Biomass and Carbon Sequestration by Juglans regia Plantations in the Karst Regions of Southwest China

To better understand carbon (C) sequestration by Juglans regia L. plantations in karst regions of southwest China, this study examined biomass increment and C storage in four different-aged J. regia stands, as well as the distribution of carbon stock among the various ecosystem components. Tree and ecosystem biomass increased with stand age. Aboveground biomass (AGB) represented 64.79% of the total biomass, belowground tree biomass comprised 22.73%, and shrubs and herbs totaled 11.38%, whereas only a small amount (1.11%) was associated with soil litter. Soil organic C (SOC) content in the top soil of plantations aged 1, 5, 9, and 13 years was 33.37, 53.15, 33.56, and 49.78 Mg ha−1, respectively. SOC content decreased continually with increasing soil depth. Ecosystem C storage amounted to 33.49, 54.21, 46.40, and 65.34 Mg ha−1 for the 1-, 5-, 9-, and 13-year old plantations, respectively, with most (86.55%) of the ecosystem C being in the soil. Our results suggest that large-scale planting of J. regia has potential for not only vegetation restoration but also high C fixation capacity in the karst region of southwest China.


Introduction
Land use change is often considered to be an important factor influencing the global carbon (C) cycle because it changes the rate of soil erosion and vegetation biomass [1,2].Over the past 200 years, the conversion of forest and grassland to cropland has contributed to elevated atmospheric CO 2 concentrations and a warming global climate [3][4][5].To improve land quality and prevent further soil erosion, the government of the People's Republic of China implemented the nationwide Grain for Green Program (GGP) to convert cropland to grassland and forest in 1999 [6].In practice, as the most ambitious ecosystem preservation program in China, the GGP has helped return steep farmlands to forests and grass, which greatly improved ecological quality in mountainous areas.The Department of Chinese Forestry has reported that 23,344,000 ha of lands, including 15,076,000 ha of barren lands and 8,268,000 ha of croplands, has been afforested from 1999 to 2010 [7].In addition, the GGP also contributed to increased accumulation of C in both vegetation and soil in degraded ecosystems, which has received increasing attention from researchers [8][9][10].
As one of the key national zones of the GGP, karst regions (covering 5.5 × 10 5 km 2 ) in Southwest (SW) China are considered fragile because of their geochemical properties, low environmental carrying capacity, and sensitivity to disturbance [11].Moreover, karst regions of SW China have extensive areas of steep, eroded barren land and degraded cropland.Most of the farmland is on slopes ≥6 • , whereas 20% of the total farmland area is on slopes ≥25 • [12,13].In the last 50 years, forests in karst regions have been negatively affected by fast-growing populations, excessive logging, and steep-slope farming [14].Under the GGP, tree plantations were widely used to restore degraded ecosystems and maintain C balance [15,16].For instance, by the end of 2011, 3.3 × 10 6 ha of land had been planted in SW China under the GGP, which accounted for 4.97% of total GGP lands [7].Therefore, the implementation of GGP in the karst area may play a significant role in attempts to develop extensive new forests and ultimately reduce atmospheric CO 2 over the long term [11].
Juglans regia L. belongs to the family Juglandaceae and is cultivated extensively for nut production.It is widely disseminated in Asia, North and South America, Europe, South Africa, Australia, and New Zealand [17].The nuts are a rich source of polyunsaturated fats, and the leaf, bark, and kernel tissues have a rich diversity of phenolics, which play unique roles in the human diet and the biology of the tree [18].Under the GGP, multi-million hectare plantations of J. regia have been planted to reverse desertification in the karst region because of its drought tolerance and ability to grow rapidly under conditions of calcerous soils and in poor habitats [8].The fruit of the J. regia tree can be an important source of income for farmers in impoverished karst regions of SW China [19].Because of their economic and ecological benefits, J. regia plantations in Southwest China currently cover more than 3,000,000 ha, and their chemical and biological characteristics have been characterized [20].For these large plantations in SW China, it is necessary to understand the importance of J. regia stands in the regional C budget.Although there is abundant research on genome sequence, food chemistry, plant disease, and nutrient cycling in J. regia plantations [18,19,21], the biomass and carbon sequestration by J. regia plantations in the karst regions is still unknown.Therefore, a better understanding of C sequestration in J. regia plantations at a regional scale is fundamentally important for forest managers, policy makers, and researchers wishing to enhance forest C sequestration.
Although our previous study demonstrated relationships between biomass and soil nutrients in J. regia stands of different ages in depressions between karst hills [21], the uncertainty regarding C and biomass allocation of this species along different stand ages represents a major information gap that hampers efforts to estimate C pool dynamics in karst regions.Thus, the present study examined changes in biomass and carbon storage in four different J. regia plantations of different ages under the GGP in Fengshan County, Guangxi Province.The aims of this research were to (1) estimate the biomass components of a plantation ecosystem across a chronosequence of four J. regia stands (1-, 5-, 9-, and 13-years old); (2) determine the changes in the C concentrations and C stocks of ecosystem components with increasing stand age in karst regions of SW China.

Experimental Sites
The four plantation sites are located in Fengshan County (106 • 42' E-107 • 17' E, 24 • 15' N-24 • 49' N) of Guangxi Province, Southwest China.The total area of this county is 1700 km 2 , of which 70.1% belongs to the karst.The undulating land has an elevation of 600-900 m, and is characterized by broken terrain, sparse vegetation, eroded soil, and exposed bedrock.This county has a subtropical monsoon climate with an average annual temperature of 17.6 • C. The area receives an annual average of 2200 h of sunshine and 269 frost-free days, whereas the active accumulated temperature (≥10 • C) is 6426 • C. The mean precipitation is 1550 mm year −1 , with 65% falling during May to August.Evapotranspiration is approximately 1130 mm year −1 [21].
Vegetation coverage has increased under the GGP in Fengshan County, from 24.8% in 2001 to 60.3% in 2008.Indeed, this county, with its extensive cover of J. regia, has become an example of how desertification can be controlled through the planting of forests under the GGP in Guangxi Province and SW China.The area of the plantation forest was approximately 5200 ha in 2015, 2100 ha of which was covered by J. regia plantations.

Field Sampling and Measurements
We applied a chronosequence approach using J. regia plantation stands that were 1, 5, 9, and 13 years old (containing 350 to 450 trees per ha).Four plantation sites for all age classes were randomly selected in this study.Based on a following site inspection, three 20 m × 50 m plots for each age class site were established in August 2015 (Table 1).Buffers were maintained between the plots and established plots for each stand age.To reduce the effects of environmental factors, all plots within each location were adjacent to each other (no more than 200 m) and characterized by similar terrain, topography, understory vegetation, and soil.All plots belonged to the GGP regions and had the same organic amendments, fertilizer trials, tillage, and weed control treatments before plantation.According to the standards of the Centre for Tropical Forest Science [22], we established the plots and divided them into ten 10 m × 10 m subplots.Tree density, height (H), diameter at breast height (DBH), and shrub and herb parameters (density, type, biomass) were recorded for each entire plot and all three subplots.Plant density was estimated according to the plant number per subplot.H and DBH were measured by hypsometer and tape measure, respectively.Three trees of average size (i.e., height, stem, and crown diameter), located near the middle of each plot and as close to each other as possible, were selected for destructive sampling to determine tree biomass.Each selected tree was separated into foliage, stem, branch, root, and fruit [4,23].The coarse root (>2 mm) of each tree in a radius between 1.5 m and 3.5 m (depending on tree size) was excavated and washed, and roots were separated by hand into size classes based on diameter (≤2 mm and >2 mm).The biomass of the coarse root per hectare was estimated by multiplying the mean weight of the coarse root volume by the number of trees in each hectare.Fine roots (≤2 mm) around 24 selected trees (in a radius of 5 m) at different depths (0-10 cm, 10-20 cm, 20-30 cm, 30-50 cm, and 50-100 cm) were sampled using a soil corer (90 mm inside diameter).The fine root biomass per hectare was estimated by multiplying the weights of the fine roots per core 0.006392 m 2 by 1,564,455.57.Total root biomass per hectare was estimated by adding the biomass of the coarse root per hectare to the fine root biomass per hectare.The fresh weights of ecosystem components (foliage, stem, branch, root, and fruit) were measured in situ, and six subsamples of each component from each collected plant were dried at 65 • C to estimate dry biomass density (Mg ha −1 ) and C concentration.
The biomass of the understory vegetation, dominated by Cipadessa cinerascens (Pell.)Hand-Mazz, Vitex negundo Linn, Imperata cylindrical, Ageratum conyzoides, and Apluda mutica, was weighed by destructive harvesting of shrubs and herbs within three 2 × 2 m microplots within each 20 m × 50 m plot.We also collected all litter from three 1 × 1 m herbaceous quadrats in each 20 m × 50 m plot.
Additionally, the aboveground biomass of understory vegetation and litter were taken back to the laboratory for drying and reweighing.
We collected ten composite samples of soil in the middle of the subplots (10 m × 10 m) of each plot (20 m × 50 m).Soil samples at depths of 0-10, 10-20, 20-30, 30-50, and 50-100 cm were collected using a soil auger corer (70 mm inside diameter) to calculate soil C storage.Three soil cores per subplot were taken and thoroughly mixed into one composite sample.Because the soil depth in the karst region is lower than that in the non-karst region [16], the soil samples at depths of 50-100 cm depended on the soil depth in different plots.For each plot, 50 soil composite samples (10 composite samples per layer × five layers) were collected.Additionally, to determine soil bulk density at different depths (0-10 cm, 10-20 cm, 20-30 cm, 30-50 cm, and 50-100 cm) in each location, we collected six soil samples at each depth using a 70 mm height × 52 mm diameter ring.After removal of rocks and plant residues from the samples, the bulk density at each soil depth was determined by weighing the samples after drying at 105 • C. In the laboratory, 200 g of soil per auger sample was sieved with a 2-mm sieve and mixed.To determine soil organic carbon (SOC) concentration, 0.2 g of sieved soil per auger sample was measured in a KCr 2 O 7 -H 2 SO 4 oil bath using a vario macro elemental analyzer (Elementar Analysensysteme GmbH, Berlin, Germany).To eliminate the influence of rocks on the soil C storage measurement in the karst region, the percentage of covered soil was also measured by the method described above [16].In addition, the site characteristics (e.g., elevation, slope, and soil depth, cation exchange capacity, and coarse rock fragments) were determined by the methods described by Du et al. [16] and Song et al. [24].

Data Analysis
The effects of stand age on C concentration, C storage, and biomass accumulation of ecosystem components were compared using one-way ANOVA, followed by Fisher's LSD method for testing the null hypothesis.α = 0.05 was set as the significance level for all statistical analyses.All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS 12.0, Chicago, IL, USA).In addition, the C storage in ecosystem components, such as herbaceous plants, shrubs, trees, and litter, was calculated by multiplying the C concentration of each component by its mass per hectare.The C storage in each soil layer (CS, Mg ha −1 ) was determined according to SOC concentration (Cc, g kg −1 ), sampled depth (D, cm), bulk density (BD, g cm −3 ), and percentage of covered soil (PCS, %) using the equation: CS = Cc × D × BD × PCS/10.The SOC stock was the sum of each soil layer.

Biomass Distribution in Ecosystem Components
In plantations of J. regia, belowground biomass (BGB), aboveground biomass (AGB), and total tree biomass (TTB) increased significantly with stand age (Table 2).BGB and AGB increased from 0.08 and 0.18 Mg ha −1 , respectively, in the 1-year-old stand to 5.63 and 21.01 Mg ha −1 , respectively, in the 13-year-old stand.Similarly, the biomass of tree components also changed with stand age (Table 2).For the 1-year-old stand, biomass increased in the following order: foliage < root < stem.For the 13-year old stand, the increase in biomass production was in the following order: foliage < fruit < root < branch < stem.The ratio of root biomass to TTB in the 1-year-old stand was higher than that in the 5-, 9-, and 13-year-old stands.
There were no shrubs or herbs found in the understory of 1-and 5-year old J. regia plantations because both are usually removed by farmers.The biomass of shrub, herb, and litter in the 9-year old stand was not significantly different than that of the 13-year old stand.The overall biomass of the ecosystem also increased markedly with stand age.

Carbon Concentration and Accumulation in Biomass
Carbon concentration in trees, shrubs, herbs, and litter was age-independent (Table 3).On average, the C concentration in fruit was the highest among all ecosystem components, whereas that in the branch fraction was the lowest.Among all tree components, fruit showed the highest C concentration, with a mean value of 486.4 g kg −1 .In contrast, the C concentration in the tree branches had the lowest values.The mean C concentrations of foliage, stems, and roots were 464.7, 461.3, and 452.3 g kg −1 , respectively.The mean C concentration of litter was 476.2 g kg −1 .
Table 3. Carbon concentration (g kg −1 ) in trees, understory vegetation, and litter for 1-, 5-, 9-, and 13-year-old J. regia plantations (n = 3).The average value of C stored in the AGB was 0.08 Mg ha −1 for the 1-year-old plantation, and 0.78, 6.42, and 9.51 Mg ha −1 for the 5-, 9-, and 13-year-old plantations, respectively (Table 4).Root biomass C storage steadily increased from 0.04 Mg ha −1 in the 1-year-old plot to 0.28, 1.88, and 2.57 Mg ha −1 in the 5-, 9-, and 13-year-old plots, respectively, representing 33.3%, 26.4%, 20.2%, and 21.3%, respectively, of the total tree biomass (TTB) C storage in the four stand age classes (Figure 1).Most of the carbon in the biomass was stored in the TTB, shrubs, and herbs, whereas only a small amount was stored in litter (Figure 1).Biomass C content in fruit also increased with stand age.The contribution of fruit C storage to total biomass C content varied from 0 for the 1-and 5-year-old stands to 9.25% for the 13-year-old stand (Figure 1).Moreover, the C storage in shrub, herb, and litter was not age-dependent.The total biomass C in trees, understory vegetation, and litter increased significantly with stand age.

Carbon Concentration and Storage in Soil
Statistically significant differences were observed for SOC values between different aged stands at different soil depths (Figure 2; Table 5).SOC concentration decreased with soil depth irrespective of stand age (Figure 2).The SOC concentration at 0-10 cm ranged from 16.96 g/kg to 26.46 g/kg, which was much higher than that at other soil depths.The C concentration of the soil at 0-10 cm, 10-20 cm, and 20-30 cm increased significantly with stand age, whereas the C concentration at 30-50 cm and 50-100 cm did not.In the karst area, SOC storage in J. regia plantations did not increase with stand age because soil depth, soil bulk density, coarse rock fragments, and rocks in the soil can affect the total mass of the soil.The mean SOC storage in the top soil (less than 100 cm in the karst soil) in the 1-, 5-, 9-, and 13-year-old stands was 33.37, 53.15, 33.56, and 49.78 Mg ha −1 , respectively.More than 79.5% of SOC was stored within the upper 0.5 m of the soil profile from each stand (Table 5).

Carbon Concentration and Storage in Soil
Statistically significant differences were observed for SOC values between different aged stands at different soil depths (Figure 2; Table 5).SOC concentration decreased with soil depth irrespective of stand age (Figure 2).The SOC concentration at 0-10 cm ranged from 16.96 g/kg to 26.46 g/kg, which was much higher than that at other soil depths.The C concentration of the soil at 0-10 cm, 10-20 cm, and 20-30 cm increased significantly with stand age, whereas the C concentration at 30-50 cm and 50-100 cm did not.In the karst area, SOC storage in J. regia plantations did not increase with stand age because soil depth, soil bulk density, coarse rock fragments, and rocks in the soil can affect the total mass of the soil.The mean SOC storage in the top soil (less than 100 cm in the karst soil) in the 1-, 5-, 9-, and 13-year-old stands was 33.37, 53.15, 33.56, and 49.78 Mg ha −1 , respectively.More than 79.5% of SOC was stored within the upper 0.5 m of the soil profile from each stand (Table 5).In stands of all ages, most of the ecosystem C accumulated in the soil rather than in the biomass (Figure 3).In the 1-year-old plantation, 99.64% of the ecosystem C was stored in the soil, and this proportion decreased to 76.19% in the 13-year-old plantation.Carbon accumulation in biomass increased with stand age.The contribution of biomass C to ecosystem C increased with stand age from 0.36% for the 1-year-old stand to 23.81% for the 13-year-old stand (Figure 3).

Biomass of J. regia Plantations of Different Ages
In this study, the biomass of J. regia plantations across the chronosequence indicated that tree biomass increased with stand age.This pattern was similar to that of many previous reports [25][26][27], and is consistent with expected patterns of biomass accumulation.Our results revealed a total tree biomass (TTB) of 0.26 Mg ha −1 for the 1-year-old stand and 26.01 Mg ha −1 for the 9-year-old stand of J. regia (Table 2), which was higher than that previously reported for other tree species.For example, Cheng et al. measured a TTB of 0.05 Mg ha −1 for a 1-year-old stand and 16.70 Mg ha −1 for 10-year-old stands within Zanthoxylum bungeanum plantations [13].Similarly, the TTB for a 10-year-old mixed forest of Betula luminifera and Populus euramevicana was only 4.03 Mg ha −1 , and even less (1.12 Mg ha −1 ) for a plantation of Cupressus funebris [28,29].These comparisons demonstrate that afforestation of the karst region with J. regia under the GGP has greater potential to accumulate biomass than afforestation with these other species.

Carbon Storage in the J. regia Plantation Ecosystem
Ecosystem C storage amounted to 33.49, 54.21, 46.40, and 65.34 Mg ha −1 for the 1-, 5-, 9-and 13-year old plantations, respectively.In stands of all ages, most of the ecosystem C accumulated in the soil rather than in the biomass (Figure 3).In the 1-year-old plantation, 99.64% of the ecosystem C was stored in the soil, and this proportion decreased to 76.19% in the 13-year-old plantation.Carbon accumulation in biomass increased with stand age.The contribution of biomass C to ecosystem C increased with stand age from 0.36% for the 1-year-old stand to 23.81% for the 13-year-old stand (Figure 3).The ratio of belowground to aboveground biomass decreased with stand age, which was consistent with the results of our previous study [30].In the karst region, shallow soil and the soilrock mixture from lime soil may be responsible for the smaller root biomass of adult trees [13].Thus, these results may not be consistent with some reports of adult forests that have large amounts of root biomass in non-karst regions [29].However, the large ratio of belowground (root) biomass to total biomass (0.27-0.50) of adult trees also highlights the importance of accounting for roots in the accurate estimation of the total biomass and carbon of plantation forests [21].
In the karst region, abundant rainfall, sunlight, and sparse tree cover may favor the growth of understory vegetation [13].Here, our data showed that the understory biomass ranged from 6.40 Mg ha −1 in the 13-year-old stand to 7.27 Mg ha −1 in the 9-year-old stand.These values were higher than other previously reported values of 0.87-3.55Mg ha −1 and 1.61-3.76Mg ha −1 [26,27].In addition, the understory biomass may not correspond to stand age.In fact, it may depend more on farming activities, soil conditions, stand-specific canopy, stand density, and forest types, which could affect nutrient availability, water, and light for the development of the understory [31].

Biomass C of J. regia Plantations of Different Ages
C concentration is usually regarded as a fixed value (0.5) to convert dry biomass into carbon storage [30].In this study, the C concentration for components of J. regia ranged from 0.422 to 0.494 (Table 3).The mean value of 0.464 was similar to the C concentration previously observed for Acacia auriculiformis (0.466), but lower than that for Masson pine (0.577) and tea-oil Camellia (0.503) [32].
Previous studies showed that C concentrations of plantation forests are often influenced by geographical location, climate, soil conditions, wood type, and tree components [13,33].Thus, all of these factors might be used to accurately calculate C stock when measuring C concentrations under

Biomass of J. regia Plantations of Different Ages
In this study, the biomass of J. regia plantations across the chronosequence indicated that tree biomass increased with stand age.This pattern was similar to that of many previous reports [25][26][27], and is consistent with expected patterns of biomass accumulation.Our results revealed a total tree biomass (TTB) of 0.26 Mg ha −1 for the 1-year-old stand and 26.01 Mg ha −1 for the 9-year-old stand of J. regia (Table 2), which was higher than that previously reported for other tree species.For example, Cheng et al. measured a TTB of 0.05 Mg ha −1 for a 1-year-old stand and 16.70 Mg ha −1 for 10-year-old stands within Zanthoxylum bungeanum plantations [13].Similarly, the TTB for a 10-year-old mixed forest of Betula luminifera and Populus euramevicana was only 4.03 Mg ha −1 , and even less (1.12 Mg ha −1 ) for a plantation of Cupressus funebris [28,29].These comparisons demonstrate that afforestation of the karst region with J. regia under the GGP has greater potential to accumulate biomass than afforestation with these other species.
The ratio of belowground to aboveground biomass decreased with stand age, which was consistent with the results of our previous study [30].In the karst region, shallow soil and the soil-rock mixture from lime soil may be responsible for the smaller root biomass of adult trees [13].Thus, these results may not be consistent with some reports of adult forests that have large amounts of root biomass in non-karst regions [29].However, the large ratio of belowground (root) biomass to total biomass (0.27-0.50) of adult trees also highlights the importance of accounting for roots in the accurate estimation of the total biomass and carbon of plantation forests [21].
In the karst region, abundant rainfall, sunlight, and sparse tree cover may favor the growth of understory vegetation [13].Here, our data showed that the understory biomass ranged from 6.40 Mg ha −1 in the 13-year-old stand to 7.27 Mg ha −1 in the 9-year-old stand.These values were higher than other previously reported values of 0.87-3.55Mg ha −1 and 1.61-3.76Mg ha −1 [26,27].In addition, the understory biomass may not correspond to stand age.In fact, it may depend more on farming activities, soil conditions, stand-specific canopy, stand density, and forest types, which could affect nutrient availability, water, and light for the development of the understory [31].

Biomass C of J. regia Plantations of Different Ages
C concentration is usually regarded as a fixed value (0.5) to convert dry biomass into carbon storage [30].In this study, the C concentration for components of J. regia ranged from 0.422 to 0.494 (Table 3).The mean value of 0.464 was similar to the C concentration previously observed for Acacia auriculiformis (0.466), but lower than that for Masson pine (0.577) and tea-oil Camellia (0.503) [32].Previous studies showed that C concentrations of plantation forests are often influenced by geographical location, climate, soil conditions, wood type, and tree components [13,33].Thus, all of these factors might be used to accurately calculate C stock when measuring C concentrations under different edaphic and climatic conditions.
The overall within-stand biomass C increased with stand age, from 0.12 Mg C ha −1 in the 1-year-old stand to 12.08 Mg C ha −1 in the 13-year-old stand (Table 4).The accumulation of C in the total tree biomass changed with stand age, and the pattern of change was similar to that of previous studies [10,13].Accumulation of biomass C in younger stands was more rapid than that in older stands.In this study, the highest rate of C accumulation was found in 5-9-year-old stands (Table 4).This indicates that J. regia is well able to adapt to the water deficiency stress that is common in karst regions [21].The J. regia plantations in the present study stored more biomass C than other plantations in similar karst regions.For example, the tree C stock of Z. bungeanum increased from 0.02 to 7.56 Mg C ha −1 in the 1-10 stand ages [13], which is less than that in the present study.
Previous research showed that the C stock of understory vegetation decreased with increasing stand age [34].However, the C stock of the understory vegetation in this study was not significantly affected by stand age.This discrepancy might be explained by farmer activity that removed both herbs and shrubs in the understory of the 1-and 5-year-old J. regia plantations.Interestingly, we found that herbs dominated the understory vegetation C pool in 9-and 13-year old J. regia stands, and the proportion of herb C relative to the total understory vegetation C was approximately 60%.In addition, the understory vegetation C in the four stands ranged from 0 to 3.26 Mg ha −1 , which is a smaller amount than that previously reported for a same-aged Eucalyptus plantation [16].Litter C is highly susceptible to human disturbances, decomposition rate, and litter input [35], which may explain why there was no association between litter C and stand age in this study.

Soil Organic Carbon (SOC) Storage in J. regia Plantations
In this study, SOC storage did not increase with stand age (Table 5).Previous research on C storage in mineral soils has been inconsistent.Many studies have reported no significant increase in SOC stocks with stand age [36,37], whereas some studies showed increasing SOC stocks in the early period after forestation [38].This discrepancy may be due to differences in the C balance between fine root production and soil respiration rates, which could overshadow the effect of stand age [4].Additionally, the SOC stocks in our study were considerably lower than those previously reported in non-karst regions (16,27).In karst soils under J. regia plantations, over 80% of ecosystem C is stored in the top 50 cm of the soil.This means that, although it is vulnerable to human disturbance and soil erosion, the top soil in the karst area is a major C pool.In practice, implementation of good plantation management practices in the GGP is necessary to promote carbon sequestration [13].
We found that the soil C concentration at 0-30 cm increased with stand age (Figure 2), which was consistent with other studies [23,26].The soil C concentrations in the top soil layers increase as the stands age, resulting from an increase in slow decomposition and litter input [39].Therefore, our results suggest that destruction of topsoil from human disturbances is detrimental to C sequestration in the karst region.

Influence of GGP on C Sequestration in the Karst Region
Our studies suggest that the GGP will enhance ecosystem carbon sequestration due to afforestation in plantations.Biomass C storage in J. regia plantations can rapidly increase from 0.12 Mg ha −1 for the 1-year-old stand to 12.08 Mg ha −1 for the 13-year-old stand, which is similar to that found in other studies [13,14,40,41].If J. regia was planted in all karst region under the GGP, more than 3.3 × 10 7 Mg C could have been sequestered between 1999 and 2010.In the long term, the conversion of farmland or abandoned land to J. regia plantations in the karst region would increase farmer incomes, protect fragile karst region ecology, and offset greenhouse gas emissions.However, we also note that pseudoreplication, the failure to establish a control plot, and limitations of root biomass data may seriously affect the reliability of the present results.Collection of additional data in other similar plantations is necessary to provide support for these observations.Moreover, we are also aware that the cultivation of a single species might reduce biodiversity and increase vulnerability of plantations to diseases and pests.Thus, the ecological issues related to pure forest, mixed forest, and agroforestry in the GGP also should be considered in future research.

Conclusions
The results presented here demonstrate changes in biomass components for the plantation ecosystem across a chronosequence of four Juglans regia stands and illustrate the changes in the C concentration and C stock of ecosystem components with increasing stand age.The biomass in trees and ecosystems increased with stand age, whereas an increase in the biomass of shrubs, herbs, and litter was not observed.Most of the carbon in the biomass was stored in the aboveground tree biomass, tree roots, shrubs, and herbs, whereas only a small amount was associated with soil litter.Soil organic C (SOC) in the top soil of plantations was not age-dependent.SOC content decreased continually with increasing soil depth.Ecosystem C storage was dominated by the SOC, with this component representing between 72.33% and 99.64% of ecosystem carbon.Therefore, planting of J. regia has potential for not only vegetation restoration but also high C fixation capacity in the karst region of Southwest China.

Figure 1 .
Figure 1.Percentage distribution of biomass C storage in J. regia plantations of four different ages.

Figure 1 .
Figure 1.Percentage distribution of biomass C storage in J. regia plantations of four different ages.

Forests 2017, 8 , 103 8 of 13 Figure 2 .
Figure 2. Soil carbon content in different depths in J. regia plantations of four different ages.Note: Error bar indicates S.E

Figure 2 .
Figure 2. Soil carbon content in different depths in J. regia plantations of four different ages.Note: Error bar indicates S.E.

Figure 3 .
Figure 3. Percentage distribution of ecosystem C storage in J. regia plantations of four different ages.

Figure 3 .
Figure 3. Percentage distribution of ecosystem C storage in J. regia plantations of four different ages.

Table 1 .
Site characteristics and stand characteristics of four age classes of J. regia plantations (n = 3).
BGB, belowground biomass.Note: Data show the mean ± S.E. and different lower-case letters indicate significant difference at p < 0.05 (one-way ANOVA and LSD test).There are no shrubs and herbs in the understory of 1-, and 5-year old J. regia plantations due to their removal by local farmers.

1-Year-Old 5-Year-Old 9-Year-Old 13-Year-Old Mean
Data show the mean ± S.E. and different lower-case letters indicate significant difference at p < 0.05 (one-way ANOVA and LSD test). Note:
Note: Data show the mean ± S.E. and different lower-case letters indicate significant difference at p < 0.05 (one-way ANOVA and LSD test).

Table 5 .
Soil Data show the mean ± S.E and different lower-case letters indicate significant difference at p < 0.05 (one-way ANOVA and LSD test).