**1. Introduction**

Soil organic carbon (SOC) is the main part of terrestrial carbon reservoirs and an important part of soil fertility. As of 2004, the size of soil carbon pool was 3.3 times that of atmospheric pool and 4.5 times that of biotic pool [1]. Carbon sequestration implies transferring atmospheric carbon dioxide into long-lived pools and achieving the purpose of mitigating CO2 emissions. The role of organic carbon is self-evident, but macronutrients such as nitrogen (N), phosphorus (P), and potassium (K) are equally as important as carbon (C) [2]. Metabolic processes in soils are often nutrient limited by N and P [3,4]. Sequestering soil carbon relies on the availability of stabilizing elements such as N and P, which are essential for stabilizing organic C pool [5]. Changing the N and P availabilities in soils may alter the magnitude of the imbalance between SOC decomposition and formation processes [6]. It is estimated that 80 million tons (Mt) of N, 20 Mt of P, and 15 Mt of K are required to sequester 1 Gt of C in world soils [1].

Soil organic carbon, nitrogen, phosphorus, potassium, and their stoichiometry, as critical regulatory indicators, have been shown to modulate forest ecosystem functions [7,8].

**Citation:** He, J.; Dai, Q.; Xu, F.; Yan, Y.; Peng, X. Variability in Soil Macronutrient Stocks across a Chronosequence of Masson Pine Plantations. *Forests* **2022**, *13*, 17. https://doi.org/10.3390/f13010017

Academic Editors: Fuzhong Wu, Zhenfeng Xu and Wanqin Yang

Received: 16 November 2021 Accepted: 20 December 2021 Published: 23 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Nitrogen cycle processes are closely coupled with other macronutrients, and the content and availability of N directly influence plant growth and the net primary productivity of terrestrial ecosystems [9,10]. Phosphorus can only be released slowly by weathering of parent material, internal nutrient recycling, and reallocation within the soil profile in a natural environment, and it may have become the limiting nutrient in almost all terrestrial ecosystems [11]. Potassium is another essential nutrient required for plants [8]. The application of K fertilizer could increase water use efficiency, promote carbon sequestration, and reduce runoff and soil loss under plant cover [12]. Furthermore, the stoichiometry of macronutrients is a useful indicator of the intensity of nutrient flux from deadwood to the soil [13].

An age series of Masson plantations may alter community structure and soil properties following reforestation, which in turn influences soil macronutrient stocks and stoichiometric traits [14]. There is a complex response of soil macronutrient stocks and stoichiometry to vegetation restoration. The soil C stock and C:N:P stoichiometry increased, but the N stocks first increased and then decreased with forest restoration age, while the P stocks showed little variation in a study by Xu et al. [15]. However, the P stock may become a limiting factor for C sequestration as forests age [16]. The N, P, and K stocks in the 20 cm soil layer did not differ significantly between years [17]. Similarly, many studies have indicated that soil nutrient variability is likely to be small with forest stand age [18,19]. Some researchers have reported that the soil P availability declined continuously with the development of larch plantations [20]. Therefore, there still exists some controversy regarding the effects of stand age on soil macronutrient stocks.

Plantations can provide both economic and environmental benefits, and ecosystems play important roles in maintaining regional ecological protection functions, including curbing erosion, accelerating the geochemical cycling of elements, and ultimately increasing productivity. Due to the previous extensive destruction of forests and the recognition of important ecological service functions, many provinces have established a pattern of rapid afforestation of progressively larger regions over the past 10–15 years [21]. As a native tree species, Masson pine (*Pinus massoniana* Lamb.) has a wide distribution range and large forest area in central and southern China and is one of the main plantation varieties; it is always referred to as a pioneer tree species due to its rapid growth and strong adaptability [22,23]. Accurate estimation of C, N, P, and K stocks in Masson plantation ecosystems is crucial because they are the most important macronutrient pools in Earth systems, and there still exists some controversy regarding the effects of stand age on soil macronutrient stocks. In addition, plantations of different stand ages significantly influence soil physicochemical properties by changing the interior environment, litter inputs, root exudates, and so on [24]. An improved understanding of how soil physicochemical properties will affect C, N, P, and K stocks at various stand ages and soil depths may enable more accurate prediction of soil macronutrient stocks in plantation ecosystems.

Many of the C stock estimates of terrestrial ecosystems have been reported by researchers [25–28], and all of which are essential for understanding C-cycling patterns and their influences on terrestrial ecosystems. However, more studies on the response of N, P, and K stocks to plantation stand age are necessary, particularly those that attach less importance to K stock research. Furthermore, the response of soil organic carbon to soil properties has been observed by many authors [29–31], but few studies have shown the substantial drivers of soil properties on N, P, and K [32], and knowledge of the potential mechanisms remains limited. To estimate how soil macronutrient stocks change, four sample stands of Masson pine were compared across a 7-, 14-, 25-, and 30-year chronosequence. In addition, we investigated the effects of soil physicochemical properties on the storage of macronutrients. Therefore, the objectives of this study were to (1) estimate the soil macronutrient stocks and their changes with stand age and (2) determine the influence of soil physicochemical properties on the C, N, P, and K stocks at various stand ages and soil depths.

#### **2. Materials and Methods**
