1. Introduction
Forests are the largest terrestrial carbon (C) sinks due to their high capacity to store large amounts of C in plants and soils (Hua et al., 2022). Human overuse of land has reduced forest area and C sinks in forests over the past 30 years (FAO, 2020; Yuan and Chen, 2012; Hua et al., 2022). Therefore, afforestation and the protection of existing forests are important measures to improve the C sequestration capacity of terrestrial ecosystems, which can mitigate climate change (Bonner et al., 2013; Chen et al., 2019). China has made a significant contribution to global greening, with intensive afforestation projects (e.g., plantations) accounting for 42% of this contribution (Chen et al., 2019). The nutrient status in subtropical plantations changes with increasing stand age, and understanding the association between nutrient status and stand age would be helpful for devising strategies to sustain and improve ecosystem stability and C sequestration capacity (Feng et al., 2017; Yang et al., 2021).
Subtropical forests in China possess a great diversity of plants and provide high net C productivity (Yu et al., 2014). Nutrient limitation is defined as an increase in plant productivity with an increase in nutrient availability (Elser et al., 2007). Nitrogen (N) and/or phosphorus (P) often limit plant productivity in most terrestrial ecosystems (Vitousek and Howarth, 1991; Elser et al., 2007; Vitousek et al., 2010). Leaf N:P ratios can indicate the nutrient limitation of plants (Gusewell, 2004; Bui and Henderson, 2013; Liu et al., 2013a). In tropical and subtropical areas, young plantations are generally vulnerable to N limitation, while older plantations are limited by P (Fan et al., 2015; Wang and Zheng, 2021; Yang et al., 2021). Hence, P could become increasingly limiting to plant growth with increasing stand age. Yang et al. (2021) have reported that N limitation decreases while P limitation increases with increasing stand age in subtropical southeast China. Therefore, it is important to focus on the change in the nutrient balance with forest age in the subtropics.
Nutrient limitation in plantation forests is related to soil nutrient availability. Changeable N and P concentrations in plants can result in a change in the N:P ratio, which exerts a significant impact on plant productivity and the process of litter decomposition (Feng et al., 2019b). The C:N ratios and C:P ratios in litter and soil have been recognized as quality indicators of organic matter and its decomposition rate (Paul et al., 2002; Zhang et al., 2013; Liu et al., 2017; Yang et al., 2021). Soil N and P concentrations change with the stand age of forests due to factors including the changeable litter biomass, enzyme activities, and microbial community (Deng et al., 2013; Lucas–Borja et al., 2016; Moreno–Mateos et al., 2017; Feng et al., 2019a; Feng et al., 2019b). Increased stand ages result in higher litter biomass, which elevates soil N concentrations (Moreno–Mateos et al., 2017; Feng et al., 2019b). Higher microbial biomass, abundance, and diversity caused by increasing stand ages enhance the soil available N concentrations (Deng et al., 2013; Moreno–Mateos et al., 2017). In addition, higher enzyme activities, including polyphenol oxidase (PhOx) and N-acetylglucosaminidase (NAG) activities, contribute to increasing the available N concentrations through accelerating litter decomposition (Moreno–Mateos et al., 2017; Feng et al., 2019b; Dong et al., 2021). However, lower soil total P and available P concentrations are expected to be found in older forests than in younger forests (Frizano et al., 2002; Feng et al., 2017; Chen et al., 2018; Yang et al., 2021), which is driven by higher P accumulation in plants (Kitayama et al., 2000; Vitousek et al., 2010; See et al., 2015). Lower soil available P concentrations under increasing stand ages may be due to reduced acid phosphomonoesterase (AP) activity (Wang et al., 2019; Yang et al., 2020). Microbial biomass N (MBN) and P (MBP) can also reflect the status of soil N and P, and MBN and MBP can be converted into soil available N and P for plants (Lucas–Borja et al., 2016). Some studies have shown that soil MBN and MBP increase with stand ages due to increasing litter biomass and enzyme activity (Feng et al., 2019a; Feng et al., 2019b; Wang et al., 2019). However, it is not clear whether soil available P decreases and available N increases with increasing plantation age in N-rich and P-deficient subtropical regions, and thus increasing the P-limitation of plantation forests.
Castanopsis hystrix plantation occupies more than 50, 000 ha in southern China and provides high C storage and economic benefits (He et al., 2013; Zhou et al., 2013; Xu et al., 2020). However, the vegetation dynamics and the relationship between soil development and vegetation inC. hystrix plantations have rarely been studied. Previous studies have investigated the implications of increasing stand ages on the alternations in nutrient concentrations and stoichiometry in single components of many plantations, but have not explained the ecosystem-level (leaf–litter–soil–microorganism) nutrient limitation mechanisms (Feng et al., 2019a; Feng et al., 2019b; Wang and Zheng, 2021; Yang et al., 2021). This study examined the impacts of increasing stand ages (6-, 10-, 15-, 20-, 30-, and 34-year-old stands) on the nutrient concentrations and stoichiometry in the leaves, litter, soil, microorganisms, and soil enzyme activities in a C. hystrix plantation in southern China. The main objectives of this study were to (a) investigate the impacts of increasing stand ages on the nutrient concentrations and stoichiometry in the leaves, litter, soil, and microorganisms of the C. hystrix plantation; (b) determine the mechanisms regulating the nutrient concentrations in the soil, microorganisms, and plants. High N deposition with 30–50 kg N ha−1  yr−1 has been reported in some studies (Fang et al., 2008; Hietz, 2011), and P deficiency is often found in forest ecosystems in southern China. Thus, in the present study, it was hypothesized that increasing stand ages in the C. hystrix plantation would (a) decrease aboveground productivity and (b) increase leaf N:P ratios due to stable leaf N concentrations and decreased P concentrations, and that (c) the reduced productivity might be related to increased P consumption.