1 Introduction

Forest ecosystems store globally about 80% of the aboveground carbon and over 50% of soil organic carbon (SOC) (Batjes 1996), and so make an important contribution to the regulation of the global carbon balance and to the mitigation of climate change. Although there is a continuing decline in the area of natural forest, afforestation/reforestation is on the increase, thereby promoting carbon sequestration and restoring the quality of degraded land (Kelty 2006; Wei, Li et al. 2013; Xie, Guo et al. 2013). Both plantation species composition and stand age are major determinants of the carbon budget (Chen, Zhang et al. 2011; Tang and Li 2013; Wei, Li et al. 2013; Xie, Guo et al. 2013). Rapidly growing plantation species fix considerable quantities of carbon over the short term (Laclau 2003; Peichl and Arain 2006; Chen, Zhang et al. 2011; Xie, Guo et al. 2013): Eucalyptus and Acacia plantations have been estimated to accumulate around 2 kg m−2 SOC over a 17-year period, while over a 29-year growing period, Norway spruce (Picea abies) and oak (Quercus sp.) managed only 0.9 and 0.2 kg m−2, respectively (Bhattacharya, Kim et al. 2016). Tree species native to the subtropics are typically rather intolerant of degraded soil, so species belonging to the genera Eucalyptus and Acacia, which are both tolerant and fast growing, are favored for reforestation (Yang, Liu et al. 2009; Chen, Zhang et al. 2011).

A rational assessment of the size of the carbon pool in a plantation requires the whole ecosystem to be considered. It has been suggested that most of the carbon accumulated in forest ecosystems is due to their high accumulation of biomass (Dou, Deng et al. 2013; Cheng, Lee et al. 2015), and that SOC should increase naturally with stand age following the reforestation of degraded soil (Xie et al., 2013). The dependency of SOC on plantation age is not universally acknowledged, since there are examples in which it has been shown to decrease somewhat during the establishment phase, before gradually increasing with stand age (Kasel, Singh et al. 2011; Assefa, Rewald et al. 2017). Since Acacia spp., unlike Eucalyptus spp., are able to fix atmospheric nitrogen, their growth results in an improvement to soil fertility; as a result, the accumulation of SOC under Acacia dominated plantations has been found to be generally superior to that achievable under Eucalyptus (Kasel, Singh et al. 2011). Some authors have argued that the high productivity of Eucalyptus and Acacia plantations results in a substantial export of nutrients in forest products such as nitrogen and phosphorus, and thus a slight decrease or no change in SOC was observed over time (Santana, Knicker et al. 2015; Cook, Binkley et al. 2016). Eucalyptus and Acacia plantations used for wood production are typically maintained for less than 10 years (Attiwill 1994; Inagaki, Kamo et al. 2010), so there is little evidence regarding the extent of their longer term capacity to store carbon. Furthermore, little attention has been given to the amount of SOC stored deeper in the soil profile in these two plantation types (Kasel, Singh et al. 2011; Souza-Alonso, Guisande-Collazo et al. 2015; Cook, Binkley et al. 2016), even though it has been claimed that, globally, the proportion of soil carbon present in the top 30 cm of the soils accounts for only around 30% or so of the carbon present in the top 100 cm (Batjes 2014).

The working hypotheses of the present research were threefold: (a) SOC can be boosted significantly as a result of afforestation, and especially, the accumulation of SOC is more effective under a nitrogen fixing species such as Acacia than under Eucalyptus; (b) a significant proportion of the SOC is held at depth in the soil profile; and (c) the two tree species are both effective in sequestering carbon. To test these hypotheses, the objectives of the study were to characterize the accumulation and allocation of total ecosystem carbon (TEC) in a sample of Eucalyptus (E. urophylla) and Acacia (A. mangium) plantations located in the Pearl River Delta (PRD) region of southern China, and establish the effect of species, stand age, and biomass on the SOC level in the top 100 cm of the soil. The wider purpose of the research was to provide a baseline for forest management, in particular with a view to enhancing the carbon sequestration capacity of forest ecosystems and identifying the species best suited for afforestation in the PRD.

2 Materials and methods

2.1 Local climate and soils

The PRD lies in the Chinese province of Guangdong (21° 31′–23° 10′ N, 112° 45′–113° 50′ E). Its climate is subtropical and monsoonal, characterized by hot, humid summers and mild winters. The region has a mean annual rainfall of 1600 mm, with the main rainy season occurring during July and August. The mean annual temperature is approximately 21 °C, while the daytime temperature exceeds 30 °C on around 120 days per year. The chosen sites, at which 10 were planted exclusively with Acacia and 11 exclusively with Eucalyptus, with both tree species adjacent to each other, lie in hilly land in the neighborhood of the cities Guangzhou, Zhuhai, and Heshan. The Eucalyptus plantations were measured in the period November–December 2010 and the Acacia ones between December 2010 and January 2011. The local red soils were, prior to the establishment of the plantations, covered with a combination of small shrubs and grass. A full description of the soils’ physicochemical characteristics are given in Table 1. The plantations fell into three age groups: young (less than 6 years), middle-aged (6–15 years), and mature (over 16 years). Further descriptive details of the sites are given by Zhang et al. (2012) and are reproduced in Table 1.

Table 1 A summary description of the study Eucalyptus and Acacia plantations
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2.2 Biomass and soil sampling

At each site, the canopy biomass captured within a 30 m × 30 m quadrat was measured. The understorey biomass (shrubs, ferns and grasses) was estimated from a destructive harvest of the central 1 m2 of each of five 2 m × 2 m quadrats. The contribution of the litter layer was obtained from a set of five 1 m × 1 m quadrats arranged along the diagonal of the main 30 m × 30 m quadrat. Felled dead wood and standing dead trees of trunk diameter > 2.5 cm and height > 1 m were included within the coarse woody debris (CWD) fraction. The Zhang et al. (2012) estimate of the relative contribution to the overall biomass of the various fractions (trees, shrubs, herbaceous species, litter, and CWD) was adopted. Estimates of the SOC, total nitrogen content (TN), and hydrolyzable nitrogen (HN) in the top 100 cm of the soil profile were obtained from samples taken from each of an upper, mid, and lower slope location: an equal number of 500 g soil aliquots was taken from the top 10 cm of the profile, from the next 10 cm (10–20 cm), from 20–30, 30–40, 40–50, 50–75, and 75–100 cm, producing a total of 231 soil samples (11 sites × three locations × seven soil depths) from the Eucalyptus plantations and 210 (10 × 3 × 7) from the Acacia plantations. Samples of undisturbed soil from each of the various layers were also collected using a soil sample ring kit (100 cm3), and the cutting ring method was used to measure soil bulk density (Lu 1999).

2.3 Laboratory analyses

SOC contents were obtained from air-dried (at room temperature) soil samples after the removal of surface organic matter and fine roots. Samples from each layer within a site were mixed in equal proportion and passed through a 0.149-mm sieve. The carbon content of the soil and biomass components was determined using a wet oxidation method (Forestry Standards of the People’s Republic of China method LY/T1237-1999). Soil pH was measured using a Toledo Five Easy pH meter (Mettler, Giessen, Germany). Three replicates of the surface soil (0–10 cm) were assessed for their TN content using a Foss 2300 Kjeltec Analyzer Unit (Foss Analytical AB, Höganäs, Sweden). A 0.0050–1.0000 g sample of either soil or biomass was digested in 5 mL 0.8 M K2Cr2O7 and 5 mL 18 M H2SO4, held at 170–180 °C for 5 min, and titrated with 0.2 M FeSO4·7H2O. An estimate for the quantity of biomass carbon was obtained by multiplying the weighed biomass by its derived proportion of carbon. The quantity of carbon present in the canopy was estimated by assuming that 50% of the biomass was carbon (Lieth and Whittaker 1975; Razakamanarivo, Grinand et al. 2011), while the quantity stored in the various soil layers was calculated by multiplying soil bulk density by both the depth of the layer and the concentration of carbon present. The TEC was obtained by summing the quantities of carbon stored in the biomass and in the soil.

2.4 Statistical analysis

The means and associated standard deviations of the proportion of carbon and quantity of carbon stored in the biomass and soil, as well as the TN contents were calculated. A two-way analysis of variance was used to test for the effect of tree type and stand age on the proportion or the quantity of carbon stored in the biomass and in the soil, as well as the distribution of SOC and TN content within the top 100 cm of the soil. Regression analyses were performed between TEC content and stand age, SOC content, and the amount of biomass represented in the CWD and litter layer. All calculations and analyses were carried out using the software package SPSS v17.0 (SPSS Inc., Chicago, IL, USA).

Data availability

Data presented are available at https://doi.org/10.6084/m9.figshare.5853420.v3 (Song et al. 2018).”

3 Results

3.1 TEC stock

The stock of TEC increased with stand age in both plantation types (Fig. 1): the associated coefficients of determination (R2) were 0.87 for the Eucalyptus and 0.93 for the Acacia plantations. The quantity of TEC was significantly greater in the mature than in the middle-aged or the young plantations, ranging from 79.8 to 204.9 t ha−1 under Eucalyptus and from 117.4 to 226.8 t ha−1 under Acacia.

Fig. 1
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The relationship between total ecosystem carbon and stand age in both a Eucalyptus and an Acacia plantation

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3.2 Biomass carbon stock

Both stand age and plantation type had a major impact on the accumulation of carbon and on its distribution between the various components (Table 2). The middle-aged and mature plantations both produced significantly more total biomass carbon than the young ones (P < 0.01). The contribution of the understorey layer, litter layer, and CWD were all nonnegligible. This was especially the case for the Acacia stands, in which the proportion was 8% in both the middle-aged and mature plantations, compared to, respectively, 5 and 4% for the Eucalyptus stands.

Table 2 Variation in the size of the TEC stock and its components in the plantations (Mg ha−1)
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3.3 Correlations between biomass and SOC

In this study, significant positive correlations were established between SOC storage and total biomass (Fig. 2), between SOC storage and litter biomass (Fig. 3), and between SOC storage and the CWD biomass (Fig. 4); the relationships were particularly strong for the Acacia-based plantations. The accumulation of litter and CWD biomass was more effective in the Acacia plantations (respectively, 4.0–15.5 and 0.9–13.4 t ha−1) than in the Eucalyptus ones (respectively, 2.0–9.6 and 0–2.6 t ha−1), which implies that the volume of raw material contributing to SOC is greater in the former plantation type.

Fig. 2
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The relationship between soil organic carbon and total biomass in both a Eucalyptus and an Acacia plantation

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Fig. 3
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The relationship between soil organic carbon and the biomass stored in the litter in both a Eucalyptus and an Acacia plantation

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Fig. 4
figure 4

The relationship between soil organic carbon and the biomass stored in the coarse woody debris in both a Eucalyptus and an Acacia plantation

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3.4 SOC stock

SOC stock increased with stand age (Fig. 5), with associated R2 values of 0.57 for the Eucalyptus stands and 0.75 for the Acacia stands. A significant positive correlation existed between the SOC storage in the top 100 cm of the soil and both stand age and forest type (P < 0.05, Table 2). The Acacia trees were more effective accumulators of SOC than the Eucalyptus ones: SOC storage under the former ranged from 74.1 to 132.7 t ha−1, while under the latter, the range was only 50.3–102.7 t ha−1. The overall contribution of SOC to TEC was highly significant: 60% in the case of Acacia and 53% in the case of Eucalyptus.

Fig. 5
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The relationship between soil organic carbon and stand age in both a Eucalyptus and an Acacia plantation

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3.5 The soils’ organic carbon and nitrogen content, carbon/nitrogen ratio, and bulk density

The variation in SOC content between the various soil layers in the two plantation types is summarized in Table 3. The SOC content in the top 100 cm of the soil increased markedly with stand age, ranging from 3.7 to 8.5 g kg−1 under Eucalyptus, and from 5.6 to 11.7 g kg−1 under Acacia. SOC content decreased gradually with increasing soil depth. At all of the sites, significantly (P < 0.01) more SOC was concentrated in the upper 20 cm of the soil than was found deeper in the profile. The SOC content of the top 10 cm was 3.6–5.3 (Eucalyptus) and 3.2–4.8 (Acacia) times higher than that present in the lowest 25 cm (75–100 cm), while the respective increases in the 10–20 cm layer were in the range 2.0–2.8 and 2.6–3.0. The TN content and carbon/nitrogen ratio both rose gradually with stand age, ranging from, respectively, 329.5–693.8 t kg−1 and from 10.4–11.7 (Eucalyptus), and from 497.9–949.2 t kg−1 and 11.2–12.4 (Acacia). In both plantation types, the TN content and the carbon/nitrogen ratio decreased gradually with soil depth, independently of stand age: both variables were at their highest in the surface soil layer, ranging from 1346.2–211.4 t kg−1 and from 14.1–8.9 (Eucalyptus) and from 1857.2–259.4 t kg−1 and 15.1–9.7 (Acacia) (Table 4, Fig. 6). The soils’ bulk density changed in the opposite sense with both stand age and soil depth. Independently of stand age, the bulk density of the upper (0–20 cm) layer was significantly lower than that of the lower (50–100 cm) layer: the range was 1.2–1.6 g cm−3 under Eucalyptus, and 1.2–1.8 g cm−3 under Acacia (Fig. 7).

Table 3 The effect of stand age and tree species on the accumulation of SOC (g kg−1)
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Table 4 Variation in TN across the top 100 cm of the plantation soils (mg kg−1)
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Fig. 6
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The carbon/nitrogen ratio of soil of different depths for different aged Eucalyptus (a) and Acacia (b) stands

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Fig. 7
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The bulk density of soil at different soil depths under Eucalyptus (a) and Acacia (b) plantations

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4 Discussion

4.1 The effect of tree species and stand age on stored SOC

The soil’s carbon pool reflects a balance between its loss through decomposition and its gain through the decomposition of leaf litter and CWD (Dou, Deng et al. 2013; Chen, Yu et al. 2017), the growth of the understory and root expansion (Cheng, Lee et al. 2015). There are two considerations which could explain the positive effect of stand age on SOC storage: the first relates to the increasing size of the carbon input from the understory layer, the litter, and the CWD as the plantation ages (Zhang, Guan et al. 2012; Zhao, Kang et al. 2014); the second is a consequence of the very low level of SOC (25 t ha−1) present in the upper 30 cm of the soil prior to afforestation, giving the opportunity for a large increase as the trees aged (Chen, Yu et al. 2017). The ability of Acacia plants to biologically fix nitrogen through their symbiosis with Rhizobium spp. should favor SOC accumulation (Yang, Liu et al. 2009). A correlation of determination > 0.8 obtained between carbon and nitrogen storage in both plantation types (Fig. 8), and the fact that the level of nitrogen was higher under Acacia than under Eucalyptus (Table 1) supports the notion that increasing the nitrogen status of the soil enhances the storage of SOC. The outcome validates the superiority, in terms of SOC accumulation, of nitrogen fixing over nonfixing tree species (hypothesis a), and is consistent with related studies comparing the effect on SOC of nitrogen fixing trees and nonfixing ones (Resh, Binkley et al. 2002). Overall, assuming a similar land use history and a similar climate, the accumulation of SOC in a given soil type is mainly dependent on the nature of the long-term vegetation present (Yang and Wang 2005; Xian, Zhang et al. 2009): thus, tree species and tree age, both individually and in combination, are strong determinants of the size of the SOC stock.

Fig. 8
figure 8

The relationship between soil organic carbon and total nitrogen content in both a Eucalyptus (a) and an Acacia (b) plantation

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4.2 Effect of biomass on SOC storage

Early stand development in a poor soil requires as rapid as possible a buildup of SOC in order for the plants to thrive. One aim of reforestation is to generate a large volume of biomass over a long period, in order to boost SOC in the PRD (Xie, Guo et al. 2013). Our study has confirmed that SOC content is positively correlated with biomass, which agrees with related studies based on the performance of other plantation tree species used for reforestation purposes in southern China (Zhang 2009; Dou, Deng et al. 2013; Wang, Guan et al. 2014). For example, Dou et al. (2013) found that reforetation of P. massoniana under eroded red soil after 18–30 years significantly increased soil C levels; the C accumulation following the reforestation appeared to be due to a combination of large biomass input and low C decomposition (Dou, Deng et al. 2013). Wang et al. (2014) have shown that SOC in the top 100 cm of a variety of mangrove forest soils varied from 184 to 279 t ha−1, in line with the biomass of the canopy layer (60–385 t ha−1). Similarly, Zhang (2009) have demonstrated that the SOC content of the uppermost 100 cm of the soil profile under a stand of evergreen broad-leaved trees ranged from about 85 t ha−1 in a young forest to 151 t ha−1 in a mature one, again reflecting increases in the biomass represented in the litter and CWD.

4.3 The vertical distribution of SOC

The SOC content of the surface layer in the present mature plantations was comparable to that measured in 7-year-old plantations of either subtropical Eucalyptus (~ 20 g kg−1) or Acacia (~ 25 g kg−1) provided with a dressing of phosphate fertilizer before planting (Santana, Knicker et al. 2015). The vertical distribution of SOC in the soil is in line with that observed in similar studies based on either tropical or subtropical Eucalyptus (Mishra, Sharma et al. 2003; Cao, Fu et al. 2010) or Acacia (Lemma, Kleja et al. 2006; Laik, Kumar et al. 2009) plantations, as well as in other plantation types monitored in southern China (Dou, Deng et al. 2013). Other factors affecting SOC were the soil’s TN content, bulk density, and carbon to nitrogen ratio (Table 5); their vertical distribution also determined the significantly high SOC present in surface soil layer (P < 0.01). It is therefore clearly important to maintain the SOC at and immediately below the soil surface.

Table 5 Correlations between SOC content and soil properties in Eucalyptus and Acacia plantations of different stand age
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Most studies that estimated the SOC level focused on the organic layer and surface horizons in the upper 30 cm of the soil. However, the roots of plant and thus input of organic matter extend to deep subsoil horizons, and soil carbon level would be greatly underestimated if one does not include the amounts stored in subsoil (Rumpel and Kögel-Knabner 2011; Wiesmeier, Prietzel et al. 2013; Zhang, Wang et al. 2015). For instance, Zhang et al. (2015) claimed that the proportion of SOC in a cropping soil 30–200-cm depth can reach more than 70% of total SOC (Zhang, Wang et al. 2015). In this study, we found that the content of SOC at deeper horizons (30–100 cm) is still high, especially for mature forests (> 4 g kg−1). Subsoil SOC showed significant correlation with stand age and forest type (P < 0.05) (Table 3). Hypothesis (b) was confirmed: a substantial presence of SOC was detected up to 50 cm below the soil surface in the Eucalyptus plantations (P < 0.05), and the SOC content of mature plantation was significant higher than that of the middle-aged and young ones (P < 0.05), showing that carbon is able to accumulate at a deeper level than can readily be explained by the presence of a greater root biomass (Bauhus, Khanna et al. 2000). Meanwhile, under Acacia, the upper 75 cm of the soil profile also showed a significant SOC accumulation with stand age (P < 0.05), perhaps reflecting the activity of nitrogen fixing bacteria (Inagaki, Kamo et al. 2011), as is shown in Table 4. However, because in this study the soils were not sampled below a depth of 100 cm, it is not possible to conclude definitively that carbon sequestration was limited to the top 100 cm of the soil profile. Thus, future attempts to document soil carbon under tree plantations will need to monitor the status of the soil at depths below 100 cm.

4.4 Implications for carbon sequestration achieved by fast-growing plantation trees

Sequestering carbon in the soil, ultimately in the form of stable humus, may well prove to be a more productive method for mitigating the negative effects of climate warming than its more temporary storage in the form of biomass (Thuille and Schulze 2006; Farley 2007). The measured SOC content within the top 100 cm of soil under mature Eucalyptus (102.66 t ha−1) and Acacia (132.70 t ha−1) trees was rather lower than the estimated means derived from both national (193.6 t ha−1) (Zhou, Yu et al. 2000) and global (189.0 t ha−1) data (Dixon, Solomon et al. 1994). Even within the red soils typical of subtropical climates, the present estimates are considerably lower than what was derived from an analysis of a 20-year-old Mytilaria laosensis plantation (174.8 t ha−1) (Liu and Liu 2012). However, they are substantially higher than the reported levels in both a 24-year-old Pinus massoniana plantation (38.2 t ha−1) and a secondary forest (68.5 t ha−1) (Xie, Guo et al. 2013). The TEC contents of both the mature Eucalyptus (204.9 t ha−1) and Acacia (226.8 t ha−1) plantations were also below that of both nationally based (258.8 t ha−1) (Zhou, Yu et al. 2000) and globally based (275.0 t ha−1) means (Dixon, Solomon et al. 1994). Comparisons with other plantation types in the PRD suggest that, on the basis of TEC, both Eucalyptus and Acacia are less effective accumulators of carbon than certain native evergreen broad-leaved species (Sun and Guan 2014), but are more effective than Pinus massoniana (Zhang, Xu et al. 2011). Overall, Eucalyptus and Acacia are both able to sequester carbon with a satisfactory level of efficiency and so are well suited for afforestation purposes in the PRD. Given that stand age had a marked effect on TEC stock in both plantation types (Table 1; Fig. 1), the conclusion is that the use of a rapidly growing tree species for the afforestation of a poor soil should represent an effective forest management practice where the aim is to sequester carbon as fast as possible (hypothesis c).

The consideration of succession is important when estimating the size of a plantation’s carbon pool. As is shown in Table 6, the quantity of carbon accumulated in over the period 2000–2003 in PRD was 22.8 Mt., which corresponded to 26%–36% of the total carbon (1.29 × 108 t) emitted as a result of the burning of fossil fuels in Guangdong Province over the period 2000–2003 (China Statistics Press 2005; SFA 2005). Stands of mature Eucalyptus and Acacia account for just 24% of the total plantation area in the PRD: whereas they have accumulated some 8.0 Mt carbon, maintaining them for some additional years should further increase their contribution to carbon sequestration.

Table 6 Implications for carbon sequestration by Eucalyptus and Acacia plantations in Guangdong Province
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5 Conclusions

The marked increase in TEC stock achieved over time by the Eucalyptus and Acacia plantations has confirmed that the afforestation of degraded soils can contribute positively to carbon sequestration. The largest single component of TEC was SOC. The major factors determining SOC accumulation in the top 100 cm of the soil profile were the species of tree, the stand age, and the biomass input. The accumulation of SOC under Acacia was more effective than under Eucalyptus. Protection of the SOC present in the surface layer of the soil is very important in the context of carbon sequestration, especially for Acacia plantations. The deeper soil layers (below 50 cm for Eucalyptus, and below 75 cm for Acacia) also represent a substantial carbon sink. The indications are that the afforestation of poor soils using rapid growing tree species should be an effective management practice for improving carbon budgets and compensating for the carbon emitted as a result of the burning of fossil fuel.