Article Carbon, Nitrogen and Phosphorus Stoichiometry in Natural and Plantation Forests in China Lin Li 1 , Lei Liu 2, *, Zhen Yu 2 , Josep Peñuelas 3,4 , Jordi Sardans 3,4 , Qifei Chen 2 , Jiangbing Xu 2 and Guoyi Zhou 2,5, * 1 School of Environment Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China; 20191112008@nuist.edu.cn 2 Institute of Ecology, School of Applied Meteorology, Nanjing University of Information Science & Technology, Nanjing 210044, China; zyu@nuist.edu.cn (Z.Y.); qifei_chen@nuist.edu.cn (Q.C.); jbxu@nuist.edu.cn (J.X.) 3 Global Ecology Unit CREAF-CSIC-UAB, CSIC, 08193 Bellaterra, Catalonia, Spain; josep.penuelas@uab.cat (J.P.); j.sardans@creaf.uab.cat (J.S.) 4 Centre de Recerca Ecològica i Aplicacions Forestals (CREAF), 08193 Cerdanyola del Vallès, Catalonia, Spain 5 South China Institute of Botany, Chinese Academy of Sciences, Guangzhou 510650, China * Correspondence: liulei@nuist.edu.cn (L.L.); gyzhou@nuist.edu.cn (G.Z.) Abstract: Ecological stoichiometry is essential for understanding the biogeochemical cycle in forest ecosystems. However, previous studies of ecological stoichiometry have rarely considered the impacts of forest origins, which could help explain why to date so much uncertainty has been reported on this subject. In this study, we tried to reduce this uncertainty by examining carbon (C), nitrogen (N) and phosphorus (P) in roots, litter and soil in both natural and plantation forests throughout China. The sampled forest sites were divided into three groups according to the identified succession stages: early (ES), middle (MS) and late (LS) stages. Our results show that soil C, N and P concentrations were significantly higher in natural (NF) than in plantation (PL) forests. As succession/growth Citation: Li, L.; Liu, L.; Yu, Z.; proceeded, P concentrations significantly increased in litter, roots and soil in NF, while the opposite Peñuelas, J.; Sardans, J.; Chen, Q.; Xu, occurred in PL. These results indicate that NF are able to use P more efficiently than PL, especially in J.; Zhou, G. Carbon, Nitrogen and the LS. Furthermore, the higher root N:P ratio indicates that the growth of PL was limited by P in Phosphorus Stoichiometry in Natural both MS and LS. Our results also suggest that geographical and climatic factors are not the dominant and Plantation Forests in China. Forests 2022, 13, 755. https:// factors in the differences in P between NF and PL, and, even more clearly and importantly, that doi.org/10.3390/f13050755 native forests with native species are more capable of conserving P than planted forests, which are frequently less diverse and dominated by fast-growing non-site native species. These results will Academic Editors: Jianping Wu, Jie help improve biogeochemical models and forest management throughout the world. Zhao, Songze Wan and Frank S. Gilliam Keywords: C N P stoichiometry; natural forests; plantations; succession stage; climatic factors Received: 15 April 2022 Accepted: 9 May 2022 Published: 13 May 2022 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Carbon (C) is one of the base materials that support vegetation growth [1], in which published maps and institutional affil- nitrogen (N) and phosphorus (P) are the two most important limiting elements [2,3]. The iations. ratios of carbon to nitrogen (C:N), carbon to phosphorus (C:P) and nitrogen to phosphorus (N:P) can therefore be used to explore the relationships and feedback between organisms and the environment [4], as well as biogeochemical processes [2]. C, N and P in forest ecosystems are exchanged between plants, litter and soil [5]. However, the determination Copyright: © 2022 by the authors. of stoichiometry in root–litter–soil systems still needs clarifying. The importance of roots, Licensee MDPI, Basel, Switzerland. in particular, is often ignored, even though they play a vital role in connecting plants’ This article is an open access article aboveground and underground structures [6]. More than 90% of the N and P nutrients in distributed under the terms and vegetation are returned to the soil through litter [7]. conditions of the Creative Commons China possesses abundant forest resources with rich forest types spread over a climatic Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ gradient that ranges from cold temperate to tropical [8]. These forests, either naturally 4.0/). regenerated (natural forests, NF) or artificially planted (plantation forests, PL), greatly differ Forests 2022, 13, 755. https://doi.org/10.3390/f13050755 https://www.mdpi.com/journal/forests Forests 2022, 13, 755 2 of 17 in terms of both species composition and management. As previously reported, China’s PL provide fewer ecosystem services than NF since they are monocultures, and as a result of inappropriate management, there is excessive use of fast-growing pioneer successional species, use of non-native species such as Pinus and Eucalyptus, less biodiversity, and declining forest productivity [9,10]. Thus, there is an urgent need to explore differences in nutrient cycling between NF and PL, a process that is seldom reported but which is essential for improving forest management. Despite intensive studies that have investigated the stoichiometric characteristics of plant tissues [11–13] and controlling factors at different temporal-spatial scales [3,12,14–16], debate continues as to the influence of climatic factors on stoichiometric ratios. For example, by compiling published data relating to global experiments conducted in natural environments, Yuan and Chen [17] found that the ratio of terrestrial plant N:P decreases with greater atmospheric concentrations of CO2 , increased rainfall, and P fertilization, but increases with warming, drought, and N fertilization. By comparison, Reich and Oleksyn [3] used a global data set of 5087 observations of leaf N and P to demonstrate that leaf N and P increase from the tropics towards cooler and drier mid-latitudes, and that the N:P ratio increases with mean annual temperature (MAT) and towards the equator. Yet, Townsend [18] found no relationship between the N:P ratios and either latitude or mean annual precipitation (MAP) in the tropics. In addition to these discrepancies, the interactions between root– litter–soil are often not taken into account in studies of the stoichiometric characteristics in forests, even though they could give rise to important feedbacks between the above- and below-ground components of ecosystems [19]. As such, the internal mechanisms of C, N and P balances in different forest stands during succession/growth stages still need to be further explored. In this study, we examined the C, N and P concentrations and stoichiometric ratios in roots, litter and soil in different forest stands in China’s forest ecosystems. We predicted that the C, N and P concentrations and their stoichiometric ratios in roots, litter and soil are likely to be closely linked. Therefore, our objectives were to (1) quantify the patterns of C, N and P concentrations and stoichiometric characteristics in roots, litter and soil during succession stages in NF and PL throughout China’s forest ecosystems; and (2) determine the factors affecting C, N and P concentrations and ecological stoichiometry in roots, litter and soil, and the differences between NF and PL. 2. Materials and Methods 2.1. Samples and Measurement Root, litter and soil samples were collected in China during the years 2011–2015. Samplings and laboratory analyses were all conducted following a consistent standardized protocol [20,21]. We divided the country into three grid sizes (100, 400 and 900 km2 ) based on vegetation distribution using a 1: 1,000,000 vegetation map. A grid size of 100 km2 was designed for tropical and subtropical regions with rich species diversity, and 400 and 900 km2 were for temperate and alpine vegetation regions where species diversity is relatively poor. Then we overlaid the grid maps with administrative maps and randomly select 3% to 5% of the grid for the field surveys by considering forest origin, age and type [22]. At each site, a 1000-m2 plot was established (600 m2 in some cases in plantations), each of which consisted of 10 subplots (10 m × 10 m) for the field survey. In each plot, plant roots were collected from the commonest tree species (diameter breast height ≥ 5 cm, 1–5 species). We also set up three 1 m × 1 m quadrats in each subplot for litter sampling. After removing the litter layer, soil samples at 0–50 cm were collected in each plot with a soil auger. Fertilized sites, sites with missing measurements in any depth of 0–50 cm in the soil matrix, and missing root of tree samples were filtered, and in total of 1112 sites were retained for analysis (Figure 1). All the root and litter samples were oven-dried at 65 ◦ C to a constant weight and ground into fine powder with a plant sample grinder. Soil samples were air dried. C and N concentrations in the root, litter and soil were analyzed using an elemental analyzer (2400 II CHNS; Perkin-Elmer, Boston, MA, USA). P concentrations Forests 2022, 13, x FOR PEER REVIEW 3 of 18 retained for analysis (Figure 1). All the root and litter samples were oven-dried at 65 °C to Forests 2022, 13, 755 a constant weight and ground into fine powder with a plant sample grinder. Soil 3samples of 17 were air dried. C and N concentrations in the root, litter and soil were analyzed using an elemental analyzer (2400 II CHNS; Perkin-Elmer, Boston, MA, USA). P concentrations were determined using the molybdate/ascorbic acid method after H2SO4-H2O2 digestion. were determined using the molybdate/ascorbic acid method after H2 SO4 -H2 O2 digestion. The sampled The sampledsites siteswere were divided divided into into twotwo groups, groups, natural natural (NF)(NF) and plantations and plantations (PL) for- (PL) forests. Three successional stages or growth stages were identified for each (early, middle and middle ests. Three successional stages or growth stages were identified for each (early, late and lateDetails stages). stages). ofDetails of the identification the identification of the successional of the successional and growthand growth stages can bestages foundcanin be found Yu et al.in[22]. Yu et al. [22]. Dominant Dominant species of species differentofsuccessional different successional stages inforest stages in different different forest origins origins were wereinshown shown detail in detailA1. in Table in Table A1. Figure 1. Sampling Figure 1. Samplinglocations locationsofofthe thenatural naturalforests and forests plantations and in this plantations study. in this study. 2.2. Climate Data 2.2. Climate Data Climate data (MAT and MAP) were obtained from the National Ecosystem Research Climate Network data (http://data.cma.cn/en, of China (MAT and MAP) were obtained accessed from on 16 the National January 2021).Ecosystem Research Network of China (http://data.cma.cn/en, accessed on 16 January 2021). 2.3. Statistical Analyses 2.3. Statistical The meanAnalyses and standard error of the investigated variables (including C, N and P contents, The and mean C:N, andC:P and N:Perror standard ratios) of of theroot, litter and variables investigated soil mixtures were calculated (including C, N and P separately. Independent-samples T-tests were used to examine the differences contents, and C:N, C:P and N:P ratios) of root, litter and soil mixtures were calculated between the C, N and P concentrations and ratios in the different forest types. One-way separately. Independent-samples T-tests were used to examine the differences between analyses of variance (ANOVA) were used to examine the differences in C, N and P concentrations the C, N and P concentrations and ratios in the different forest types. One-way analyses and the ratios between of variance (ANOVA)the different were usedsuccession/growth stages. Pearson’s to examine the differences correlation in C, N was used and P concentrations to analyze the association between the studied variables (C, N and P concentrations, and and the ratios between the different succession/growth stages. Pearson’s correlation was geographical and climatic factors). All values were considered statistically significant at used to analyze the association between the studied variables (C, N and P concentrations, p < 0.05 and the results are reported as the means ± standard errors. All statistical analyses and geographical and climatic factors). All values were considered statistically significant were performed using SPSS v22.0 (IBM Crop.) (Chicago, IL, USA), and the statistical data at p 0.05). However, litter N concentrations were significantly higher in PL than in NF. In contrast, 1, p < 0.05); root P concentrations were significantly lower in PL than in NF (Table 1, p < litter P concentrations were significantly higher in NF than PL (Table 1, p < 0.05). Soil C, N and P concentrations were significantly lower in PL than in NF (Table 1, p < 0.05). Forests 2022, 13, 755 4 of 17 Table 1. Concentrations of C, N, and P in roots, litter and soil in NF and PL in forest ecosystems in China. C Concentration N Concentration P Concentration Parameter Forest Origin (g·kg−1 ) (g·kg−1 ) (g·kg−1 ) NF 443.30 ± 1.87 B 4.4 ± 0.12 B 0.73 ± 0.03 A Root PL 451.80 ± 2.41 A 6.64 ± 0.26 A 0.56 ± 0.03 B NF 412.71 ± 2.69 A 10.48 ± 0. 13 B 0.93 ± 0.03 A Litter PL 410.40 ± 4.05 A 10.96 ± 0.19 A 0.71 ± 0.02 B NF 22.00 ± 0.83 A 1.56 ± 0.05 A 0.48 ± 0.02 A Soil PL 16.83 ± 0.69 B 1.14 ± 0.06 B 0.39 ± 0.01 B Note: Capital letters indicate significant differences in C, N and P concentrations in roots, litter and soil between different forest origins (p < 0.05); n = 667 and 445 for NF and PL, respectively. The C:N, C:P, and N:P ratios did not significantly differ between NF and PL in soils (Table 2, p > 0.05). For roots and litter, the N:P ratios were significantly higher in PL than in NF (Table 2, p < 0.05). By comparison, the C:N and C:P ratios in NF and PL only significantly differed in roots and litter (Table 2, p < 0.05), respectively. Table 2. Stoichiometry of the C, N, and P in roots, litter and soil in NF and PL in forest ecosystems in China. Forest Parameter C:N Ratio C:P Ratio N:P Ratio Origin NF 128.71 ± 2.70 A 1352.19 ± 62.18 A 11.86 ± 0.45 B Root PL 109.79 ± 4.16 B 1421.91 ± 50.82 A 16.56 ± 0.65 A NF 46.19 ± 1.02 A 674.38 ± 20.95 B 16.19 ± 0.42 B Litter PL 43.74 ± 1.11 A 853.67 ± 36.52 A 19.22 ± 0.48 A NF 17.58 ± 0.75 A 64.18 ± 5.76 A 4.20 ± 0.40 A Soil PL 19.21 ± 1.27 A 60.64 ± 2.62 A 3.79 ± 0.18 A Note: Capital letters indicate significant differences in the C, N and P stoichiometry of roots, litter and soil between the different forest origins (p < 0.05); n = 667 and 445 for NF and PL, respectively. Although the C concentrations were not significantly different in the three succession stages, both in NF and PL and in roots and litter (Figure 2a,d, p > 0.05) soil C concentrations significantly increased with succession/growth stages in both NF and PL (Figure 2g, p < 0.05). Root N concentrations generally decreased in NF during succession (Figure 2b, p < 0.05) but did not change significantly in PL (Figure 2b, p > 0.05). However, compared to the ES and MS, soil N increased significantly in NF and decreased significantly in PL in the LS (Figure 2h, p < 0.05). P concentrations generally increased in NF during succession in roots, litter and soil but significantly decreased in PL (Figure 2c,f,i, p < 0.05). The C:N, C:P and N:P ratios did not significantly differ in NF in roots during succession (Figure 3, p > 0.05). Root C:P and N:P ratios generally increased in PL during succession (Figure 3b,c, p < 0.05). Litter C:P and N:P ratios significantly increased with age in PL but decreased in NF (Figure 3e,f, p < 0.05). Soil C:N and C:P ratios significantly increased in PL (Figure 3g,h, p < 0.05). The difference in soil N:P ratios in NF and PL during succession was not significant (Figure 3i, p < 0.05). NF 600 a b 1.0 c Aa Aa PL Root N (g·Kg-1) 8 Aa Aa Aa Root P(g·Kg-1) Ba Aa Root C(g·kg-1) 500 Aa Aa Aa Aa Aa 0.8 Aa Ba 400 6 Ba Bab 0.6 300 Bb Bb Forests 4 0.4 Forests2022, 2022,13, 13,x755 FOR 200 PEER REVIEW 55 ofof1718 100 2 0.2 0 0 0.0 600 d 15 e 1.2 f NF Aa Aa ) ) 600 a b 1.0 c (g·Kg-1)-1) -1 -1 ) ) 500 Aa Aa Aa Aa Aa 1.0 Ab Aa -1 -1 PL Aa 8 Aa Aa Ba Aa Aa P(g·Kg Ba Aa Aa Aa Aa AaBab N N(g·Kg Aa Aa Aa C(g·kg 500 Ba Aa 400 Aa 10 0.8 0.8 Aa Ba Bb P(g·Kg C(g·kg 400 6 Ba 300 Bab 0.6 300 Bb Bb 54 Litter 200 0.4 0.4 Litter Litter 200 Root Root 100 0.2 Root 100 2 0.2 0 0 0.0 0 0 0.0 600 g Aa 15 e 2.5 h 1.2 fi 0.8 30 d Aa Aa N(g·Kg-1-1)) Aa Aa C(g·kg-1-1) ) Soil P(g·Kg-1) 500 Aa Ab 2.0 Aa Aa Aa Aa 1.0 AbAa Ba Aa Aa Aa Ba Aa Aa Ba 0.6 Aa SoilN(g·Kg Ab SoilC(g·kg Bab Ab Aab 20 400 Ac Bab 10 Ab 1.5 Aa 0.8 Ac Bb Ab Aab Bb 0.4 Bb 300 1.0 0.6 10 Litter 200 5 0.4 0.2 Litter 100 Litter 0.5 0.2 0 0.0 0.0 0 ES MS LS 0 ES MS LS 0.0 ES MS LS g Figure 2.AaC, N and h 2.5P concentrations in roots 0.8 i 30 Aa (a–c), litter (d–f) and soil (g–i) Aa in different succession Soil N(g·Kg-1) Soil P(g·Kg-1) Soil C(g·kg-1) Ab stages in NFBa and2.0 PL. Note: TheAberror bars indicate0.6 standard Aa errors of the means. Capital letters Ab Aab 20 Ac Ab Bab indicate significant Ab 1.5differences Aab in Aa concentrations Bb in 0.4 Ac different forest originsBbat the same succession 1.0 lowercase letters indicate significant differences in concentrations in the same stage (p < 0.05), while 10 0.2 0.5 forest origin at different succession stages (p < 0.05); n = 149, 344, 174, 102, 199 and 144 for NF−ES, 0 0.0 NF−MS, NF−LS, PL−ES, 0.0 PL−MS and PL−LS, respectively. ES MS LS ES MS LS ES MS LS FigureThe Figure 2. C:N, 2. C, C, N C:PPand N and N:P ratios in concentrations did not(a–c), roots roots significantly (a–c), litter(d–f) litter differ (d–f) in soilNF andsoil and ininroots (g–i) (g–i) during indifferent different succes- succession succession stages sion stages in in NF (Figure NF and PL. 3, p >PL. and Note: 0.05). Note: TheC:P Root The error errorandbars N:P bars indicate standard ratiosstandard indicate generally errors ofofthe increased errors themeans. inmeans. Capitalsucces- PL during Capital letters letters indicate sion significant (Figure 3b,c, differences p < 0.05). in Litterconcentrations C:P and N:P in different ratios forest significantlyorigins at increasedthe indicate significant differences in concentrations in different forest origins at the same succession same with succession age in PL stage but (p < 0.05), decreased stage while (p < 0.05), in lowercase NFlowercase while letters (Figure 3e,f, indicate p indicate letters significant < 0.05). significant Soil C:N and differences in C:P ratios differences concentrations in the significantlyinincreased in concentrations same the same forest origin at different succession stages (p < 0.05); n = 149, 344, 174, 102, 199 and 144 for NF−ES, in PL (Figure forest origin at3g,h, p < succession different 0.05). The stages difference in soil (p < 0.05); n =N:P 149, ratios in 102, 344, 174, NF and PL 144 199 and during for NFsucces- −ES, NF−MS, NF−LS, PL−ES, PL−MS and PL−LS, respectively. sion NF−was MS, NF not−significant LS, PL−ES, PL (Figure −MS and3i, pPL<−0.05). LS, respectively. The C:N, C:P and N:P ratios did not significantly differ in NF in rootsNF during succes- 200 a N:P ratioscgenerally increased 2000 b 25 sion (Figure 3, p > 0.05). Root C:P and Aa Aain PLPLduring succes- Root C/N ratio Root N/P ratio Root C/P ratio 150 Aa Aa 1500p 0.05 b R22=0.54 p<0.05 c R22=0.22 p<0.05 d R22=0.52 p<0.05 e R22=0.48 p<0.05 4 a R R22=0.01 =0.00 p>0.05 p>0.05 b R R22=0.54 =0.50 p<0.05 p<0.05 c R22=0.22 R =0.14 p<0.05 p<0.05 d R R22=0.52 =0.52 p<0.05 p<0.05 e R R22=0.48 =0.48 p<0.05 p<0.05 NFatatESES R R2=0.48 R p<0.05 3 R2=0.00 =0.03 p>0.05 p<0.05 R2=0.50 R =0.19 p<0.05 p<0.05 R2=0.14 R =0.07 p<0.05 p<0.05 R22=0.52 R p<0.05 =0.18 p<0.05 =0.23 p<0.05 3 R2=0.03 p<0.05 R2=0.19 p<0.05 R2=0.07 p<0.05 R =0.18 p<0.05 R2=0.23 p<0.05 )) contentininNF -1 -1 2 (g·kg 2 (g·kg P Pcontent 1 1 0 0 22 24 26 28 30 32 34 36 90 95 100 105 110 115 400 800 1200 1600 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 422 24 26 28 30 232 34 36 90 95 100 105 110 115 400 800 1200 1600 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 4 f R2 =0.26 p<0.05 g R22=0.03 p>0.05 h R22=0.16 p<0.05 i 2 R2 =0.14 p<0.05 j R22=0.01 p>0.05 f R R22=0.26 =0.47 p<0.05 p<0.05 g R R22=0.03 p>0.05 h R R22=0.16 =0.22 p<0.05 p<0.05 i R R22=0.14 =0.34 p<0.05 p<0.05 j R R22=0.01 =0.04 p>0.05 p<0.05 contentininPLPLatatESES =0.06 p<0.05 R R2=0.47 p<0.05 R 3 =0.22 p<0.05 R2=0.06 R =0.00 p<0.05 p>0.05 R2=0.22 R =0.08 p<0.05 p<0.05 R22=0.34 =0.28 p<0.05 p<0.05 R2=0.04 R =0.10 p<0.05 p<0.05 3 R2=0.22 p<0.05 R2=0.00 p>0.05 R2=0.08 p<0.05 R =0.28 p<0.05 R2=0.10 p<0.05 )) -1 -1 2 (g·kg 2 (g·kg P Pcontent 1 1 0 0 20 25 30 35 40 45 50 100 105 110 115 120 0 400 800 1200 1600 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 20 25 30 35 40 45 50 100 105 110 115 120 0 400 800 1200 1600 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure 5. P concentration changes with latitude (a,f), longitude (b,g), mean mean annual precipitation precipitation 5. P concentration changes with latitude (a,f), longitude (b,g), mean annual Figure 5. annual precipitation (MAP) (MAP) (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in the (MAP) (c,h), (c,h), mean mean annual annual temperature temperature (MAT) (MAT) (d,i) (d,i) and and altitude altitude (e,j) (e,j) in in different different forest forest origins origins in in the the early stage (ES). The numbers of forest early stage (ES). The numbers of forest sites are 149 and 102 for NF−ES and −ES and PL−ES, respectively. PL−ES,respectively. forest sites sites are are 149 149 and and 102 102 for for NF NF−ES and PL−ES, respectively. Root Litter Soil 10 Root Litter Soil 10 a R22=0.10 p<0.05 b R22=0.00 p>0.05 c R22=0.12 p<0.05 d R22=0.25 p<0.05 e R22=0.07 p<0.05 a R R22=0.10 p<0.05 b R R22=0.00 =0.01 p>0.05 c R R22=0.12 =0.13 p<0.05 d R R22=0.25 p<0.05 e R22=0.07 R p<0.05 MS 8 =0.19 p<0.05 p>0.05 p<0.05 =0.22 p<0.05 =0.06 p<0.05 NFatatMS R R2=0.13 R =0.08 p<0.05 8 R2=0.19 R =0.04 p<0.05 p<0.05 R2=0.01 p>0.05 =0.06 p<0.05 p<0.05 R2=0.22 R =0.14 p<0.05 p<0.05 R2=0.06 R =0.11 p<0.05 p<0.05 R2=0.04 p<0.05 R2=0.06 p<0.05 R2=0.08 p<0.05 R2=0.14 p<0.05 R2=0.11 p<0.05 6 )) 6 -1 -1 contentininNF (g·kg 4 (g·kg 4 P Pcontent 2 2 0 0 20 25 30 35 40 45 90 95 100 105 110 115 120 125 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 4 20 25 30 35 40 45 90 95 100 105 110 115 120 125 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 4 f R22=0.21 p<0.05 g R22=0.10 p<0.05 h R22=0.16 p<0.05 i R22=0.20 p<0.05 j R22=0.00 p>0.05 f R R22=0.21 p<0.05 g R R22=0.10 p<0.05 h R R22=0.16 p<0.05 i R R22=0.20 p<0.05 j R R22=0.00 p>0.05 MS =0.33 p<0.05 =0.02 p<0.05 =0.24 p<0.05 =0.37 p<0.05 =0.00 p>0.05 contentininPLPLatatMS 3 R2=0.02 R p<0.05 R22=0.24 R p<0.05 R2=0.37 R p<0.05 R 3 R2=0.33 R =0.18 p<0.05 p<0.05 =0.05 p<0.05 =0.16 p<0.05 =0.29 p<0.05 R2=0.00 =0.02 p>0.05 p<0.05 R2=0.18 p<0.05 R2=0.05 p<0.05 R =0.16 p<0.05 R2=0.29 p<0.05 R2=0.02 p<0.05 )) -1 -1 2 (g·kg 2 (g·kg 1 P Pcontent 1 0 0 20 25 30 35 40 45 100 105 110 115 120 125 1300 400 800 1200 1600 2000 0 5 10 15 20 25 0 3000 6000 9000 12000 20 25 30 35 40 45 100 105 110 115 120 125 1300 400 800 1200 1600 2000 0 5 10 15 20 25 0 3000 6000 9000 12000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure 6. P concentration changes with latitude (a,f), longitude (b,g), mean annual precipitation (MAP) (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in the middle stage (MS). The numbers of forest sites are 344 and 199 for NF−MS and PL−MS, respectively. Forests 2022, 13, x FOR PEER REVIEW 8 of 18 Figure 6. P concentration changes with latitude (a,f), longitude (b,g), mean annual precipitation Forests 2022, 13, 755 8 of 17 (MAP) (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in the middle stage (MS). The numbers of forest sites are 344 and 199 for NF−MS and PL−MS, respectively. Root Litter Soil 5 a R2=0.01 p>0.05 b R2=0.38 p<0.05 c R2=0.19 p<0.05 d R2=0.35 p<0.05 e R2=0.32 p<0.05 R2=0.07 p<0.05 R2=0.34 p<0.05 R2=0.27 p<0.05 R2=0.53 p<0.05 R2=0.35 p<0.05 P content in NF at LS 4 R2=0.05 p<0.05 R2=0.05 p<0.05 R2=0.14 p<0.05 R2=0.09 p<0.05 R2=0.07 p<0.05 (g·kg-1) 3 2 1 0 20 25 30 35 40 45 50 95 100 105 110 115 120 400 800 1200 1600 2000 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 3 f R2=0.02 p>0.05 g R2=0.03 p<0.05 h R2=0.08 p<0.05 i R2=0.03 p<0.05 j R2=0.00 p>0.05 R2=0.16 p<0.05 R2=0.02 p>0.05 R2=0.17 p<0.05 R2=0.23 p<0.05 R2=0.02 p>0.05 P content in PL at LS R2=0.27 p<0.05 R2=0.04 p<0.05 R2=0.22 p<0.05 R2=0.38 p<0.05 R2=0.10 p<0.05 2 (g·kg-1) 1 0 20 25 30 35 40 95 100 105 110 115 120 300 600 900 1200150018002100 5 10 15 20 25 0 1000 2000 3000 4000 5000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure 7.7. P concentration changes with latitude (a,f), longitude (b,g), mean mean annual annual precipitation precipitation (MAP) (c,h), (MAP) (c,h), mean mean annual annual temperature temperature (MAT) (MAT) (d,i) (d,i) and and altitude altitude (e,j) (e,j) in in different different forest forest origins origins in in the the late stage late stage (LS). (LS). The The numbers numbers of of forest forest sites sites are are 114 114 and and 144 144 for for NF NF−LS −LS andand PL−LS, PL−LS,respectively. respectively. 4. Discussion 4.1. 4.1. C, C, N N and and P P Concentrations Concentrations and and Stoichiometry Stoichiometry in in Roots, Roots, Litter Litter and and Soil Soil in in Different Different Forests Forests Ecosystem Ecosystem C, C, NNandandPPare aretransferred transferred andand recycled recycled in plants, in plants, litterlitter andand soil [23], soil [23], and and play an important role in maintaining the nutrient balance in forest ecosystems. For For play an important role in maintaining the nutrient balance in forest ecosystems. ex- example, ample, litter litter nutrients nutrients are are important important inputs inputs for and for soil soil help and sustain help sustain the growth the growth of plants of plants [24]. Roots are not only an organ used by plants to absorb [24]. Roots are not only an organ used by plants to absorb nutrients such as N and P [25], nutrients such as N and P but[25], arebutalsoareanalso an important important source source of nutrients of nutrients themselves.themselves. Indeed,Indeed, Vogt etVogt et al. al. [26] [26] found found that the carbon and nutrients (N and P) returned to the soil through the turnover of that the carbon and nutrients (N and P) returned to the soil through the turnover of fine fine roots exceeded the ground litter in forest ecosystems on a global scale. roots exceeded the ground litter in forest ecosystems on a global scale. Our results showed that C, N and P concentrations were closely correlated between Our results showed that C, N and P concentrations were closely correlated between roots, litter and soil in both NF and PL (Figure 4), and significantly differed between roots, litter and soil in both NF and PL (Figure 4), and significantly differed between dif- different forest stands in China’s forest ecosystems. Root C and N concentrations in PL ferent forest−1stands in China’s forest ecosystems. Root C and N concentrations in PL (451.8 (451.8 g·kg and 6.64 g·kg−1 ) were significantly higher than those in NF (443.3 g·kg−1 g·kg−1 and 6.64 g·kg −1) were significantly higher than those in NF (443.3 g·kg−1 and 4.4 and 4.4 g·kg−1 ), and were also higher than the levels (417.8 g·kg−1 for root C, 4.87 g·kg−1 g·kg−1), and were also higher than the levels (417.8 g·kg−1 for root C, 4.87 g·kg−1 for root N) for root N) for China’s terrestrial ecosystems reported by Tang et al. [24]. By comparison, for China’s terrestrial ecosystems reported by Tang et al. [24]. By comparison, root P con- root P concentrations in PL (0.56 g·kg−1 ) were significantly lower than in NF (0.73 g·kg−1 ) centrations in PL (0.56 g·kg−1) were significantly lower than in NF (0.73 g·kg −1) (p < 0.05), (p < 0.05), and were smaller than the average root P in fine roots (0.78 g·kg−1 ) at a global and were level, smaller by as reported thanYuantheetaverage al. [27].root ThisPmay in fine roots (0.78 be related to theg·kg −1) at a global level, as different morphological reported by Yuan characteristics et al.and of roots [27].the This may beenvironment different related to the different bases of themorphological two forest stands character- [28,29]. Previous hypotheses have also claimed greater plant diversity with deeper roots,Previ- istics of roots and the different environment bases of the two forest stands [28,29]. since ous hypotheses more soil animals havecanalso claimed capture andgreater exploitplant P fromdiversity the deeperwith deeper soil [30]. roots, Liusince et al.more[31] soil animals have reported canforcapture surfaceand soilexploit layersP(0–10 from cm) the deeper C (31.01soil g·kg[30]. −1 ),Liu N et al. [31] (1.71 g·kghave −1 ) and re- ported for surface − 1 soil layers (0–10 cm) C (31.01 g·kg −1), N (1.71 g·kg−1) and P (0.39 g·kg−1) P (0.39 g·kg ) concentrations in China’s forests that are close to the C, N and P concentra- concentrations tions in the PL in inour China’s study. forests that are We found thatclose to N soil C, theandC, NP inandNFP were concentrations significantly in higher the PL in our study. than those in PL. We found that soil C, N and P in NF were significantly higher than those in PL. Forest restoration is the key factor affecting soil C, N and P concentrations [23]. In Forest we this study, restoration found that, is theas key factor affecting succession progressed, soil C,theNCand P concentrations concentrations in roots [23]. andIn this study, we found that, as succession progressed, the C concentrations litter did not change significantly, while the soil C concentrations increased significantly in roots and litter didboth in not change NF andsignificantly, PL (p < 0.05).while Morethe soil C concentrations specifically, increased significantly the soil C concentrations in both were obviously NF andinPL higher (p 0.05) but did significantly increase from 12.12 to 19.67 in PL. Such changes indicate that P is not the limiting nutrient in the ES in PL, but, rather, gradually becomes the limiting factor as the forest grows; this also suggests that the P retention capacity is worse in PL than in NF. As well, N gradually became the limiting nutrient in the LS in NF. In general, forests are usually limited by N in the early stage of succession and will become restricted by both N and P, or by just P, in later stages, a process that is more evident in PL (as indicated by our study). Soil C:P ratios can act as indicators of the potential of microbial mineralization in soil organic matter to release P or to absorb and store environmental P [41]. In our study we found that the soil C:P ratios in PL increased significantly as the succession progressed (p < 0.05) but did not change significantly in NF (p > 0.05). Decreasing P concentrations might therefore reduce the P availability in PL, which may further inhibit microbial activity and reduce long-term C storage capacity [42,43]. Wardle et al. [44] also found that in most long-term observational studies the N:P ratio in litter rose as the age of the soil substrate increased. This indicates that there are similar degradation trends in forest ecosystems from tropical to temperate zones, which generate a decrease in soil P availability as the succession advances. In our study, trends were observed to be more intense in PL than in NF. The frequent use of fast- growing, mostly non-native trees in plantations, above all in wet tropical and subtropical areas, tends to impoverish soil nutritional quality, which is especially evident in the case of P conservation in the plant–soil system [9,10]. Native forests, on the other hand, are generally more diverse and have several adaptations for coping with limited P availability, mainly in the late successional stages when there is a greater capacity for retaining P in the plant soil system [9,18,22]. Forests 2022, 13, 755 10 of 17 4.2. Factors Affecting C, N and P Concentrations and Their Stoichiometry in Different Succession Stages in NF and PL Recent analyses have shown that temperature [3,12], precipitation [18,27], functional group [31] and soil and litter nutrients [18,19,45,46] are strongly correlated with plant nutrients and stoichiometry. However, most of these previous studies have focused on changes at global or regional scales using published data and did not consider forest origins. Our results show that the changes in C concentrations along latitude, longitude, MAP and altitude gradients were similar in NF and PL. Tang et al. [20] report that vegetation and litter C concentrations decreased with increasing MAT. However, our study shows that during the three stages MAT had different effects on root and litter C concentrations in NF (negative) and PL (positive). This may be because of the differences in tree species composition in forest stands [47]. Sardans and Peñuelas [48] have demonstrated that water resources are an important driving force for increasing plant nutrient-use efficiency in ecosystems, and droughts will habitually change the internal N and P cycles in terrestrial ecosystems. Precipitation decreases from the southeast to the northwest in China, a pat- tern that shapes the distribution of the soil water content and further limits soil nutrient availability [49]. Certain studies report that climatic factors only have a weak impact on plant N concentrations, which are mainly determined by plant functional groups [31]. Moreover, the Temperature-Plant Physiological Hypothesis also indicates that higher N and P content help enhance the metabolic activity and growth rate of plants, which offsets the low impact of temperature at high latitude by decreasing the biochemical reaction rate of N-rich enzymes and P-rich RNA in plants [3,50]. The results of our study show that latitude and longitude affect the distribution of vegetation N content through temperature and precipitation. Moreover, MAP and MAT showed opposite effects on the root and litter N content between NF and PL, while they only had a significantly effect on the soil N content in the middle and later stage of NF. These results also suggested that plant species, rather than climatic variation, was the major determinant of plant N concentrations [51]. Like previous studies [3,12,18], climatic factors (MAT and MAP) were significantly and negatively correlated with P concentrations in both NF and PL (p < 0.05), but were related to the strong weathering and eluviation of soil in high MAT and MAP areas [52]. As succession progressed, P concentrations in root, litter and soil increased with greater longitude in the ES and MS stages in PL, but then decreased with longitude in the LS stages. However, we found that MAT, MAP, latitude, longitude and altitude all had the same effect on P concentrations in NF and PL in the LS, which indicates that geographical and climatic factors were not the reasons for differences in P concentrations between NF and PL. Yu et al. [53] have proved that high long-term N deposition significantly reduces total soil TP concentrations but has no significant influence on total soil TN concentrations, which has thus increased the C:P and N:P ratios over the past 60 years in subtropical China. These studies provide strong evidence that China’s total soil P concentrations have been decreasing over the past few decades, which has led to P limitations and higher N:P ratios in its forest ecosystems [31]. However, our results also support the idea that, given P limitation in forest ecosystems, natural forests (NF) seem to be more able to retain P than plantations (PL), which are frequently dominated by fast-growing non-native species. The overarching reason, as pointed out by Yu et al. [22], is that natural native vegetation during succession grows, retains and accumulates P from deeper soil layers and has a greater P-retention capacity [9,18,22]. However, we found that MAT, MAP, latitude, longitude and altitude all had the same effect on P concentrations in NF and PL in the LS, thus indicating that geographical and climatic factors were not the reasons for the differences in P concentrations between NF and PL. Instead, our results strongly suggest that species composition is the main cause of the different evolution of P limitation in soil in PL and in NF. Forests 2022, 13, 755 11 of 17 5. Conclusions Ecological stoichiometry in forest ecosystems were different between forest origins. In this study, we found that P concentrations significantly decreased in the litter, root and soil in plantation forests (PL) as succession/growth proceeded, while the opposite occurred in natural forests (NF). Our results indicate that NF maybe more capable as growth progresses of storing P nutrients than PL. Different plant N:P ratios in NF and in PL during the progress of succession/growth also suggests that the difference between forest origins should be considered in the future research. Our study also reveals that geographical and climatic factors were not the dominant drivers of the differences in P concentrations between NF and PL in China, although more studies are still required to confirm this finding. Author Contributions: Conceptualization, G.Z., L.L. (Lei Liu) and L.L. (Lin Li).; methodology, L.L. (Lin Li). and Q.C.; formal analysis, L.L. (Lin Li); writing—original draft preparation, L.L. (Lin Li) and L.L. (Lei Liu).; writing—review and editing, Z.Y., J.P., J.S., J.X. and G.Z.; supervision, L.L. (Lei Liu) and G.Z.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the China National Science Foundation (No. 42130506), the Startup Foundation for Introducing Talent of NUIST (No. 2019r065) and Ministry of Science and Technology of China (No. G2021014071L). J.P. and J.S. were financially supported by the Spanish Government project PID2019-110521GB-I00, the Catalan government project SGR2017-1005, and the Fundación Ramón Areces project CIVP20A6621. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: By 2022, the data will available for download at the website: http: //dhf.cern.ac.cn/meta/metaData, accessed on 16 January 2021. Acknowledgments: We thank field data collection efforts from Jingyun Fang, Guirui Yu, Gengxu Wang, Keping Ma, Shenggong Li, Sheng Du, Shijie Han, Youxin Ma, Deqiang Zhang, Shizhong Liu, Guowei Chu, Qianmei Zhang and Yuelin Li. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations The following abbreviations are used in this manuscript: C Carbon N Nitrogen P Phosphorus NF Natural forests PL Plantation forests ES Early stage MS Middle stage LS Late stage MAT Mean annual temperature MAP Mean annual precipitation Forests 2022, 13, 755 12 of 17 Appendix A Table A1. Tree species of each growth stages in NF and PL. Forest Growth n N Dominant Species Origin Stages Quercus wutaishansea Mary, Populus simonii Carr, Betula platyphylla Suk., Betula albosinensis Burk., Quercus variabilis Bl., Quercus acutissima Carruth., Pinus tabuliformis Carr., Pinus massoniana Lamb., Quercus glandulifera var. brevipetiolata Nakai, Abies fabri (Mast.) Craib, Quercus semicarpifolia Smith, ES 149 Pinus yunnanensis Franch., Cyclobalanopsis glauca (Thunb.) Oerst., Lithocarpus glaber (Thunb.) Nakai, Pinus densata Mast., Picea spinulosa (Griff.) A. Henry, Cunninghamia lanceolata (Lamb.) Hook., Abies georgei Orr var. smithii (Viguie et Gaussen) Cheng et L. Fraxinus rhynchophylla Hance, Quercus mongolica Fisch. ex Ledeb, Juglans mandshurica Maxim., Populus davidiana Dode, Betula dahurica Pall., Quercus aliena Bl. var. acuteserrata Maxim. ex Wenz., Robinia pseudoacacia L., Betula platyphylla Suk., Quercus wutaishansea Mary, Pinus armandii Franch., Pinus tabuliformis Carr., Picea crassifolia Kom., Sabina przewalskii Kom., Larix gmelinii (Rupr.) Kuzen., Quercus acutissima Carruth., Acer ginnala Maxim., Quercus variabilis Bl., Juniperus formosana MS 344 Hayata, Quercus aliena Bl. var. acuteserrata Maxim. ex Wenz., Vernicia fordii (Hemsl.) Airy Shaw, Populus tomentosa Carr, Pinus tabuliformis var. henryi (Masters) C. T. Kuan, Cunninghamia lanceolata (Lamb.) Hook., Pinus massoniana Lamb., Bothrocaryum controversum, Abies fabri (Mast.) Craib, NF 667 Pinus yunnanensis Franch., Quercus semicarpifolia Smith, Abies georgei Orr var. smithii (Viguie et Gaussen) Cheng et L., Picea spinulosa (Griff.) A. Henry Ulmus pumila L., Larix gmelinii (Rupr.) Kuzen., Quercus variabilis Bl., Cyclobalanopsis glauca (Thunb.) Oerst., Quercus aliena Bl. var. acuteserrata Maxim. ex Wenz., Pinus tabuliformis Carr., Picea crassifolia Kom., Pinus armandii Franch., Platycladus orientalis (L.) Franco, Quercus wutaishansea Mary, Betula albosinensis Burk., Quercus acutissima Carruth., Quercus aliena Bl. var. acuteserrata Maxim. ex Wenz., Pinus massoniana Lamb., Liquidambar formosana Hance, Quercus glandulifera var. brevipetiolata LS 174 Nakai, Pinus yunnanensis Franch., Schima superba Gardn. et Champ., Loropetalum chinense (R. Br.) Oliver, Castanopsis fargesii Franch., Bothrocaryum controversum, Castanopsis sclerophylla (Lindl.) Schott., Cinnamomum porrectum (Roxb.) Kosterm., Castanopsis hystrix J. D. Hooker et Thomson ex A. De Candolle, Castanopsis carlesii (Hemsl.) Hayata., Cunninghamia lanceolata (Lamb.) Hook., Abies georgei Orr, Abies delavayi Franch., Abies georgei Orr var. smithii (Viguie et Gaussen) Cheng et L., Pinus densata Mast., Quercus semicarpifolia Smith, Picea spinulosa (Griff.) A. Henry Larix gmelinii (Rupr.) Kuzen., Populus gansuensis C. Wang et H. L. Yang, Robinia pseudoacacia L., Platycladus orientalis (L.) Franco, Pinus tabuliformis Carr., Populus simonii Carr, Betula albosinensis Burk., Populus euramericana cv.‘I-214’, Pinus massoniana Lamb., Abies fabri (Mast.) Craib, Quercus ES 102 semicarpifolia Smith, Cinnamomum camphora (L.) Presl., Pinus yunnanensis Franch., pinus elliottii, Eucalyptus robusta Smith, Cunninghamia lanceolata (Lamb.) Hook., Acacia mangium Willd., Schima superba Gardn. et Champ., E. urophylla × E. grandis Larix gmelinii (Rupr.) Kuzen., Pinus sylvestris var. mongolica Litv., Picea asperata Mast., Pinus tabuliformis Carr., Larix gmelinii var. principis-rupprechtii (Mayr) Pilger, Robinia pseudoacacia L., Larix kaempferi (Lamb.) Carr., Pinus thunbergii Parlatore, Quercus acutissima Carruth., Styphnolobium PL 445 MS 199 japonicum (L.) Schott, Populus tomentosa Carr, Quercus aliena Bl. var. acuteserrata Maxim. ex Wenz., Populus tomentosa Carr, Quercus variabilis Bl., Cunninghamia lanceolata (Lamb.) Hook., Cupressus funebris Endl., Eucalyptus robusta Smith, Acacia mangium Willd. Pinus tabuliformis Carr., Larix gmelinii (Rupr.) Kuzen., Populus davidiana Dode, Pinus densiflora Sieb. et Zucc., Platycladus orientalis (L.) Franco, Pinus thunbergii Parlatore, Populus simonii Carr, Populus cathayana Rehd., Populus tomentosa Carr, Larix kaempferi (Lamb.) Carr., Cunninghamia lanceolata LS 144 (Lamb.) Hook., Pinus massoniana Lamb., pinus elliottii, Cinnamomum camphora (L.) Presl., Quercus variabilis Bl., Loropetalum chinense (R. Br.) Oliver, Acacia mangium Willd., Schima superba Gardn. et Champ., Liquidambar formosana Hance ulus cathayana Rehd., Populus tomentosa Carr, Larix kaempferi LS 144 (Lamb.) Carr., Cunninghamia lanceolata (Lamb.) Hook., Pinus mas- soniana Lamb., pinus elliottii, Cinnamomum camphora (L.) Presl., Quercus variabilis Bl., Loropetalum chinense (R. Br.) Oliver, Acacia Forests 2022, 13, 755 mangium Willd., Schima superba Gardn. et Champ., Liquidambar 13 of 17 formosana Hance Root Litter Soil 800 700 a R2=0.00 p>0.05 b R2=0.15 p<0.05 c R2=0.10 p<0.05 d R2=0.13 p<0.05 e R2=0.09 p<0.05 R2=0.22 p<0.05 R2=0.14 p<0.05 R2=0.00 p>0.05 R2=0.07 p<0.05 R2=0.10 p<0.05 C content in NF at ES 600 R2=0.02 p>0.05 R2=0.07 p<0.05 R2=0.03 p<0.05 R2=0.03 p<0.05 R2=0.12 p<0.05 500 (g·kg-1) 400 300 200 100 0 25 30 35 90 95 100 105 110 115 120 400 800 1200 1600 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 900 800 f R2=0.00 p>0.05 g R2=0.00 p>0.05 h R2=0.01 p>0.05 i R2=0.00 p>0.05 j R2=0.02 p>0.05 R2=0.15 p<0.05 R2=0.00 p>0.05 R2=0.09 p<0.05 R2=0.09 p<0.05 R2=0.01 p>0.05 C content in PL at ES 700 R2=0.01 p<0.05 R2=0.00 p>0.05 R2=0.03 p>0.05 R2=0.01 p>0.05 R2=0.11 p<0.05 600 (g·kg-1) 500 400 300 200 100 0 20 25 30 35 40 45 50 100 105 110 115 120 0 300 600 900 120015001800-5 0 5 10 15 20 0 1000 2000 3000 4000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure A1. C content changes with latitude (a,f), longitude (b,g), mean annual precipitation14(MAP) Forests 2022, 13, x FOR PEER REVIEW Figure A1. C content changes with latitude (a,f), longitude (b,g), mean annual precipitation of 18 (c,h), mean (MAP) annual (c,h), meantemperature (MAT) (d,i) annual temperature (MAT)and(d,i) altitude (e,j) in different and altitude forest origins (e,j) in different in the early forest origins stage (ES). in the The early number stage of forest (ES). The number sites are 149 of forest sitesand are 102 for NF−ES 149 and and 102 for NF −ESPL−ES, and PLrespectively. −ES, respectively. Root Litter Soil 800 700 a R2=0.01 p<0.05 b R2=0.02 p<0.05 c R2=0.00 p>0.05 d R2=0.02 p<0.05 e R2=0.05 p<0.05 R2=0.05 p<0.05 R2=0.00 p>0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 C content in NF at MS 600 R2=0.00 p>0.05 R2=0.12 p<0.05 R2=0.07 p<0.05 R2=0.12 p<0.05 R2=0.28 p<0.05 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 45 50 95 100 105 110 115 120 125 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 R2=0.05 p<0.05 800 f R2=0.17 p<0.05 g R2=0.03 p<0.05 h R2=0.05 p<0.05 i R2=0.05 p<0.05 j R2=0.01 p>0.05 R2=0.04 p<0.05 R2=0.20 p<0.05 R2=0.17 p<0.05 R2=0.05 p<0.05 C content in PL at MS 700 R2=0.00 p>0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.41 p<0.05 600 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 45 100 105 110 115 120 125 1300 400 800 1200 1600 20000 5 10 15 20 25 0 3000 6000 9000 12000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure A2. Figure C content A2. changes C content with changes latitude with (a,f), latitude longitude (a,f), (b,g), longitude mean (b,g), meanannual precipitation annual (MAP) precipitation (c,h), mean (MAP) annual (c,h), meantemperature (MAT)(MAT) annual temperature (d,i) and altitude (d,i) (e,j) in and altitude different (e,j) forest in different origins forest in the origins middle in the stage (MS). middle The(MS). stage number of forestofsites The number forestare 344are sites and 344199andfor 199NF−MS for NF−and PL−MS, MS and respectively. PL−MS, respectively. Root Litter Soil 800 700 a R2=0.00 p>0.05 b R2=0.12 p<0.05 c R2=0.08 p<0.05 d R2=0.15 p<0.05 e R2=0.17 p>0.05 R2=0.18 p<0.05 R2=0.11 p<0.05 R2=0.00 p>0.05 R2=0.06 p<0.05 R2=0.15 p<0.05 C content in NF at LS 600 R2=0.05 p<0.05 R2=0.28 p<0.05 R2=0.10 p<0.05 R2=0.21 p<0.05 R2=0.35 p>0.05 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 45 50 95 100 105 110 115 120 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 800 700 f R2=0.01 p>0.05 g R2=0.00 p>0.05 h R2=0.00 p>0.05 i R2=0.01 p>0.05 j R2=0.03 p<0.05 R2=0.17 p<0.05 R2=0.08 p<0.05 R2=0.19 p<0.05 R2=0.26 p<0.05 R2=0.00 p>0.05 C content in PL at LS 600 R2=0.01 p<0.05 R2=0.10 p>0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.18 p<0.05 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 100 105 110 115 120 400 800 1200 1600 2000 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 100 C 0 20 25 30 35 40 45 100 105 110 115 120 125 1300 400 800 1200 1600 20000 5 10 15 20 25 0 3000 6000 9000 12000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure A2. C content changes with latitude (a,f), longitude (b,g), mean annual precipitation (MAP) Forests 2022, 13, 755 (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in 14 theofmiddle 17 stage (MS). The number of forest sites are 344 and 199 for NF−MS and PL−MS, respectively. Root Litter Soil 800 700 a R2=0.00 p>0.05 b R2=0.12 p<0.05 c R2=0.08 p<0.05 d R2=0.15 p<0.05 e R2=0.17 p>0.05 R2=0.18 p<0.05 R2=0.11 p<0.05 R2=0.00 p>0.05 R2=0.06 p<0.05 R2=0.15 p<0.05 C content in NF at LS 600 R2=0.05 p<0.05 R2=0.28 p<0.05 R2=0.10 p<0.05 R2=0.21 p<0.05 R2=0.35 p>0.05 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 45 50 95 100 105 110 115 120 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 800 700 f R2=0.01 p>0.05 g R2=0.00 p>0.05 h R2=0.00 p>0.05 i R2=0.01 p>0.05 j R2=0.03 p<0.05 R2=0.17 p<0.05 R2=0.08 p<0.05 R2=0.19 p<0.05 R2=0.26 p<0.05 R2=0.00 p>0.05 C content in PL at LS 600 R2=0.01 p<0.05 R2=0.10 p>0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.18 p<0.05 500 (g·kg-1) 400 300 200 100 0 20 25 30 35 40 100 105 110 115 120 400 800 1200 1600 2000 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure FigureA3.A3. C content changes C content with changes latitude with (a,f), latitude longitude (a,f), (b,g), longitude mean (b,g), annual mean precipitation annual (MAP) precipitation Forests 2022, 13, x FOR PEER REVIEW 15 of 18 (c,h), (MAP) (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins late mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in the stage (LS). in the lateThe number stage of number (LS). The forest sites are 114 of forest sitesand 144and are 114 for NF−LS and 144 for NF PL−LS, −LS and PLrespectively. −LS, respectively. Root Litter Soil 30 a R2=0.00 p>0.05 b R2=0.08 p<0.05 c R2=0.07 p<0.05 d R2=0.10 p<0.05 e R2=0.03 p<0.05 25 R2=0.06 p<0.05 R2=0.06 p<0.05 R2=0.04 p<0.05 R2=0.10 p<0.05 R2=0.05 p<0.05 N content in NF at ES R2=0.01 p>0.05 R2=0.08 p<0.05 R2=0.01 p>0.05 R2=0.03 p<0.05 R2=0.10 p<0.05 20 (g·kg-1) 15 10 5 0 25 30 35 90 95 100 105 110 115 120 400 800 1200 1600 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 40 f R2=0.09 p<0.05 g R2=0.00 p>0.05 h R2=0.13 p<0.05 i R2=0.04 p>0.05 j R2=0.00 p>0.05 R2=0.06 p<0.05 R2=0.00 p>0.05 R2=0.06 p<0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 N content in PL at ES 30 R2=0.00 p>0.05 R2=0.02 p>0.05 R2=0.00 p>0.05 R2=0.05 p<0.05 R2=0.20 p<0.05 (g·kg-1) 20 10 0 20 25 30 35 40 45 50 100 105 110 115 120 0 300 600 900 120015001800-5 0 5 10 15 20 0 1000 2000 3000 4000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure FigureA4. N content A4. changes N content with changes latitude with (a,f), latitude longitude (a,f), (b,g), longitude mean (b,g), meanannual precipitation annual (MAP) precipitation (c,h), mean (MAP) annual (c,h), meantemperature (MAT) (d,i) annual temperature and(d,i) (MAT) altitude (e,j) in different and altitude forest origins (e,j) in different in the early forest origins stage (ES). in the The early number stage of forest (ES). The number sites are 149 of forest sitesand are 102 for NF−ES 149 and and 102 for NF −ESPL−ES, and PLrespectively. −ES, respectively. Root Litter Soil 40 a R2=0.09 p<0.05 b R2=0.09 p<0.05 c R2=0.01 p<0.05 d R2=0.00 p>0.05 e R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.03 p<0.05 R2=0.09 p<0.05 R2=0.06 p<0.05 N content in NF at MS R2=0.03 p<0.05 R2=0.00 p>0.05 R2=0.16 p<0.05 R2=0.10 p<0.05 R2=0.13 p<0.05 R2=0.30 p<0.05 30 (g·kg-1) 20 10 0 20 25 30 35 40 45 50 95 100 105 110 115 120 125 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 R2=0.08 p<0.05 f R2=0.07 p<0.05 g R2=0.00 p>0.05 h R2=0.09 p<0.05 i R2=0.05 p<0.05 j R2=0.01 p>0.05 R2=0.02 p<0.05 R2=0.04 p<0.05 R2=0.03 p<0.05 R2=0.07 p<0.05 N content in PL at MS 30 R2=0.01 p>0.05 R2=0.11 p<0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.02 p<0.05 (g·kg-1) 20 10 0 0 20 25 30 35 40 45 50 100 105 110 115 120 0 300 600 900 120015001800-5 0 5 10 15 20 0 1000 2000 3000 4000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure A4. N content changes with latitude (a,f), longitude (b,g), mean annual precipitation (MAP) Forests 2022, 13, 755 15 of (c,h), mean annual temperature (MAT) (d,i) and altitude (e,j) in different forest origins in the17early stage (ES). The number of forest sites are 149 and 102 for NF−ES and PL−ES, respectively. Root Litter Soil 40 a R2=0.09 p<0.05 b R2=0.09 p<0.05 c R2=0.01 p<0.05 d R2=0.00 p>0.05 e R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.03 p<0.05 R2=0.09 p<0.05 R2=0.06 p<0.05 N content in NF at MS R2=0.03 p<0.05 R2=0.00 p>0.05 R2=0.16 p<0.05 R2=0.10 p<0.05 R2=0.13 p<0.05 R2=0.30 p<0.05 30 (g·kg-1) 20 10 0 20 25 30 35 40 45 50 95 100 105 110 115 120 125 0 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 R2=0.08 p<0.05 f R2=0.07 p<0.05 g R2=0.00 p>0.05 h R2=0.09 p<0.05 i R2=0.05 p<0.05 j R2=0.01 p>0.05 R2=0.02 p<0.05 R2=0.04 p<0.05 R2=0.03 p<0.05 R2=0.07 p<0.05 N content in PL at MS 30 R2=0.01 p>0.05 R2=0.11 p<0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.02 p<0.05 (g·kg-1) 20 10 0 20 25 30 35 40 45 100 105 110 115 120 125 1300 400 800 1200 1600 2000 0 5 10 15 20 25 0 3000 6000 9000 12000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure Figure A5.A5. N content N content changes changes with with latitude latitude (a,f), (a,f), longitude longitude (b,g), (b,g), mean mean annual annual precipitation precipitation (MAP) Forests 2022, 13, x FOR PEER REVIEW 16 of 18 (MAP) (c,h), (c,h), mean meantemperature annual annual temperature (MAT)(MAT) (d,i) (d,i) and and altitude altitude (e,j) (e,j) in in different different forest forest origins origins in the in the middle stage (MS). middle The(MS). stage number of forestofsites The number forestare 344are sites and 344199 andfor for NF−and 199NF−MS PL−MS, PL−MS, MS and respectively. respectively. Root Litter Soil 30 a R2=0.04 p<0.05 b R2=0.07 p<0.05 c R2=0.06 p<0.05 d R2=0.10 p<0.05 e R2=0.06 p<0.05 25 R2=0.01 p>0.05 R2=0.27 p<0.05 R2=0.12 p<0.05 R2=0.21 p<0.05 R2=0.28 p<0.05 N content in NF at LS R2=0.02 p<0.05 R2=0.27 p<0.05 R2=0.13 p<0.05 R2=0.24 p<0.05 R2=0.38 p<0.05 20 (g·kg-1) 15 10 5 0 20 25 30 35 40 45 50 95 100 105 110 115 120 400 800 1200 1600 2000 -15 -10 -5 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 30 f R2=0.05 p<0.05 g R2=0.03 p>0.05 h R2=0.08 p<0.05 i R2=0.07 p<0.05 j R2=0.01 p>0.05 R2=0.01 p>0.05 R2=0.00 p>0.05 R2=0.02 p>0.05 R2=0.00 p>0.05 N content in PL at LS R2=0.06 p<0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.00 p>0.05 R2=0.04 p<0.05 20 (g·kg-1) 10 0 20 25 30 35 40 100 105 110 115 120 400 800 1200 1600 2000 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 Latitude (°) Longitude (°) MAP (mm) MAT (℃) Altitude (m) Figure FigureA6. 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