochemical properties on the variability in soil macronutrient stocks within each Masson pine plantation were revealed by stepwise multiple regression. Soil aggregates promote the stabilization of organic matter by regulating the availability of oxygen and water; hence, the formation and stability of aggregates are crucial for the SOC stock [53,54]. In our research, the proportion of >0.25 mm was the key factor for the C stock in the 14-, 25-, and 30-year-old plantations. The stocks of C and K in the 7-year-old plantation were controlled by the available P. Similar results were reported by Zhong et al. [55], who reported that lower availability of P could result in a decrease in leaf photosynthetic capacity and nutrient concentrations, which adversely affects the organic carbon input to soils from litter. Ks and SBD showed a positive correlation with N and P stocks in the 14-year-old plantation. The SBD is an important parameter for the calculation of N and P stocks, and a higher Ks indicates better hydrothermal conditions, which is beneficial for promoting microorganisms to biologically sequester N and P [56].

The available N, P, and K were the key factors controlling macronutrient stocks in the 7-year-old plantation. In addition, the available P and organic matter were the most dominant controls for the N stock in the 25- and 30-year-old plantations. The implication of these results is that the available nutrients were not only the principal factor affecting the soil macronutrient stocks in the 7-year-old plantation but also the key factor driving the distribution of N stock within the 25- and 30-year-old plantations. Soil acidification is prone to the leaching of elements and thus these soils tend to be nutritionally poor [57]. This might be a good explanation for the positive correlations between C stock and pH in the 30-year-old plantation. Moreover, soil aggregate stability could be the primary parameters affecting K stock across a chronosequence of Masson pine plantations.

The method of redundancy analysis was used to analyze the correlations between macronutrient stocks and soil physicochemical properties at various soil depths. Macronutrient stocks in the soil surface layer could be affected by many factors, and the variance could be explained mainly by the available K and P:K ratio in our research. The present research showed that the C:P ratio was significantly higher than the other ratios within each plantation, followed by the C:N ratio, whereas the N:K and P:K ratios were at the lowest level regardless of soil depth. Litter input is an important means for vegetation to regulate soil nutrients [58] through the priming effect and nutrient input [59]. Therefore, the observed accumulation of C, N, and P stocks in the 0 −20 cm soil layer was higher than that in the 20 −40 cm soil layer, which exacerbated the K limitation in this soil layer. Phosphorus was also an important limiting factor in our research. Therefore, K availability to plants and the balance of P and K were more essential to macronutrient stocks in the surface soil with abundant nutrients. However, Ks had an adverse effect on macronutrient stocks. Root growth and root exudates can improve soil structure in the 20 −40 cm soil layer, and in addition to the P:K ratio, the stability of aggregates is strongly associated with C, N, and K stocks in this soil layer. Studies have also reported that soil aggregate stability has positive effects on C and N sequestration [60], but research on K is rare. For the total soil system, we found that organic matter explained most of the variation in soil macronutrient stocks, followed by the P:K ratio and available K. Specifically, the C stock was affected by the available nutrients, organic matter, and stoichiometry; and the N and P stocks were influenced by the available nutrients and organic matter. In addition, the C, N, and P stocks may decrease with soil acidification. The K stock was positively correlated with aggregate

stability, which requires further study. Therefore, the response of soil macronutrient stocks to stand age and soil depth relies upon different soil physicochemical properties, and P and K may be the critical limitations of Masson pine plantation ecosystems in our study area.

**Author Contributions:** Conceptualization, Q.D.; data curation, J.H.; investigation, F.X.; methodology, F.X.; writing—original draft, J.H.; writing—review and editing, Q.D., Y.Y. and X.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by China Postdoctoral Science Foundation (2020M673296), Natural Science Foundation of China (42167044, 42007054), the High-level Innovative Talents in Guizhou Province of Guizhou Province (Qian Ke He Platform Talents (2018)5641), the Science and Technology Projects of Guizhou Province (Qian Ke He Platform Talents (2017)5788), the first-class discipline Construction Project of Guizhou Province (GNYL (2017)007), and the Cultivation project of Guizhou University (Cultivation (2019) No.10 of Guizhou University).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** All authors declared that they have no conflict of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
