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Soil stoichiometry and carbon storage in long-term afforestation soil affected by understory vegetation diversity
Ecological Engineering 74 (2015) 415–422
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Soil stoichiometry and carbon storage in long-term afforestation soil affected by understory vegetation diversity Fazhu Zhao a,b , Di Kang a,b , Xinhui Han a,b, *, Gaihe Yang a,b, **, Gaihe Yang a,b , Yongzhong Feng a,b , Guangxin Ren a,b a b
College of Agronomy, Northwest A&F University, Yangling, 712100 Shaanxi, China The Research Center of Recycle Agricultural Engineering and Technology of Shaanxi Province, Yangling, 712100 Shaanxi, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 March 2014 Received in revised form 23 October 2014 Accepted 9 November 2014 Available online xxx
The afforestation of abandoned land could offer opportunities to sequester soil organic carbon (SOC), promote nutriment elements cycling, improve plant diversity in the plantation understory and provide ecosystem services. The objectives of this study were to identify plant diversity in the plantation understory, quantify the changes in SOC and total nitrogen (TN) storage in deep soil, assess the SOC, TN, and total phosphorus (TP) stoichiometries, and investigate their relationships in the Loess Plateau Region (LPR) undergoing long-term afforestation. Soil samples were collected at a soil depth of 0–200 cm under 30-yr old Robinia pseudoacacia L. and adjacent abandoned sites, and SOC, TN and TP were determined in different soil depth. Additionally, plant composition and diversity in the plantation understory were evaluated. The results showed that land subjected to long-term afforestation had greater plant coverage, plant density, richness index (R) and Shannon–Wiener diversity (H) compared to abandoned land communities (P < 0.05). SOC, TN and TP contents in afforested sites were significantly increased in surface soil (0–30 cm) as well as in the underlying soil (100 cm) compared to the corresponding abandoned land sites (P < 0.05) in most cases. Meanwhile, SOC, TN, and TP stoichiometry in afforested areas were higher than those of abandoned lands and significantly related to understory vegetation diversity (P < 0.05). In addition, lands subjected to long-term afforestation effectively increased SOC and TN storages compared to abandoned land at soil depths of 0–30 cm and 100–200 cm and were also significantly related to understory vegetation diversity (P < 0.05). These findings demonstrating that afforestation not only affects SOC and TN stocks in surface soil, but also strongly influences that in deep soil. And it is also indicating that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Long-term afforestation C:N:P stoichiometry Carbon storage Deep soil Loess Plateau
1. Introduction Afforestation areas, which occupy approximately 0.20 billion ha worldwide, could effectively help mitigate increasing atmospheric CO2 (Zhong et al., 2013). Afforestation also affects the establishment of natural vegetation and understorey ecology via resource competition (i.e. Bremer and Farley, 2010), allelopathy (i.e. Zhang et al., 2010). In addition, plant diversity in the plantation understory plays important roles in improving stimulating the soil nutrient cycling and maintaining soil quality after afforestation (Halpern 1995; Wang et al., 2011). Brockerhoff et al. (2003)
* Corresponding author. Tel.: +86 02987092265. ** Corresponding author. E-mail addresses: [email protected] (F. Zhao), [email protected] (D. Kang), [email protected] (X. Han), [email protected] (G. Yang), [email protected] (Y. Feng), [email protected] (G. Ren). http://dx.doi.org/10.1016/j.ecoleng.2014.11.010 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
reported that wider tree spacing during plantation establishment supports better maintenance of understorey vegetation. Zhang et al. (2014) found that the first four years after the establishment of afforestation are associated with lower plant diversity, however, plant diversity was improved after long-term afforestation. Meanwhile, understanding plant diversity in the plantation understory is critical to investigate the ecological functions of plantations and improve their management, especially after long-term afforestation. Following afforestation, changes inevitably occur in soil physical and chemical elements, particularly in the three main elements: carbon, nitrogen and phosphorus (Adams et al., 2001; Wei et al., 2009). Many studies (i.e. Walker and Adams, 1958; Post et al., 1982; Melillo et al., 2003; Zhong et al., 2013) have indicated that soil carbon, nitrogen, and phosphorus are often closely related. Cleveland and Liptzin (2007) found a well constrained C:N:P ratio of microbial biomass in 0–10 cm
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organic-rich soil globally. Tian et al. (2010) documented that soil C:N, C:P, and N:P ratios in organic-rich topsoil could be good indicators of soil nutrient status during soil development. Zhang et al. (2014) also demonstrated that vegetation cover, plant communities, geomorphology, and seawall all affected C, N, and P stoichiometry in soils. Despite progress made in terrestrial ecosystem restoration methods and analyses (Han et al., 2005; Manzoni et al., 2010) the stoichiometrical characteristics of C, N, and P in soils have yet to be fully described (Swift et al., 1998; Manzoni and Porporato, 2009), especially in the Loess Plateau Region (LPR) of China. Approximately 75% of total terrestrial C is stored in the world’s soils (Henderson 1995), and forest soils hold approximately 40% of C belowground (Dixon et al., 1994). Therefore, even if afforestation only slightly affects soil C stocks at the local level, it could have a significant effect on the global C budget (Paul et al., 2002). Sean et al. (2012) illustrated that changes in soil organic carbon (SOC) due to afforestation are negatively related to mean annual precipitation and positively correlated with plantation age. Nave et al. (2012) also demonstrated that afforestation has significant positive effects on SOC sequestration in the United States, although these effects require decades to manifest and primarily occur in the uppermost portion of the mineral soil profile. Thus, long-term afforestation is a better management option for increasing terrestrial C sequestration. It has been reported that more than 50% of total SOC is stored in the subsoil (at a depth below 50 cm) (Amundson, 2001), and at least 61% of the total soil C is stored below a depth of 30 cm depth in the northern circumpolar permafrost region (Guo and Gifford, 2002). In recent decades, many studies have illustrated that subsoil C may be even more important than topsoil C as a source or sink for CO2 than topsoil C (VandenBygaart et al., 2010; Rumpel and KögelKnabner, 2011). Thus, considering the potential role of SOC as an atmospheric CO2 sink it is important to understand whether the long-term afforestation affects large amounts of SOC in the subsoil or deep soil. However, the C and N storages in deep soil layers are not fully understood in LPR of China to date. The LPR is an important geological region that influences the global carbon cycle (Wang et al., 2010a,b). Vegetation coverage in the LPR is relatively low due to its harsh environment (Wei et al., 2012). Soil erosion and desertification in LPR reduced net primary productivity by 12 kg C ha1 year1 (Bai and Dent, 2009). Since the 1950s, the Chinese government has made great efforts to control soil erosion and restore ecosystems (Fu et al., 2002). More than 9.27 million ha of abandoned farmland (investment of more than 28.8 billion USD and with the involvement of 0.12 billion farmers) have been afforested in this region through the “Grain to Green Program” (GTGP), which has implemented large-scale ecological rehabilitation since 1999 (Lü et al., 2012). However, few studies have reported the plant diversity, soil C:N:P stoichiometry, carbon storage in deep soil and their relationships after long-term afforestation in this region until recently. Thus, this study aimed to: (a) analyze the plant diversity in the understory after long-term afforestation; (b) illustrate the soil C:N:P stoichiometry and assess SOC and N storage in different soil depths; and (c) evaluate the relationship between plant diversity, soil C:N:P stoichiometry, and soil C and N storages after long-term afforestation.
2. Methods and materials 2.1. Research area The study was conducted in the Wuliwan catchment (36 460 4200 –36 460 2800 N, 109130 4600 –109 160 0300 E), which is located in Ansai county in the central region of LPR (see Fig. 1). Ansai is a
Fig. 1. Location of the Loess Plateau and the study site.
typical county characterized by a semi-arid climate and a hilly loess landscape in the Loess Plateau. it has an annual average temperature of 8.8 C, and an average annual precipitation of 505 mm. 60% of the precipitation which occurs between July and September (300 mm in dry years while >700 mm in wet years). Accumulated temperatures above 0 C and 10 C are 3733 C and 3283 C, respectively. On average, there are approximately 157 frost-free days and 2415 h of sunshine each year. Arable farming mostly occurs on sloping lands without irrigation. The loess parent material at the site has an average thickness of approximately 50–80 m and the soil in this region is Calciustepts soil (Gong et al., 1999). Sand (2–0.05 mm) and silt (0.05–0.002 mm) account for approximately 29.22% and 63.56% of the material at a soil depth of 0–20 cm, respectively. The soil is highly erodible, with an erosion modulus of 10,000–12,000 Mg km2 year1 before the restoration efforts began in this region (Liu, 1999). After 30 years of vegetation restoration the area of forested lands, the area of the area of forestland increased significantly from 5% to 40% (Xue et al., 2009). The Wuliwan catchment is one of the experimental sites of the Institute of Soil and Water Conservation, Chinese Academy of Science (CAS). The major agricultural land use type in the LPR is slope cropland. Agricultural management in this region, including the major crop types grown, has not been changed significantly since the 1970s. After more than 30 years of comprehensive management, the ecological environment of the catchment has been significantly improved (Zhang et al., 2007). Beginning in late 1970s, slope cropland was replanted with forest, mainly Robinia pseudoacacia L. to control soil erosion. Abandoned cropland was also generated during this period due to its extremely low productivity and long distance from farmers’ residences (Li et al., 2004). Despite that wild grasslands and shrub lands were usually found on steep slopes, they were often used for firewood collection resulting in reduced vegetation to barrenness for long periods (80 year).
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2.2. Experimental design
2.3. Plant species identification and species diversity index
The main forest plantation in the Wuliwan catchment is R. psendoacacia (30-yr-old). The canopy closure, mean diameter at breast height (DBH), and mean height of forest are approximately 70%, 13.31 cm, and 6.36 m, respectively. And the major species were Lespedeza dahurica, Stipa bungeana in afforestation land and Achillea capillaries, S. bungeana in abandon land. In September 2013, based on land use history and using the line transects method, we randomly selected ten 20m 20 m plots in each of the afforested and abandoned lands as portions of the experiment sites. All sites had same physiographical conditions, same slope aspects, and same elevation (1250 m).
The identification of plant species was carried out in situ. Unidentified specimens were identified by plant taxonomists after being collected and dried with a plant press. Five 1 m 1 m quadrats were established in each plot (a total of 50 quadrats in the study area). Vegetation surveys of herbaceous plants in the plantation understory were done by tallying stem quantity and plant height for each species, and then coverage of each plant species was estimated visually. Coverage was taken as the average percentage of ground surface covered by the shadow of the foliage in each quadrats. Species richness is the number of species in each quadrat (Deng et al., 2014). The richness index (R), Shannon–Wiener diversity index (H) and evenness index (E) of the afforestation and
Fig. 2. Effect of afforestation and abandoned land on coverage (a), plant density (b), height (c), evenness index (d), richness index (e) and Shannon–Wiener diversity index (f) of the undergrowth vegetation. The box represents SE, the whisker shows min and max, the small square is the mean, and the horizontal line is the median. Significances between afforestation and abandoned are indicated by symbols: **P < 0.01, *P < 0.05; and ns, no significant difference.
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abandoned land communities were calculated using following equations: Richness index (R), Shannon–Wiener diversity index (H), evenness index (E): R¼S
H¼
S X ðPi Ln Pi Þ
(1)
(2)
i¼1
E¼
H Ln S
(3)
where S = total number of species in each community; H = Shannon–Wiener diversity index; Pi = density proportion of species “i”, and Ln = natural log. 2.4. Soil sampling Soil samples at 15 soil depths (0–10 cm, 10–0 cm, 20–30 cm, 30–40 cm, 40–50 cm, 50–60 cm, 60–70 cm, 70–80 cm, 80–90 cm, 90–100 cm, 100–120 cm, 120–140 cm, 140–160 cm, 160–180 cm and 180–200 cm) were collected using stainless steel cylinder with an inner diameter of 5 cm. After removing the litter layer, ten soil cores were collected in an “S” type pattern at each depth of every plot. Samples were collected at least 80 cm away from trees. All samples were sieved through a 2 mm screen to remove roots and other debris. Soil samples were air-dried and stored at room temperature for the determination of soil chemical properties. A ring tube (5 cm diameter) was used to determine the bulk density in each soil depth, where soil samples had been collected for
chemical analysis. SOC and TN storages were calculated as follows: 1d (4) 101 SOCDðTNDÞ ¼ C SOC;TN r H 100 where SOCD (TND) = SOC, (N) storage (Mg ha1); CSOC,TN = the content (g kg1) of SOC or TN; r = the bulk density (g cm3); H = the soil horizon thickness (cm), and d = the fraction (%) of gravels with size of >2 mm in soil. Because the soil gravel size of loess in China is mostly below 2 mm, this fraction was assumed to be 0 (Wang et al., 2010a,b). The soil total C, N and P concentrations (g kg1) were transformed to the unit of mmol kg1, and C:N, C:P and N:P ratios for each type were calculated as molar ratios (atomic ratio). 2.5. Laboratory analysis SOC content (g kg1), TN content (g kg1) and total soil phosphorus content were determined using the K2Cr2O7 oxidation method, Kjeldhal method and the Mo-Sb Antispectrophotography method, respectively (Bao, 2000). 2.6. Statistical analyses All statistical analyses were carried out with SPSS 17.0. Analysis of variance (ANOVA) and Duncan’s multiple range test (DMRT) at a 5% level of significance were used to compare the difference among community coverage, height, plant density, Richness index (R), Shannon–Wiener diversity (H), evenness index (E), ration of C:N (RCN), ration of C:P (RCP), ration of N:P (RNP), SOC, TN contents and storage among different land use types or soil depths. Pearson linear correlation coefficients analysis was used to estimate the relationships among the characteristics at each site.
Fig. 3. Effect of afforestation and abandoned land on vertical distributions of SOC (a), TN (b), TP (c), RCN (d), RCP (e) and RNP (f). The error bars are the standard errors. Significances between afforestation and abandoned are indicated by symbols: ***P < 0.001, **P < 0.01, *P < 0.05; ns, no significant difference.
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3. Results 3.1. Community coverage, height and plant density In this study, afforestation had a larger positive effect on plant communities compared to the abandoned land. It is shown that long-term afforestation had greater plant coverage (P < 0.01) and plant density (P < 0.05), which are higher than that of abandoned land community by 14.2% and 32.0%, respectively (Fig. 2a and b). However, there was no significant difference in plant heights between afforested and abandoned land communities (Fig. 2c). In addition, the afforested community had greater richness index (R) and Shannon–Wiener diversity (H) (P < 0.05) (Fig. 2e and f), and lower evenness index (E) (Fig. 2d) compared to the abandoned land community. 3.2. SOC, TN, and TP contents and Soil C:N:P stoichiometry Long-term afforestation had significant effects on SOC, TN and TP contents. afforested significantly increased SOC content in the depth of 0–30 cm than abandoned land (P < 0.001) and were significantly greater at depths of 0–100 cm and in the underlying soil depth (140 cm) for the afforested sites than the abandoned sites (P < 0.05) (Fig. 3a). Long-term afforested also significantly increased TN content in the depths of 10–40 cm (P < 0.001), 0–10 cm, 30–40 cm, 100–120 cm, 140–160 cm (P < 0.01), 40–50 cm, and 160–180 cm (P < 0.05) (Fig. 3b). Moreover, there was no significant difference in TP contents between afforested and abandoned land at a depth of 100–120 cm, although TP contents were significantly greater under afforestation at a depth of
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0–100 cm and in the underlying soil depth (140 cm; P < 0.05; Fig. 3c). Additionally, SOC and TN contents decreased sharply with increasing soil depth in both afforested and abandoned lands, while the changes in TP content were negligible (Fig. 3a–c). Afforested significantly increased RCN value at the depths of 0–30 cm (P < 0.001), 30–50 cm (P < 0.01), 60–80 cm, and 140– 180 cm (P < 0.05) compared to that of abandoned land (Fig. 3d). Averaged RCN value was higher for the afforested sites than the abandoned land sites by 12.78 in the depth of 0–50 cm. In addition, the RCP values of the afforested sites at depths of 0–30 cm, 40–70 cm, and 140–180 cm (25.72; P < 0.001), (12.49; P < 0.01), and (2.63; P < 0.05) were greater than those of the abandoned sites (Fig. 3e). The RNP value for afforested sites was 0.68 greater than that of the abandoned land for a the depth of 0–50 cm (P < 0.05), and significant differences in the underlying soil depth (50 cm) appeared in a few cases (Fig. 3f). Although the RNP values of the afforested sites were higher than that of abandoned land sites, they were lower than 14 in the entire soil profile. 3.3. Storage and sequestration of SOC and TN Long-term afforested sites had significant effects on SOC and TN storages at different soil depths (Fig. 4). Compared with abandoned land sites, the SOC storages in afforested sites at depths of 0–30 cm, 40–70 cm and 140–200 were significantly higher by an average of 8.10 Mg ha1 (P < 0.001), 3.43 Mg ha1 (P < 0.05) and 2.97 Mg ha1 (P < 0.05) (Fig. 4a), respectively. Soil TN storages were Similar to SOC storages, which were significantly greater by 0.39 Mg ha1, 0.16 Mg ha1 and 0.38 Mg ha1 in the depths of 0–30 cm (P < 0.001), 40–50 cm and 120–180 cm (P < 0.05) (Fig. 4b),
Fig. 4. Effect of afforestation and abandoned land on soil C storage (a), soil N storage (b), soil C sequestration (c) and soil N sequestration (d). The error bars are the standard errors. Significances between afforestation and abandoned are indicated by symbols: ***P < 0.001, **P < 0.01, *P < 0.05; ns, no significant difference.
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diversity dynamics after afforestation. Zhang et al. (2014) reported that total plant species number, density, and diversity in plantations increased in the first three years after plantation establishment, then stabilized or decreased in the next 1–2 years before increasing significantly over the following years. This is due to the microclimatic changes of limited light availability, severe competition for water and nutrients, and allelopathy decreasing the understory plant biodiversity in the 4–5 years after plantation establishment (Zhang et al., 2010, 2014). However, better development of soil organic layers with plantation age, increased deadfall, better light environment over time could effectively increase plant biodiversity (Bremer and Farley, 2010; Zhang et al., 2014). Similar results were also reported by Deng et al. (2014).
respectively. The SOC and TN sequestration values ranged from 0.30–8.32 and 0.10–0.27 Mg ha1, respectively (Fig. 4c and d). The highest SOC and TN sequestration values were 8.62 Mg ha1 and 0.54 Mg ha1, appeared at the depths of 0–10 cm and 100–120 cm, respectively. 3.4. Relationships among the characteristics of afforestation and abandoned land Significant correlations among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index, C:N, C:P, N:P, SOC storage and TN storage were observed in most cases of afforestation (P < 0.05) (Table 1), whereas significant correlations among them were observed in a few cases of abandoned land. Moreover, C:N, C:P were significant correlations among plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in afforestation land. And SOC storage was also significant correlations among plant diversity, evenness index, Shannon–Wiener diversity index, C:N, C:P, and N: P. However, there were no significant correlations between C:N, C: P, N:P, SOC storage and plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in abandoned land. This implies that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios.
4.2. Effect of long-term afforestation on SOC, TN, and TP contents and SOC, TN storage Long-term afforestation can greatly influence soil quality, as well as SOC and TN cycling (Paul et al., 2002; Grünzweig et al., 2007). Our results showed that afforestation significantly increased the contents of SOC and TN (Fig. 3a–c). The results essentially agreed with those in a previous study (i.e. Eaton et al., 2008; Fu et al., 2010) A possible explanation is that the lower residue input into the soil in abandoned land leads to lower SOC and TN contents. Afforestation is one of the major factors that affect SOC variation and global carbon balance (Lal, 2008). Our results indicated that afforestation increased SOC, TN storage more effectively than abandoned land (Fig. 4). The results were consistent with previous studies such as Murty et al. (2002). Zhou et al. (2011) also reported vegetation recovery reduced SOC and TN losses from wind erosion due to increasing plant cover and productivity. Also, increase organic matter input (litter, dead roots, mycorrhizae, and exudates) to the soil leads to an increase of SOC and TN through vegetation recovery (Prietzel and Bachmann, 2012). Recently it was reported that the depth of sampling is an important factor for the measurement of changes in SOC and TN stocks (VandenBygaart et al., 2010) and land use could influence subsoil C pools (Strahm et al., 2009). Our results found that SOC and TN stocks in afforestation were higher than that in abandoned
4. Discussion 4.1. Effect of long-term afforestation on community coverage, height and plant diversity Afforestation of agricultural or abandoned land represents a major change in land use (Zhang et al., 2014). Land use change has been recognized as the most important driver of changes in biodiversity in the current century (Brockerhoff et al., 2003). Our results showed that land undergoing long-term afforestation had greater plant coverage (P < 0.01), plant density (Fig. 2a and b), Richness index (R) and Shannon–Wiener diversity (H) (P < 0.05) compared to the abandoned land communities (Fig. 2e and f). This may be due to the changed microclimatic conditions, which become the driving factor of plant species composition and Table 1 Pearson linear correlation coefficients among the characteristics at each site. Ha
Cb
Rc
Pd
Afforestation C R P E SH C:N C:P N:P C storage N storage
0.45 0.60 0.09 0.71* 0.11 0.40 0.43 0.45 0.41 0.53
0.52 0.71* 0.33 0.58 0.72* 0.66* 0.57 0.34 0.72*
0.31 0.65* 0.38 0.78* 0.48 0.55 0.23 0.56
0.18 0.38 0.71* 0.76* 0.74* 0.78* 0.80*
Abandoned C R P E SH C:N C:P N:P C storage N storage
0.52 0.06 0.36 0.12 0.12 0.05 0.04 0.17 0.00 0.16
0.43 0.22 0.15 0.35 0.31 0.31 0.23 0.33 0.35
0.41 0.45 0.78** 0.06 0.01 0.17 0.06 0.12
0.47 0.08 0.06 0.17 0.30 0.16 0.28
Ee
SHf
C:N
C:P
N:P
C storage
0.32 0.66* 0.71* 0.26 0.88** 0.62
0.70* 0.80* 0.44 0.81* 0.76*
0.95** 0.91** 0.89** 0.77*
0.85** 0.93** 0.25
0.81** 0.30
0.56*
0.84** 0.05 0.18 0.54* 0.13 0.37
0.67* 0.10 0.30 0.05 0.25
0.88** 0.54* 0.54* 0.42
0.83** 0.75** 0.36
0.55* 0.31
0.38
Statistically significant values are indicated by symbols: **P < 0.01; *P < 0.05. a Height (cm); bcoverage (%); crichness index (R); dplant density (m2); eevenness index (E); fShannon–Wiener diversity (H).
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land at different soil profiles, especially in depths of 0–30 cm and 120–200 cm (Fig. 4). It is demonstrated that afforestation not only affects SOC and TN stocks in surface soil, but also largely influences that in deep soil. The results were consistent with Wang et al. (2010a,b) who reported that deep layer (50–200 cm) SOC stocks were equivalent to approximately 25% of that in the shallow layer (0–50) after vegetation recovery in LPR. It is mainly due to the fact that SOC input into subsoil is largely affected by plant roots and root exudates, dissolved organic matter, and bioturbation. In addition, one of the most important factors leading to protection of SOC in subsoil or deep soil may be the spatial separation of SOM, microorganisms and extracellular enzyme activity related to the heterogeneity of C input (Rumpel and Kögel-Knabner, 2011). 4.3. Effect of long-term afforestation on soil C:N:P stoichiometry Carbon, nitrogen and phosphorus are the three main elements that exist in relatively stable ratios in living organisms, and key characteristics of organisms and ecosystems are determined by dynamics of element ratios (Michaels 2003). C, N, and P stoichiometry in soils differ with plant communities and have high complexities (Zhang et al., 2014). Our results indicated that the C:N, C:P and N:P ratios were higher than that of abandoned land after long-term afforestation (Fig 3d–f). We speculate that vegetation covers and plant communities all affect the nutrient stoichiometry in soil. Li et al. (2012) reported that the different types of land use exhibited different soil C:N:P ratios due to differences in elevation, vegetation type and land management practices. Aponte et al. (2010) presented a Spanish dataset indicating that the average soil C:N:P ratio in forests is slightly greater than that in woodlands; this ratio varied based on the type of land use. Meanwhile, the influence of C, N, and P stoichiometry probably has two aspects. On one hand, plants change the C, N, and P ratios by absorbing or releasing them from or to soil (Zeng and Chen, 2005; He and Han, 2010). On the other hand, during greater litter decomposition processes, organic matter decomposition in afforestation releases nitrogen or phosphorus to soil which affects the soil’s C, N, and P ratios (Zhong et al., 2013). Our results also indicated that significant correlation were observed among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in most cases (Table 1) than abandoning the land, and had indicating soil C:N, C:P and N:P ratios could be a good indicator of soil nutrient status during soil development. Similar results were also reported by Tian et al. (2010). Although this study offered the most accurate estimation of understory plant diversity, soil C:N:P stoichiometry, SOC, TN storage in deep soil and their relationships after long-term afforestation in this region. To date, studies on the soil C, N, and P stoichiometry at different scales are lacking, and information about their influences on the global or regional scale are scarce, particularly in China. 5. Conclusion In this study, afforestation had a larger positive effect on plant community than abandoned land, and had significant affects on SOC, TN and TP contents. The average RCN value of the afforested sites was higher than the abandoned land sites by 12.78 in the depth of 0–50 cm. In addition, the RCP values of afforested sites higher than that of abandoned land sites in most of case. RNP values for afforested land was 0.68 greater than that of abandoned land at a depth of 0–50 cm (P < 0.05), and significant differences in the underlying soil depth (50 cm) appeared in a few cases. Compared with abandoned land, the SOC storages for afforested land at a depths of 140–200 cm were significantly higher by an average of
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2.97 Mg ha1; soil TN storages was significantly greater by 0.38 Mg ha1 in the depths at a of 120–180 cm, indicating that afforestation not only affects SOC and TN stocks in surface soils but also significantly influences them in deep soil. Significant correlations among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index, C:N, C:P, N:P implies that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios. Acknowledgments National Natural Science Foundation of China (No 41301601); Special Fund for forest-scientific Research in the Public Interest (201304312) financially supported this work. We are also grateful to the anonymous reviewers for their valuable comments. Reference Adams, M.L., Davis, M.R., Powell, K.J., 2001. Effects of grassland afforestation on exchangeable soil and soil solution aluminium. Aust. J. Soil Res. 39, 1003–1014. Amundson, R., 2001. The soil carbon budget in soils. Ann. Rev. Earth Planet. Sci. 29, 535–562. Aponte, C., Marañón, T., García, L., 2010. Microbial C, N and P in soils of Mediterranean oak forests: influence of season canopy cover and soil depth. Biogeochemistry 101, 77–92. Bai, Z.G., Dent, D., 2009. Recent land degradation and improvement in China. Ambio 38, 150–156. Bao, S.D., 2000. Soil and Agricultural Chemistry Analysis. China Agriculture Press, Beijing, China, pp. 46–57 (in Chinese). Bremer, L.L., Farley, K.A., 2010. Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness. Biodivers. Conserv. 19, 3893–3915. Brockerhoff, E.G., Ecroyd, C.E., Leckie, A.C., Kimberley, M.O., 2003. Diversity and succession of adventive and indigenous vascular understorey plants in Pinus radiata plantation forests in New Zealand. For. Ecol. Manage. 185, 307–326. Cleveland, C.C., Liptzin, D., 2007. C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass? Biogeochemistry 85, 235–252. Deng, L., Zhang, Z.N., Shangguan, Z.P., 2014. Long-term fencing effects on plant diversity and soil properties in China. Soil Till. Res. 13, 7–15. Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., Wisniewski, J., 1994. Carbon pools and flux of global forest ecosystems. Science 263, 185–190. Eaton, J.M., McGoff, N.M., Byme, K.A., Leahy, P., Kiely, G., 2008. Land cover change and soil organic carbon stocks in the Republic of Ireland 1851–2000. Climate Change 91, 317–334. Fu, B.J., Chen, L.D., Qiu, Y., Wang, J., Meng, Q.H., 2002. Land Use Structure and Ecological Processes in the Losses Hilly Area, China. Chinese Commercial Press, Beijing, pp. 1–50 (in Chinese). Fu, X.L., Shao, M.A., Wei, X.R., Robertm, H., 2010. Soil organic carbon and total nitrogen as affected by vegetation types in Northern Loess Plateau of China. Geoderma 155, 31–35. Gong, Z.T., Lei, W.J., Chen, Z.C., Gao, Y.X., Zeng, S.G., 1999. Chinese Soil Taxonomy. Science Press, Beijing, pp. 36–38. Grünzweig, J.M., Gelfand, I., Yakir, D., 2007. Biogeochemical factors contributing to enhanced carbon storage following afforestation of a semi-arid shrubland. Biogeosci. Discuss. 4, 2111–2145. Guo, L., Gifford, R.M., 2002. Soil carbon stock s and land use change: a meta analysis. Glob. Change Biol. 8, 345–360. Halpern, C.B., 1995. Plant species diversity in natural and managed forests of the pacific northwest. Ecol. Appl. 5, 913–934. Han, W.X., Fang, J.Y., Guo, D.L., Zhang, Y., 2005. Leaf nitrogen and phosphorus stoichiometry across 753 terrestrial plant species in China. New Phytol. 168, 377–385. He, J.S., Han, X.G., 2010. Ecological stoichiometry: searching for unifying principles from individuals to ecosystems. Chin. J. Plant Ecol. 31, 2–6 (in Chinese). Henderson, G.S., 1995. Soil organic matter: a link between forest management and productivity. In: Bigham, J.M., Bartels, J.M. (Eds.), Carbon Forms and Functions in Forest Soils. Soil Sci. Soc. Am. J, Madison, WI, pp. 419–435. Lal, R., 2008. Carbon sequestration. Phil. Trans. R. Soc. B 363, 815–830. Li, F.M., Song, Q.H., Jjemba, P.K., Shi, Y.C., 2004. Dynamics of soil microbial biomass C and soil fertility in cropland mulched with plastic film in a semiarid agroecosystem. Soil Biol. Biochem. 36, 1893–1902. Li, Y.J., Wu, S., Liu, J., Shen, D., Huang, Y., Su, W.W., Syers, J.K., 2012. Is the C:N:P stoichiometry in soil and soil microbial biomass related to the landscape and land use in southern subtropical China? Glob. Biogeochem. Cycles 26, 1–14. Liu, G., 1999. Soil conservation and sustainable agriculture on the Loess Plateau: challenges and prospective. Ambio 28, 663–668. Lü, Y., Fu, B., Feng, X., Zeng, Y., Chang, Liu Y., R.Y, Sun G., Wu, B.F., 2012. A policydriven large scale ecological restoration: quantifying ecosystem services changes in the Loess Plateau of China. PLoS ONE 7 (2), e31782. doi:http://dx.doi. org/10.1371/journal.pone0031782.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Soil stoichiometry and carbon storage in long-term afforestation soil affected by understory vegetation diversity Fazhu Zhao a,b , Di Kang a,b , Xinhui Han a,b, *, Gaihe Yang a,b, **, Gaihe Yang a,b , Yongzhong Feng a,b , Guangxin Ren a,b a b
College of Agronomy, Northwest A&F University, Yangling, 712100 Shaanxi, China The Research Center of Recycle Agricultural Engineering and Technology of Shaanxi Province, Yangling, 712100 Shaanxi, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 March 2014 Received in revised form 23 October 2014 Accepted 9 November 2014 Available online xxx
The afforestation of abandoned land could offer opportunities to sequester soil organic carbon (SOC), promote nutriment elements cycling, improve plant diversity in the plantation understory and provide ecosystem services. The objectives of this study were to identify plant diversity in the plantation understory, quantify the changes in SOC and total nitrogen (TN) storage in deep soil, assess the SOC, TN, and total phosphorus (TP) stoichiometries, and investigate their relationships in the Loess Plateau Region (LPR) undergoing long-term afforestation. Soil samples were collected at a soil depth of 0–200 cm under 30-yr old Robinia pseudoacacia L. and adjacent abandoned sites, and SOC, TN and TP were determined in different soil depth. Additionally, plant composition and diversity in the plantation understory were evaluated. The results showed that land subjected to long-term afforestation had greater plant coverage, plant density, richness index (R) and Shannon–Wiener diversity (H) compared to abandoned land communities (P < 0.05). SOC, TN and TP contents in afforested sites were significantly increased in surface soil (0–30 cm) as well as in the underlying soil (100 cm) compared to the corresponding abandoned land sites (P < 0.05) in most cases. Meanwhile, SOC, TN, and TP stoichiometry in afforested areas were higher than those of abandoned lands and significantly related to understory vegetation diversity (P < 0.05). In addition, lands subjected to long-term afforestation effectively increased SOC and TN storages compared to abandoned land at soil depths of 0–30 cm and 100–200 cm and were also significantly related to understory vegetation diversity (P < 0.05). These findings demonstrating that afforestation not only affects SOC and TN stocks in surface soil, but also strongly influences that in deep soil. And it is also indicating that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Long-term afforestation C:N:P stoichiometry Carbon storage Deep soil Loess Plateau
1. Introduction Afforestation areas, which occupy approximately 0.20 billion ha worldwide, could effectively help mitigate increasing atmospheric CO2 (Zhong et al., 2013). Afforestation also affects the establishment of natural vegetation and understorey ecology via resource competition (i.e. Bremer and Farley, 2010), allelopathy (i.e. Zhang et al., 2010). In addition, plant diversity in the plantation understory plays important roles in improving stimulating the soil nutrient cycling and maintaining soil quality after afforestation (Halpern 1995; Wang et al., 2011). Brockerhoff et al. (2003)
* Corresponding author. Tel.: +86 02987092265. ** Corresponding author. E-mail addresses: [email protected] (F. Zhao), [email protected] (D. Kang), [email protected] (X. Han), [email protected] (G. Yang), [email protected] (Y. Feng), [email protected] (G. Ren). http://dx.doi.org/10.1016/j.ecoleng.2014.11.010 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
reported that wider tree spacing during plantation establishment supports better maintenance of understorey vegetation. Zhang et al. (2014) found that the first four years after the establishment of afforestation are associated with lower plant diversity, however, plant diversity was improved after long-term afforestation. Meanwhile, understanding plant diversity in the plantation understory is critical to investigate the ecological functions of plantations and improve their management, especially after long-term afforestation. Following afforestation, changes inevitably occur in soil physical and chemical elements, particularly in the three main elements: carbon, nitrogen and phosphorus (Adams et al., 2001; Wei et al., 2009). Many studies (i.e. Walker and Adams, 1958; Post et al., 1982; Melillo et al., 2003; Zhong et al., 2013) have indicated that soil carbon, nitrogen, and phosphorus are often closely related. Cleveland and Liptzin (2007) found a well constrained C:N:P ratio of microbial biomass in 0–10 cm
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organic-rich soil globally. Tian et al. (2010) documented that soil C:N, C:P, and N:P ratios in organic-rich topsoil could be good indicators of soil nutrient status during soil development. Zhang et al. (2014) also demonstrated that vegetation cover, plant communities, geomorphology, and seawall all affected C, N, and P stoichiometry in soils. Despite progress made in terrestrial ecosystem restoration methods and analyses (Han et al., 2005; Manzoni et al., 2010) the stoichiometrical characteristics of C, N, and P in soils have yet to be fully described (Swift et al., 1998; Manzoni and Porporato, 2009), especially in the Loess Plateau Region (LPR) of China. Approximately 75% of total terrestrial C is stored in the world’s soils (Henderson 1995), and forest soils hold approximately 40% of C belowground (Dixon et al., 1994). Therefore, even if afforestation only slightly affects soil C stocks at the local level, it could have a significant effect on the global C budget (Paul et al., 2002). Sean et al. (2012) illustrated that changes in soil organic carbon (SOC) due to afforestation are negatively related to mean annual precipitation and positively correlated with plantation age. Nave et al. (2012) also demonstrated that afforestation has significant positive effects on SOC sequestration in the United States, although these effects require decades to manifest and primarily occur in the uppermost portion of the mineral soil profile. Thus, long-term afforestation is a better management option for increasing terrestrial C sequestration. It has been reported that more than 50% of total SOC is stored in the subsoil (at a depth below 50 cm) (Amundson, 2001), and at least 61% of the total soil C is stored below a depth of 30 cm depth in the northern circumpolar permafrost region (Guo and Gifford, 2002). In recent decades, many studies have illustrated that subsoil C may be even more important than topsoil C as a source or sink for CO2 than topsoil C (VandenBygaart et al., 2010; Rumpel and KögelKnabner, 2011). Thus, considering the potential role of SOC as an atmospheric CO2 sink it is important to understand whether the long-term afforestation affects large amounts of SOC in the subsoil or deep soil. However, the C and N storages in deep soil layers are not fully understood in LPR of China to date. The LPR is an important geological region that influences the global carbon cycle (Wang et al., 2010a,b). Vegetation coverage in the LPR is relatively low due to its harsh environment (Wei et al., 2012). Soil erosion and desertification in LPR reduced net primary productivity by 12 kg C ha1 year1 (Bai and Dent, 2009). Since the 1950s, the Chinese government has made great efforts to control soil erosion and restore ecosystems (Fu et al., 2002). More than 9.27 million ha of abandoned farmland (investment of more than 28.8 billion USD and with the involvement of 0.12 billion farmers) have been afforested in this region through the “Grain to Green Program” (GTGP), which has implemented large-scale ecological rehabilitation since 1999 (Lü et al., 2012). However, few studies have reported the plant diversity, soil C:N:P stoichiometry, carbon storage in deep soil and their relationships after long-term afforestation in this region until recently. Thus, this study aimed to: (a) analyze the plant diversity in the understory after long-term afforestation; (b) illustrate the soil C:N:P stoichiometry and assess SOC and N storage in different soil depths; and (c) evaluate the relationship between plant diversity, soil C:N:P stoichiometry, and soil C and N storages after long-term afforestation.
2. Methods and materials 2.1. Research area The study was conducted in the Wuliwan catchment (36 460 4200 –36 460 2800 N, 109130 4600 –109 160 0300 E), which is located in Ansai county in the central region of LPR (see Fig. 1). Ansai is a
Fig. 1. Location of the Loess Plateau and the study site.
typical county characterized by a semi-arid climate and a hilly loess landscape in the Loess Plateau. it has an annual average temperature of 8.8 C, and an average annual precipitation of 505 mm. 60% of the precipitation which occurs between July and September (300 mm in dry years while >700 mm in wet years). Accumulated temperatures above 0 C and 10 C are 3733 C and 3283 C, respectively. On average, there are approximately 157 frost-free days and 2415 h of sunshine each year. Arable farming mostly occurs on sloping lands without irrigation. The loess parent material at the site has an average thickness of approximately 50–80 m and the soil in this region is Calciustepts soil (Gong et al., 1999). Sand (2–0.05 mm) and silt (0.05–0.002 mm) account for approximately 29.22% and 63.56% of the material at a soil depth of 0–20 cm, respectively. The soil is highly erodible, with an erosion modulus of 10,000–12,000 Mg km2 year1 before the restoration efforts began in this region (Liu, 1999). After 30 years of vegetation restoration the area of forested lands, the area of the area of forestland increased significantly from 5% to 40% (Xue et al., 2009). The Wuliwan catchment is one of the experimental sites of the Institute of Soil and Water Conservation, Chinese Academy of Science (CAS). The major agricultural land use type in the LPR is slope cropland. Agricultural management in this region, including the major crop types grown, has not been changed significantly since the 1970s. After more than 30 years of comprehensive management, the ecological environment of the catchment has been significantly improved (Zhang et al., 2007). Beginning in late 1970s, slope cropland was replanted with forest, mainly Robinia pseudoacacia L. to control soil erosion. Abandoned cropland was also generated during this period due to its extremely low productivity and long distance from farmers’ residences (Li et al., 2004). Despite that wild grasslands and shrub lands were usually found on steep slopes, they were often used for firewood collection resulting in reduced vegetation to barrenness for long periods (80 year).
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2.2. Experimental design
2.3. Plant species identification and species diversity index
The main forest plantation in the Wuliwan catchment is R. psendoacacia (30-yr-old). The canopy closure, mean diameter at breast height (DBH), and mean height of forest are approximately 70%, 13.31 cm, and 6.36 m, respectively. And the major species were Lespedeza dahurica, Stipa bungeana in afforestation land and Achillea capillaries, S. bungeana in abandon land. In September 2013, based on land use history and using the line transects method, we randomly selected ten 20m 20 m plots in each of the afforested and abandoned lands as portions of the experiment sites. All sites had same physiographical conditions, same slope aspects, and same elevation (1250 m).
The identification of plant species was carried out in situ. Unidentified specimens were identified by plant taxonomists after being collected and dried with a plant press. Five 1 m 1 m quadrats were established in each plot (a total of 50 quadrats in the study area). Vegetation surveys of herbaceous plants in the plantation understory were done by tallying stem quantity and plant height for each species, and then coverage of each plant species was estimated visually. Coverage was taken as the average percentage of ground surface covered by the shadow of the foliage in each quadrats. Species richness is the number of species in each quadrat (Deng et al., 2014). The richness index (R), Shannon–Wiener diversity index (H) and evenness index (E) of the afforestation and
Fig. 2. Effect of afforestation and abandoned land on coverage (a), plant density (b), height (c), evenness index (d), richness index (e) and Shannon–Wiener diversity index (f) of the undergrowth vegetation. The box represents SE, the whisker shows min and max, the small square is the mean, and the horizontal line is the median. Significances between afforestation and abandoned are indicated by symbols: **P < 0.01, *P < 0.05; and ns, no significant difference.
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abandoned land communities were calculated using following equations: Richness index (R), Shannon–Wiener diversity index (H), evenness index (E): R¼S
H¼
S X ðPi Ln Pi Þ
(1)
(2)
i¼1
E¼
H Ln S
(3)
where S = total number of species in each community; H = Shannon–Wiener diversity index; Pi = density proportion of species “i”, and Ln = natural log. 2.4. Soil sampling Soil samples at 15 soil depths (0–10 cm, 10–0 cm, 20–30 cm, 30–40 cm, 40–50 cm, 50–60 cm, 60–70 cm, 70–80 cm, 80–90 cm, 90–100 cm, 100–120 cm, 120–140 cm, 140–160 cm, 160–180 cm and 180–200 cm) were collected using stainless steel cylinder with an inner diameter of 5 cm. After removing the litter layer, ten soil cores were collected in an “S” type pattern at each depth of every plot. Samples were collected at least 80 cm away from trees. All samples were sieved through a 2 mm screen to remove roots and other debris. Soil samples were air-dried and stored at room temperature for the determination of soil chemical properties. A ring tube (5 cm diameter) was used to determine the bulk density in each soil depth, where soil samples had been collected for
chemical analysis. SOC and TN storages were calculated as follows: 1d (4) 101 SOCDðTNDÞ ¼ C SOC;TN r H 100 where SOCD (TND) = SOC, (N) storage (Mg ha1); CSOC,TN = the content (g kg1) of SOC or TN; r = the bulk density (g cm3); H = the soil horizon thickness (cm), and d = the fraction (%) of gravels with size of >2 mm in soil. Because the soil gravel size of loess in China is mostly below 2 mm, this fraction was assumed to be 0 (Wang et al., 2010a,b). The soil total C, N and P concentrations (g kg1) were transformed to the unit of mmol kg1, and C:N, C:P and N:P ratios for each type were calculated as molar ratios (atomic ratio). 2.5. Laboratory analysis SOC content (g kg1), TN content (g kg1) and total soil phosphorus content were determined using the K2Cr2O7 oxidation method, Kjeldhal method and the Mo-Sb Antispectrophotography method, respectively (Bao, 2000). 2.6. Statistical analyses All statistical analyses were carried out with SPSS 17.0. Analysis of variance (ANOVA) and Duncan’s multiple range test (DMRT) at a 5% level of significance were used to compare the difference among community coverage, height, plant density, Richness index (R), Shannon–Wiener diversity (H), evenness index (E), ration of C:N (RCN), ration of C:P (RCP), ration of N:P (RNP), SOC, TN contents and storage among different land use types or soil depths. Pearson linear correlation coefficients analysis was used to estimate the relationships among the characteristics at each site.
Fig. 3. Effect of afforestation and abandoned land on vertical distributions of SOC (a), TN (b), TP (c), RCN (d), RCP (e) and RNP (f). The error bars are the standard errors. Significances between afforestation and abandoned are indicated by symbols: ***P < 0.001, **P < 0.01, *P < 0.05; ns, no significant difference.
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3. Results 3.1. Community coverage, height and plant density In this study, afforestation had a larger positive effect on plant communities compared to the abandoned land. It is shown that long-term afforestation had greater plant coverage (P < 0.01) and plant density (P < 0.05), which are higher than that of abandoned land community by 14.2% and 32.0%, respectively (Fig. 2a and b). However, there was no significant difference in plant heights between afforested and abandoned land communities (Fig. 2c). In addition, the afforested community had greater richness index (R) and Shannon–Wiener diversity (H) (P < 0.05) (Fig. 2e and f), and lower evenness index (E) (Fig. 2d) compared to the abandoned land community. 3.2. SOC, TN, and TP contents and Soil C:N:P stoichiometry Long-term afforestation had significant effects on SOC, TN and TP contents. afforested significantly increased SOC content in the depth of 0–30 cm than abandoned land (P < 0.001) and were significantly greater at depths of 0–100 cm and in the underlying soil depth (140 cm) for the afforested sites than the abandoned sites (P < 0.05) (Fig. 3a). Long-term afforested also significantly increased TN content in the depths of 10–40 cm (P < 0.001), 0–10 cm, 30–40 cm, 100–120 cm, 140–160 cm (P < 0.01), 40–50 cm, and 160–180 cm (P < 0.05) (Fig. 3b). Moreover, there was no significant difference in TP contents between afforested and abandoned land at a depth of 100–120 cm, although TP contents were significantly greater under afforestation at a depth of
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0–100 cm and in the underlying soil depth (140 cm; P < 0.05; Fig. 3c). Additionally, SOC and TN contents decreased sharply with increasing soil depth in both afforested and abandoned lands, while the changes in TP content were negligible (Fig. 3a–c). Afforested significantly increased RCN value at the depths of 0–30 cm (P < 0.001), 30–50 cm (P < 0.01), 60–80 cm, and 140– 180 cm (P < 0.05) compared to that of abandoned land (Fig. 3d). Averaged RCN value was higher for the afforested sites than the abandoned land sites by 12.78 in the depth of 0–50 cm. In addition, the RCP values of the afforested sites at depths of 0–30 cm, 40–70 cm, and 140–180 cm (25.72; P < 0.001), (12.49; P < 0.01), and (2.63; P < 0.05) were greater than those of the abandoned sites (Fig. 3e). The RNP value for afforested sites was 0.68 greater than that of the abandoned land for a the depth of 0–50 cm (P < 0.05), and significant differences in the underlying soil depth (50 cm) appeared in a few cases (Fig. 3f). Although the RNP values of the afforested sites were higher than that of abandoned land sites, they were lower than 14 in the entire soil profile. 3.3. Storage and sequestration of SOC and TN Long-term afforested sites had significant effects on SOC and TN storages at different soil depths (Fig. 4). Compared with abandoned land sites, the SOC storages in afforested sites at depths of 0–30 cm, 40–70 cm and 140–200 were significantly higher by an average of 8.10 Mg ha1 (P < 0.001), 3.43 Mg ha1 (P < 0.05) and 2.97 Mg ha1 (P < 0.05) (Fig. 4a), respectively. Soil TN storages were Similar to SOC storages, which were significantly greater by 0.39 Mg ha1, 0.16 Mg ha1 and 0.38 Mg ha1 in the depths of 0–30 cm (P < 0.001), 40–50 cm and 120–180 cm (P < 0.05) (Fig. 4b),
Fig. 4. Effect of afforestation and abandoned land on soil C storage (a), soil N storage (b), soil C sequestration (c) and soil N sequestration (d). The error bars are the standard errors. Significances between afforestation and abandoned are indicated by symbols: ***P < 0.001, **P < 0.01, *P < 0.05; ns, no significant difference.
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diversity dynamics after afforestation. Zhang et al. (2014) reported that total plant species number, density, and diversity in plantations increased in the first three years after plantation establishment, then stabilized or decreased in the next 1–2 years before increasing significantly over the following years. This is due to the microclimatic changes of limited light availability, severe competition for water and nutrients, and allelopathy decreasing the understory plant biodiversity in the 4–5 years after plantation establishment (Zhang et al., 2010, 2014). However, better development of soil organic layers with plantation age, increased deadfall, better light environment over time could effectively increase plant biodiversity (Bremer and Farley, 2010; Zhang et al., 2014). Similar results were also reported by Deng et al. (2014).
respectively. The SOC and TN sequestration values ranged from 0.30–8.32 and 0.10–0.27 Mg ha1, respectively (Fig. 4c and d). The highest SOC and TN sequestration values were 8.62 Mg ha1 and 0.54 Mg ha1, appeared at the depths of 0–10 cm and 100–120 cm, respectively. 3.4. Relationships among the characteristics of afforestation and abandoned land Significant correlations among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index, C:N, C:P, N:P, SOC storage and TN storage were observed in most cases of afforestation (P < 0.05) (Table 1), whereas significant correlations among them were observed in a few cases of abandoned land. Moreover, C:N, C:P were significant correlations among plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in afforestation land. And SOC storage was also significant correlations among plant diversity, evenness index, Shannon–Wiener diversity index, C:N, C:P, and N: P. However, there were no significant correlations between C:N, C: P, N:P, SOC storage and plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in abandoned land. This implies that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios.
4.2. Effect of long-term afforestation on SOC, TN, and TP contents and SOC, TN storage Long-term afforestation can greatly influence soil quality, as well as SOC and TN cycling (Paul et al., 2002; Grünzweig et al., 2007). Our results showed that afforestation significantly increased the contents of SOC and TN (Fig. 3a–c). The results essentially agreed with those in a previous study (i.e. Eaton et al., 2008; Fu et al., 2010) A possible explanation is that the lower residue input into the soil in abandoned land leads to lower SOC and TN contents. Afforestation is one of the major factors that affect SOC variation and global carbon balance (Lal, 2008). Our results indicated that afforestation increased SOC, TN storage more effectively than abandoned land (Fig. 4). The results were consistent with previous studies such as Murty et al. (2002). Zhou et al. (2011) also reported vegetation recovery reduced SOC and TN losses from wind erosion due to increasing plant cover and productivity. Also, increase organic matter input (litter, dead roots, mycorrhizae, and exudates) to the soil leads to an increase of SOC and TN through vegetation recovery (Prietzel and Bachmann, 2012). Recently it was reported that the depth of sampling is an important factor for the measurement of changes in SOC and TN stocks (VandenBygaart et al., 2010) and land use could influence subsoil C pools (Strahm et al., 2009). Our results found that SOC and TN stocks in afforestation were higher than that in abandoned
4. Discussion 4.1. Effect of long-term afforestation on community coverage, height and plant diversity Afforestation of agricultural or abandoned land represents a major change in land use (Zhang et al., 2014). Land use change has been recognized as the most important driver of changes in biodiversity in the current century (Brockerhoff et al., 2003). Our results showed that land undergoing long-term afforestation had greater plant coverage (P < 0.01), plant density (Fig. 2a and b), Richness index (R) and Shannon–Wiener diversity (H) (P < 0.05) compared to the abandoned land communities (Fig. 2e and f). This may be due to the changed microclimatic conditions, which become the driving factor of plant species composition and Table 1 Pearson linear correlation coefficients among the characteristics at each site. Ha
Cb
Rc
Pd
Afforestation C R P E SH C:N C:P N:P C storage N storage
0.45 0.60 0.09 0.71* 0.11 0.40 0.43 0.45 0.41 0.53
0.52 0.71* 0.33 0.58 0.72* 0.66* 0.57 0.34 0.72*
0.31 0.65* 0.38 0.78* 0.48 0.55 0.23 0.56
0.18 0.38 0.71* 0.76* 0.74* 0.78* 0.80*
Abandoned C R P E SH C:N C:P N:P C storage N storage
0.52 0.06 0.36 0.12 0.12 0.05 0.04 0.17 0.00 0.16
0.43 0.22 0.15 0.35 0.31 0.31 0.23 0.33 0.35
0.41 0.45 0.78** 0.06 0.01 0.17 0.06 0.12
0.47 0.08 0.06 0.17 0.30 0.16 0.28
Ee
SHf
C:N
C:P
N:P
C storage
0.32 0.66* 0.71* 0.26 0.88** 0.62
0.70* 0.80* 0.44 0.81* 0.76*
0.95** 0.91** 0.89** 0.77*
0.85** 0.93** 0.25
0.81** 0.30
0.56*
0.84** 0.05 0.18 0.54* 0.13 0.37
0.67* 0.10 0.30 0.05 0.25
0.88** 0.54* 0.54* 0.42
0.83** 0.75** 0.36
0.55* 0.31
0.38
Statistically significant values are indicated by symbols: **P < 0.01; *P < 0.05. a Height (cm); bcoverage (%); crichness index (R); dplant density (m2); eevenness index (E); fShannon–Wiener diversity (H).
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land at different soil profiles, especially in depths of 0–30 cm and 120–200 cm (Fig. 4). It is demonstrated that afforestation not only affects SOC and TN stocks in surface soil, but also largely influences that in deep soil. The results were consistent with Wang et al. (2010a,b) who reported that deep layer (50–200 cm) SOC stocks were equivalent to approximately 25% of that in the shallow layer (0–50) after vegetation recovery in LPR. It is mainly due to the fact that SOC input into subsoil is largely affected by plant roots and root exudates, dissolved organic matter, and bioturbation. In addition, one of the most important factors leading to protection of SOC in subsoil or deep soil may be the spatial separation of SOM, microorganisms and extracellular enzyme activity related to the heterogeneity of C input (Rumpel and Kögel-Knabner, 2011). 4.3. Effect of long-term afforestation on soil C:N:P stoichiometry Carbon, nitrogen and phosphorus are the three main elements that exist in relatively stable ratios in living organisms, and key characteristics of organisms and ecosystems are determined by dynamics of element ratios (Michaels 2003). C, N, and P stoichiometry in soils differ with plant communities and have high complexities (Zhang et al., 2014). Our results indicated that the C:N, C:P and N:P ratios were higher than that of abandoned land after long-term afforestation (Fig 3d–f). We speculate that vegetation covers and plant communities all affect the nutrient stoichiometry in soil. Li et al. (2012) reported that the different types of land use exhibited different soil C:N:P ratios due to differences in elevation, vegetation type and land management practices. Aponte et al. (2010) presented a Spanish dataset indicating that the average soil C:N:P ratio in forests is slightly greater than that in woodlands; this ratio varied based on the type of land use. Meanwhile, the influence of C, N, and P stoichiometry probably has two aspects. On one hand, plants change the C, N, and P ratios by absorbing or releasing them from or to soil (Zeng and Chen, 2005; He and Han, 2010). On the other hand, during greater litter decomposition processes, organic matter decomposition in afforestation releases nitrogen or phosphorus to soil which affects the soil’s C, N, and P ratios (Zhong et al., 2013). Our results also indicated that significant correlation were observed among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index in most cases (Table 1) than abandoning the land, and had indicating soil C:N, C:P and N:P ratios could be a good indicator of soil nutrient status during soil development. Similar results were also reported by Tian et al. (2010). Although this study offered the most accurate estimation of understory plant diversity, soil C:N:P stoichiometry, SOC, TN storage in deep soil and their relationships after long-term afforestation in this region. To date, studies on the soil C, N, and P stoichiometry at different scales are lacking, and information about their influences on the global or regional scale are scarce, particularly in China. 5. Conclusion In this study, afforestation had a larger positive effect on plant community than abandoned land, and had significant affects on SOC, TN and TP contents. The average RCN value of the afforested sites was higher than the abandoned land sites by 12.78 in the depth of 0–50 cm. In addition, the RCP values of afforested sites higher than that of abandoned land sites in most of case. RNP values for afforested land was 0.68 greater than that of abandoned land at a depth of 0–50 cm (P < 0.05), and significant differences in the underlying soil depth (50 cm) appeared in a few cases. Compared with abandoned land, the SOC storages for afforested land at a depths of 140–200 cm were significantly higher by an average of
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2.97 Mg ha1; soil TN storages was significantly greater by 0.38 Mg ha1 in the depths at a of 120–180 cm, indicating that afforestation not only affects SOC and TN stocks in surface soils but also significantly influences them in deep soil. Significant correlations among height, plant diversity, coverage, richness index, evenness index, Shannon–Wiener diversity index, C:N, C:P, N:P implies that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios. Acknowledgments National Natural Science Foundation of China (No 41301601); Special Fund for forest-scientific Research in the Public Interest (201304312) financially supported this work. We are also grateful to the anonymous reviewers for their valuable comments. Reference Adams, M.L., Davis, M.R., Powell, K.J., 2001. Effects of grassland afforestation on exchangeable soil and soil solution aluminium. Aust. J. Soil Res. 39, 1003–1014. Amundson, R., 2001. The soil carbon budget in soils. Ann. Rev. Earth Planet. Sci. 29, 535–562. Aponte, C., Marañón, T., García, L., 2010. Microbial C, N and P in soils of Mediterranean oak forests: influence of season canopy cover and soil depth. Biogeochemistry 101, 77–92. Bai, Z.G., Dent, D., 2009. Recent land degradation and improvement in China. Ambio 38, 150–156. Bao, S.D., 2000. Soil and Agricultural Chemistry Analysis. China Agriculture Press, Beijing, China, pp. 46–57 (in Chinese). Bremer, L.L., Farley, K.A., 2010. Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness. Biodivers. Conserv. 19, 3893–3915. Brockerhoff, E.G., Ecroyd, C.E., Leckie, A.C., Kimberley, M.O., 2003. Diversity and succession of adventive and indigenous vascular understorey plants in Pinus radiata plantation forests in New Zealand. For. Ecol. Manage. 185, 307–326. Cleveland, C.C., Liptzin, D., 2007. C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass? Biogeochemistry 85, 235–252. Deng, L., Zhang, Z.N., Shangguan, Z.P., 2014. Long-term fencing effects on plant diversity and soil properties in China. Soil Till. Res. 13, 7–15. Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., Wisniewski, J., 1994. Carbon pools and flux of global forest ecosystems. Science 263, 185–190. Eaton, J.M., McGoff, N.M., Byme, K.A., Leahy, P., Kiely, G., 2008. Land cover change and soil organic carbon stocks in the Republic of Ireland 1851–2000. Climate Change 91, 317–334. Fu, B.J., Chen, L.D., Qiu, Y., Wang, J., Meng, Q.H., 2002. Land Use Structure and Ecological Processes in the Losses Hilly Area, China. Chinese Commercial Press, Beijing, pp. 1–50 (in Chinese). Fu, X.L., Shao, M.A., Wei, X.R., Robertm, H., 2010. Soil organic carbon and total nitrogen as affected by vegetation types in Northern Loess Plateau of China. Geoderma 155, 31–35. Gong, Z.T., Lei, W.J., Chen, Z.C., Gao, Y.X., Zeng, S.G., 1999. Chinese Soil Taxonomy. Science Press, Beijing, pp. 36–38. Grünzweig, J.M., Gelfand, I., Yakir, D., 2007. Biogeochemical factors contributing to enhanced carbon storage following afforestation of a semi-arid shrubland. Biogeosci. Discuss. 4, 2111–2145. Guo, L., Gifford, R.M., 2002. Soil carbon stock s and land use change: a meta analysis. Glob. Change Biol. 8, 345–360. Halpern, C.B., 1995. Plant species diversity in natural and managed forests of the pacific northwest. Ecol. Appl. 5, 913–934. Han, W.X., Fang, J.Y., Guo, D.L., Zhang, Y., 2005. Leaf nitrogen and phosphorus stoichiometry across 753 terrestrial plant species in China. New Phytol. 168, 377–385. He, J.S., Han, X.G., 2010. Ecological stoichiometry: searching for unifying principles from individuals to ecosystems. Chin. J. Plant Ecol. 31, 2–6 (in Chinese). Henderson, G.S., 1995. Soil organic matter: a link between forest management and productivity. In: Bigham, J.M., Bartels, J.M. (Eds.), Carbon Forms and Functions in Forest Soils. Soil Sci. Soc. Am. J, Madison, WI, pp. 419–435. Lal, R., 2008. Carbon sequestration. Phil. Trans. R. Soc. B 363, 815–830. Li, F.M., Song, Q.H., Jjemba, P.K., Shi, Y.C., 2004. Dynamics of soil microbial biomass C and soil fertility in cropland mulched with plastic film in a semiarid agroecosystem. Soil Biol. Biochem. 36, 1893–1902. Li, Y.J., Wu, S., Liu, J., Shen, D., Huang, Y., Su, W.W., Syers, J.K., 2012. Is the C:N:P stoichiometry in soil and soil microbial biomass related to the landscape and land use in southern subtropical China? Glob. Biogeochem. Cycles 26, 1–14. Liu, G., 1999. Soil conservation and sustainable agriculture on the Loess Plateau: challenges and prospective. Ambio 28, 663–668. Lü, Y., Fu, B., Feng, X., Zeng, Y., Chang, Liu Y., R.Y, Sun G., Wu, B.F., 2012. A policydriven large scale ecological restoration: quantifying ecosystem services changes in the Loess Plateau of China. PLoS ONE 7 (2), e31782. doi:http://dx.doi. org/10.1371/journal.pone0031782.
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