rapid accumulation of carbon on severely eroded red soils through afforestation in subtropical china

Forest Ecology and Management 300 (2013) 53–59

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Forest Ecology and Managemen t

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Rapid accumulation of carbon on severely eroded red soils through afforestation in subtropical China

Jinsheng Xie 1, Jianfen Guo, Zhijie Yang, Zhiqun Huang, Guangshui Chen, Yusheng Yang ⇑College of Geographical Science, Fujian Normal University, Fuzhou 350007, China Key Laboratory for Subtropical Mountain Ecology, Fuzhou 350007, China

a r t i c l e i n f o

Article history:Available online 24 July 2012

Keywords:AfforestationBiomass carbon pools Carbon sequestration Deep soil Eroded red soil LFOC

0378-1127/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.foreco.2012.06.038

⇑ Corresponding author. Address: College of GeograUniversity, No. 32, Shangsan Road, Cangshan District+86 591 83482530; fax: +86 591 83465397.

E-mail addresses: [email protected] (J. Xie), [email protected] (Z. Yang), [email protected]@163.com (G. Chen), [email protected] (Y. Yan

1 Tel.: +86 591 83465013; fax: +86 591 83465397.

a b s t r a c t

Recovery of carbon stocks after afforestation in degraded lands provides a management practice to mitigate rising atmospheric carbon dioxide concentrations, however carbon accumulation after afforestation of severely eroded lands is poorly understood. Large areas of the red soils in subtropical China suffer from severe erosion and have very low carbon density. We investigated above- and below-ground carbon pools in bare land on a severely eroded red soil (BL), a Pinus massoniana plantation that had been established onbare land in 1981(PM) and a nearby secondary forest (SF) in southeastern China. The ecosystem carbon pool in PM (130.1 ± 7.2 Mg C ha�1) was 10 times higher than in BL (13.0 ± 1.3 Mg C ha�1), and 22% lower than that in SF (166.7 ± 7.0 Mg C ha�1) (p < 0.05). The above ground biomass carbon pool was 91.9 ± 4.8 Mg C ha�1 in PM, similar to 98.2 ± 5.5 Mg C ha�1 in SF. The soil organic carbon (SOC) pool (to1 m dep th) in PM (38.2 ± 3.4 Mg C ha�1) was 2.9 times higher than that in BL (13.0 ± 1.3 Mg C ha–1),but was significantly lower than that in SF (68.5 ± 2.5 Mg C ha–1). About 70% of the organic C to 1 m depth was stored in the top 40 cm in the two forests. The storage of light fraction organic carbon (LFOC) at the 0–60 cm depth in PM was significantly higher than that in BL, but not significantly different from that inSF. PM had significantly higher proportions of LFOC to SOC for all soil depths in compariso n with BL and SF (p < 0.05). The mean accumulation rates of ecosystem carbon pools, aboveground biomass carbon pools, and SOC pools in PM were 4.88 ± 0.25, 3.83 ± 0.16, and 1.05 ± 0.09 Mg C ha�1 yr�1, respectively.Our results indicate that afforestation of severely eroded red soils with P. massoniana may be a suc cessful forest management practice to achieve rapid carbon accumulation.

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1. Introduction

China is one of the countries suffering greatly from severe soil loss with eroded lands totaling 356 M ha, an annual soil loss of5 billion tonnes, and 5% of the total loss (60 billion tonnes) world- wide (Ministry of Water Resources in China, 2002 ). The formerly densely forested hilly red soil region of Southern China, which cov- ers 218 M ha, including 10 province s, is now known as the ‘‘red desert of Southern China’’ (Zhao, 2002 ). Changtin g County was typ- ical of erosion in hilly subtropical China and Hetian town was one of the representative towns with serious soil erosion. In 1983, total soil erosion area in this town was 15,840 ha with the area experi- encing heavy soil erosion (5000–8000 t km�2 year�1) of 9334 ha(Yang et al., 2005a ). Replanting of eroded lands with fast-growin g

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phical Science, Fujian Normal , Fuzhou 350007, China. Tel.:

[email protected] (J. Guo),edu.cn (Z. Huang), gshui-g).

tree species has been widely adopted in these areas. Replanting ofdegraded land with forest vegetation can gradually increase vegetation coverage , leading to accumulation of litter-fall mass, con- struction of root networks, and improvem ent of soil physiochemi cal properties, which in turn lead to reduced runoff and soil loss (Gao and Liu, 1992; Hou et al., 1996; Li and Shao,2006).

Pinus massonian a, native to a wide area of central and southern China, is considered a suitable species for afforestation on severely degraded lands in subtropical China, due to its fast growth and adaptabi lity to dry and nutrient-poor soils. At present, this species accounts for 14% of all plantations in subtropical China. With the recovery of vegetation and a reduction of soil loss, more organic carbon (C) has been sequestered in soil and the potential for Csequestra tion has been enhanced (Xie et al., 2008 ). Although the afforestati on of eroded lands can result in rapid accumulation ofC in the above-ground vegetation, there is little informat ion onthe effect of such afforestation on below-gr ound C stocks. This informat ion is crucial for national C monitoring systems designed to account for soil C change associated with land-use change for countries to meet Kyoto Protocol requiremen ts. The focus of this

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54 J. Xie et al. / Forest Ecology and Management 300 (2013) 53–59

study was to assess the overall impact of afforestation with P. massoniana on ecosystem C storage, by quantifying above- and belowground C pools of bare land, a P. massoniana plantation established on bare land, and of a nearby secondary forest. Another aim was toevaluate changes in the light fraction organic carbon (LFOC) in the soil of the three ecosystems . We hypothesize d that C (or soil LFOC)storage in the three ecosystems would vary substanti ally.

2. Materials and methods

2.1. Study area and site descriptio n

The study was conducted at Hetian Research Station for Erosion (25�330N, 116 �180E), which is located in Hetian town of Changtin gcounty in western Fujian Province of southern China. The region has a subtropical monsoon climate with a 30-year mean annual temperature of 18.1 �C and an annual precipitatio n of 1737 mm.The mean length of the growing season is 260 days per year. Soils are classified as red soils (humic Planosols in the FAO system) and are easily eroded. The soil parent material is medium to coarse- crystalline granite. Native evergreen broadleaf forests in this area had been clear-fell ed in the last one hundred years, which led tosevere soil erosion and long-term site degradation. To mitigate land degradation , since the 1980’s the county’s government has established forests on bare land and closed access to hillsides to al- low natural regeneration. Some ecological restoration was achieved under those efforts. More specifically from 1983 to2003, the area of soil erosion in Hetian town declined by 14.2%.

Three land use types were selected in 2005, including severely eroded land (bare land, BL), a P. massoniana plantation (PM) which was established on bare land in 1981, and a secondary forest (SF).These land use types are all located within an area of less than 1 km2. They have a homogeneous substrate (similar mineralogy,depths, and horizonation) and the soil under the plantation was similar to that of the bare land and the secondary forest. Some soil properties (0–60 cm) of the BL, PM and SF are given in Table 1.

In BL, the A- horizon had been lost and the B horizon exposed.Soil organic matter content was 0.8 g kg�1. Although largely bare of vegetation, some P. massoniana and Arundinella setosa plantswere present.

Before establishment of the P. massoniana plantation, there was an initial vegetation cover of <10% (Gao et al., 2011 ). In 1981, small parallel ditches were formed to decrease soil erosion and improve soil water availability and 1-year-old seedlings of P. massoniana were planted in the ditches at a density of 3000 stems ha�1. Cot- tonseed cake (525 kg ha�1) and pig manure (5250 kg ha�1) were

Table 1Soil properties at 0–60 cm of a bare land (BL), a Pin us massoniana plantation (PM) and a s

Land use type

Soil depths (cm)

Organic C(mg g�1)

Total N(mg g�1)

Total P(mg g�1)

Tota(mg

BL 0–5 1.73 0.42 0.09 11.55–10 1.71 0.47 0.09 12.910–20 1.67 0.36 0.07 13.020–40 1.30 0.27 0.06 12.940–60 1.20 0.24 0.06 12.8

PM 0–5 23.07 2.03 0.29 17.35–10 12.22 1.06 0.20 16.510–20 9.03 0.71 0.15 16.720–40 3.20 0.32 0.08 15.340–60 1.83 0.23 0.07 16.0

SF 0–5 27.26 2.40 0.50 20.15–10 21.83 1.97 0.26 19.410–20 17.50 1.45 0.21 18.720–40 8.46 0.72 0.14 19.340–60 4.77 0.43 0.10 18.5

added to improve the soil organic matter content, and 1050 kg ha�1 of calcium magnesium phosphat e fertilizer was ap- plied. In 1982 the shrub species, Lespedeza bicolor and Amorphafruticosa were planted, and two further applications of fertiliser were made during the first 2 years. In 2005, the density of P. massoniana stems averaged 2975 stems ha�1. The mean tree height and diameter at breast height (DBH) were 11.5 m and 12.3 cm,respectivel y. Understorey vegetation was dominated by Lespedezabicolor, Adinandra millettii, Camellia oleifera and Dicranopteris dichotoma. The depth of the A horizon varied between 1 and 3 cm.

The secondary forest was a mixed coniferous and broadlea ved forest community, which was derived from a primary broadlea ved forest. About 100 years ago, part of the primary broadlea ved forest was clear-felled after which it was conserved as ‘‘fung-shui’’ wood- land and regenerated naturally with little managemen t. In 2005,the secondar y forest was dominated by P. massonian a and Schimasuperba with average tree densities of 163 and 175 stems ha�1,respectivel y. Mean tree height for the dominan t species was 20.7 and 18.2 m and the correspondi ng mean DBH was 37.3 and 25.3 cm, respectively. Adiandra millettii , Machilus grijsii , Daphni-phyllum oldhamii and D. dichotoma were predominated in the understo rey. The A horizon depth was about 20 cm.

2.2. Plant and soil sampling

In April 2005, three 20 m � 20 m plots were established in each land use type. In PM and SF plots, tree biomass was calculated using allometric functions (Yang et al., 2002 ). The DBH (mean oftwo breast height diameter measurements in right-angled direc- tions, accuracy, 1 mm), tree height (accuracy, 0.1 m) and crown length (accuracy, 0.1 m) of all living trees (height, >1.3 m) on the three plots of each forest were measured. Using stratified random sampling based on the distribution of DBH values, 13 P. massonian atrees in PM and 15 trees from two dominant tree species (including7 P. massoniana and 8 S. superba trees) in SF were felled. Derivation of the allometric biomass functions and subsequent calculatio n ofbiomass components (stem wood, branches, foliage and roots) atthe stand-level was carried out. The number of sample branches from each sample tree was four to six and they were taken at reg- ular intervals from along the entire length of the crown. Foliage was obtained from each branch. Samples of stem wood were obtained from each tree using an 11 mm tree corer or wedges ofwood cut using a chainsaw. Root samples were obtained by exca- vation. All samples obtained were field weighed, placed into plastic bags and kept cool until they could be transported to the laboratory. Biomass estimation functions for stems and roots were B = a

econdary forest (SF).

l Kg�1)

Hydrolyzable N(mg kg�1)

Available P(mg kg�1)

Available K(mg kg�1)

C/N

5 42.13 0.81 66 4.1 5 41.90 0.78 74 3.6 0 30.22 0.69 53 4.6 1 22.84 0.66 51 4.8 8 22.20 0.65 51 5.0

2 185.54 2.55 98 11.4 2 98.11 1.83 84 11.5 6 82.53 1.12 91 12.7 6 35.06 0.83 76 10.0 3 26.25 0.70 54 8.0

2 235.79 4.21 282 11.4 0 191.82 2.27 176 11.1 5 118.69 2.10 129 12.1 6 69.81 1.35 113 11.8 2 53.80 0.97 75 11.1

Table 2Carbon storage (Mg ha�1) in various biomass compartm ents, soil and ecosystem in abare land (BL), a 24-year-old Pin us massoniana plantation (PM) and a secondary forest (SF).

BL PM SF

Stem 0 53.8 (2.4) a 61.5 (3.2) bBranch 0 14.2 (0.7) a 9.5 (0.7) bLeaf 0 6.4 (0.4) a 2.2 (0.3) bRoot 0 12.6 (0.9) a 15.7 (1.0) bUnderstory 0 3.2 (0.6) a 6.4 (0.7) bLitter 0 1.7 (0.2) a 2.9 (0.2) bTotal aboveground 0 91.9 (4.8) a 98.2 (5.5) aSoil (0–100 cm) 13.0 (1.3)a 38.2 (3.4) b 68.5 (2.5) cTotal ecosystem 13.0 (1.3)a 130.1 (7.2) b 166.7 (7.0) c

Data are means followed by standard deviations in the parentheses, n = 3. Values with the different letters on the same row denote significant differences (p < 0.05).

J. Xie et al. / Forest Ecology and Management 300 (2013) 53–59 55

(D2H)+b, (R2 = 0.97 and 0.89 respectively), while the function ofB = a (D2H)b was used to estimate the biomass of branches and foliage (R2 = 0.96 and 0.87 respectively ). In these functions, B was biomass of tree parts measure d in kg, D and H represent breast height diameter (cm) and tree height (m), a and b were two parameters.

Understorey vegetatio n biomass was determined by harvestin gabove-groun d material in five 1 m2 subplots in each 20 m � 20 mplot. Litter was also collected in each subplot. Plant and litter samples were oven dried at 70 �C for 48 h before dry weight determi- nation and chemical analysis for organic C.

Five randomly located soil pits were dug to a depth of 1 m ineach plot. After removing the litter layer, mineral soil samples were collected for the following depth intervals: 0–5, 5–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm. Soil bulk density was determined using a stainless steel bucket (Lu, 2000 ). Soil samples were air-dried and passed through a 2 mm sieve to remove rocks and plant roots.

2.3. Density fractionation and soil C analysis

Soil was physically divided into two pools based on density using a modified method of McLauchl an and Hobbie (2004). Ten grams of air dried soil were placed in a centrifuge tube with 50 ml NaI (density of 1.70 g cm�3). The tubes were shaken by hand for 3 min, then centrifuged at 1000 rpm for 15 min. The floatingmaterial was aspirated from the surface of the tubes (about the top 20 ml) and then placed into a filter unit (a funnel containing Whatman GF A/E filter paper). The shaking-cen trifugation-aspi ration process was repeated at least four times, until no floatingmaterial remained. The samples on the filter paper, designated light fraction (LF), were rinsed thoroughly with deionised water and collected. The material (heavy fraction) remaining at the bot- tom of the centrifuge tube was quantitatively washed onto a separate funnel. The heavy fraction was rinsed repeatedly with deionised water. The light and heavy materials were dried at60 �C for 48 h and ground in a mortar and pestle (Huang et al.,2011a, 2011b ).

2.4. Laboratory analyses

The biomass samples were oven-dried, ground and passed through a 1 mm sieve. Carbon concentration of plant samples,SOC and LFOC were determined by an elemental analyzer (VarioEL III, Elementar Analysensysteme GmbH, Hanau, Germany).

2.5. Calculations of C storage and C accumulation rate

The mass of C stored in tree compartme nts, understorey vegetation and litterfall was estimated by multiplying their measured biomass by their C concentr ation. Contents of mineral SOC and LFOC per unit area for each horizon were estimated by multiplyi ngmean C concentratio ns by bulk density and appropriate soil vol- ume. Storage of SOC and LFOC in the 0–100 cm profile was the sum of their contents for each horizon.

Bare land was considered as the base-line reference that repre- sented the P. massonian a ecosystem before afforestation . Thus, the annual ecosystem (biomass + soil) C accumulation rate in the 24- year-old P. massoniana plantatio n was calculated as the differenc ein ecosystem (biomass + soil) C storage between PM and BL, divided by the recovery time (24 years).

2.6. Statistical analyses

The significance of differences in ecosystem C pools among the land use types was determined using ANOVA. Before analysis, all

variables were checked for normality (Kolmogorov–Smirnov test).A limitation of this study, as with other paired-si te studies, is the lack of true spatial replicatio n. Such replication is very difficult toachieve in studies such as this. The Least Significant Difference test (LSD) was used to separate the means when ANOVA showed differences to be significant. Statistical significance was established atthe 5% level, unless otherwise mentioned. All statistical analyses were performed using SPSS 13.0 software (SPSS Inc., 2004 ).

3. Results

3.1. Ecosystem C pools and accumulation rate

Total biomass C storage in PM (91.9 ± 4.8 Mg ha�1)(mean ± S.D.) was slightly lower than that in SF (98.2 ±5.5 Mg C ha–1) (Table 2, p = 0.137). Carbon storage in stem wood,root, understory and litter in PM was significantly lower than inSF, while branch and leaf C pools in PM were significantly higher than SF (p < 0.05). Stem wood accounted for 58.5% of the total biomass C pool in PM, compared to 62.6% in SF. The C pools in the understo ry vegetation and litter compartme nts were very small in both forests (average 5.0 and 2.4% of total biomass C pools).

Significantly higher organic C storage in the soil to a depth of100 cm was found in SF than BL and PM (p < 0.001). SOC storage of the mineral soil (0–100 cm) was only 13.0 ± 1.3 Mg C ha�1 inBL (Table 2).

Ecosyste m (i.e. total biomass + soil) C pools were 13.0 ± 1.3,130.1 ± 7.2 and 166.7 ± 7.0 Mg C ha�1 in BL, PM and SF, respectively (Table 2). In the forest ecosystems, most of the C pool was in the biomass (PM: 70.6%; SF: 58.9%). The C pools in the ecosystem and soil of PM were 117.1 ± 6.0 and 25.2 ± 2.15 Mg ha�1 greater,respectively , than in BL, however these pools in PM were still significantly lower than those in SF (Table 2, p < 0.01).

The mean annual ecosystem C accumulation rate of bare land afforested with P. massoniana was 4.88 ± 0.25 Mg C ha�1 yr�1

(Fig. 1). The rate of C accumulate d in the soil (1.05 ±0.09 Mg C ha�1 yr�1) was significantly slower than the rate of Cstored in total biomass (3.83 ± 0.16 Mg C ha�1 yr�1) (Fig. 1,p < 0.001).

3.2. Changes in soil C stocks with depth

Concentr ations of soil organic C decreased with depth, with relatively low concentratio ns below 40 cm (Fig. 2). In BL, the SOC concentration of the mineral soil (0–100 cm) was low, ranging from 1.2 to 2.0 g kg�1 (Fig. 2). The organic C concentr ations above 40 cm in PM were significantly higher than those in BL (Fig. 2,p < 0.01). Concentrati ons of SOC at all soil depths were significantlyhigher in SF than in BL (p < 0.01), while significantly higher SOC

0.0

1.0

2.0

3.0

4.0

5.0

6.0

aboveground soil ecosystem

C a

ccum

ulat

ion

rate

(M

g C

ha-1

yr-1

)

Fig. 1. Carbon accumulation rate in a 24-year-old Pinus massoniana plantation. Bars indicate ±S.D., n = 3.

0

5

10

15

20

25

30

0-5 5-10 10-20 20-40 40-60 60-80 80-100Soil depth(cm)

Con

cent

ratio

n of

SO

C (

g C

kg-1

) SF

PMBL

a

b

c

a

c

b

a

b

c

a

b

c

a

b ba

ab b a ab b

Fig. 2. Soil organic carbon (SOC) concentration in a bare land (BL), a 24-year-old Pinus massoniana plantation (PM) and a secondary forest (SF). Bars that share the different letter for each soil depth are significantly different among land use types (p < 0.05). Bars indicate ±S.D., n = 3. Table 4

Dry weight of light fraction (LF), light fraction organic carbon (LFOC) concentration and storages of soil in a bare land (BL), a 24-year-old Pinus massoniana plantation (PM) and a secondary forest (SF).

Soil depth(cm) BL PM SF

Dry weight of LF (g kg�1 soil)0–5 0.14 (0.04) a 27.38 (1.01) b 14.15 (2.48) c5–10 0.08 (0.03) a 7.14 (0.42) b 7.02 (1.04) b10–20 0.05 (0.01) a 3.37 (0.85) b 3.90 (0.27) b20–40 0.03 (0.01) a 1.19 (0.25) b 1.82 (0.20) c40–60 0.00 (0.00) a 0.25 (0.10) b 0.32 (0.05) b

LFOC concentration (g C kg�1 LF)0–5 227.5 (8.9) a 322.5 (18.2) b 327.9 (17.5) b5–10 277.8 (7.4) a 316.9 (19.1) b 335.0 (20.4) b10–20 263.2 (3.2) a 314.3 (15.6) b 338.1 (16.7) b20–40 254.3 (3.4) a 318.0 (13.4) b 331.4 (15.8) b


concentratio n in SF than in PM occurred at the depths of 0–5, 5–10,10–20, 20–40 and 40–60 cm (Fig. 2, p < 0.01).

BL had significantly lower organic C storage at the soil depth of0–100 cm than PM and SF, and the organic C storage at 0–100 cmdepth in PM was significantly lower than that in SF (Table 3,p < 0.001). About 70% of the organic C to 1 m depth was stored inthe top 40 cm in the two forests. The organic C storage above 80 cm in all soil layers in SF was significantly higher than in PM(Table 3, p < 0.05). More specifically, SOC storage at 0–20 cm and 20–100 depths was 17.9 and 7.3 Mg ha�1 higher, respectivel y, inPM than in BL. These values made up 71% and 29% of the total increase of SOC in the whole soil of afforested bare land.

Table 3Soil organic carbon (SOC) storage (Mg C ha�1) in a bare land (BL), a 24-year-old Pinus massoniana plantation (PM) and a second ary forest (SF).


0–5 0.8 (0.1) a 8.2 (0.3) b 10.6 (0.2) c5–10 0.8 (0.1) a 4.9 (0.4) b 8.4 (0.2) c10–20 1.6 (0.2) a 8.0 (0.6) b 14.0 (0.4) c20–40 2.5 (0.3) a 6.2 (0.9) b 16.7 (0.8) c40–60 2.4 (0.2) a 3.6 (0.5) b 9.9 (0.4) c60–80 2.4 (0.3) a 3.6 (0.4) b 4.9 (0.3) c80–100 2.5 (0.3) a 3.7 (0.4) b 4.0 (0.2) bTotal 13.0 (1.3) a 38.2 (3.4) b 68.5 (2.5) c


3.3. Soil organic C fractions

The dry weight of LF and the LFOC concentration and storage atdifferent soil depths in PM and SF were significantly higher than inBL (Table 4, p < 0.05). Significant differences in dry weight and LFOC storage between PM and SF were also detected at 0–5 and 20–40 cm depths (p < 0.05). In the 0–5 cm layer, the dry weight and LFOC storage in PM were 1.9 times and 1.7 times higher than in SF, respectively . However, in the 20–40 cm layer significantlylower dry weight and LFOC storage were found in PM than in SF(p < 0.05). There was no significant difference in LFOC concentr a-tion between PM and SF at any soil depth.

The differences in the proportion of LFOC to SOC among the three land use types were significant for all soil depths (p < 0.001). As a general trend, the proportio n of LFOC to SOC decreased in the order of PM > SF > BL (Table 4).

4. Discussion

4.1. Ecosystem C accumula tion

The average C storage of forest vegetation of the major Chinese forest types is 57.07 Mg ha�1 (Zhou et al., 2000 ), and the average Cstorage of over-mat ure P. massoniana forest (>80 years) is62.44 Mg ha�1 (Wang and Feng, 2000 ). The C stocks of the forests in the present study are higher than the above averages. However,the soil C stocks in the forest soils of this study (PM: 38.2 Mg ha�1;SF: 68.5 Mg ha�1) are much lower than the average soil C stock inthe red soil region (122.8 Mg ha�1) (Wang and Zhou, 1999 ). Lower C storage in the studied forest soils might result from younger forest age, short restoration time and extremely low C storage in the previous land use.

The dynamics of SOC in forest soils has been investigated in various areas because the considerable SOC storage in forest soils makes

40–60 0.0 (0.0) 310.3 (10.5) b 328.6 (13.3) b

LFOC storages (Mg C ha�1)0–5 0.02 (0.01) a 3.13 (0.18) b 1.80 (0.24) c5–10 0.01 (0.00) a 0.91 (0.13) b 0.91 (0.12) b10–20 0.01 (0.00) a 0.94 (0.18) b 1.06 (0.06) b20–40 0.01 (0.00) a 0.73 (0.20) b 1.19 (0.16) c40–60 0.00 (0.00) a 0.15 (0.02) b 0.22 (0.04) bTotal 0.05 (0.01) a 5.86 (0.51) b 5.17 (0.59) b

LFOC/SOC (%)0–5 1.84 (0.09) a 38.28 (0.71) b 17.02 (1.98) c5–10 1.30 (0.24) a 18.52 (2.73) b 10.77 (1.17) c10–20 0.79 (0.24) a 11.73 (2.08) b 7.54 (0.24) c20–40 0.59 (0.05) a 11.83 (1.53) b 7.13 (0.74) c40–60 0.00 (0.00) a 4.24 (0.12) b 2.20 (0.28) cTotal 0.67 (0.07) a 18.97 (0.77) b 8.68 (0.77) c


Table 5Fine root (<5 mm) biomass (g m�2) in a bare land (BL), a 24-year-old Pinus massoniana plantation (PM) and a secondary forest (SF).


0–10 No root 368.3 (283.2) a 444.0 (379.6)a10–20 184.1 (112.5) a 165.7 (112.1)a20–40 163.7 (121.1) a 106.0 (160.0)a40–60 58.8 (62.2) a 57.2 (54.0)a60–80 65.4 (72.6) a 34.2 (45.9)aTotal 840.3 (405.9) a 807.2 (615.7)a



them an important component of the global C cycle (IPCC, 2001 ).Large variations have been reported in the direction and amount of SOC stock change after afforestati on (Guo and Gifford, 2002 ). Ashypothesize d, amounts of C (Table 2) stored differed greatly between PM and BL. Lugo et al. (1988) estimated that aboveground Caccumulation can range from as little as 0.8 to as much as15 Mg C ha�1 yr�1 during the first two decades of plantation establishment. In our study, the plantation accumulated aboveground Cat a rate of 3.8 Mg ha�1 yr�1 since establishment in the 1980s. This rate is higher than rates reported for young (0–20 years) secondary forests (Brown and Lugo, 1990 ), and similar to rates measure d inyoung tropical plantatio ns (Brown et al., 1986 ). The average soil Caccumulation rate at the 0–20 cm depth in PM was estimate d tobe 0.74 Mg C ha�1 yr�1 for 24 years. On a global scale, the C accumulation rates in surface soils (0–30 cm) after afforestation reported previously are lower than those in this study (0.34 Mg C ha�1 yr�1

in Post and Kwon, 2000 ; 0.14 Mg C ha�1 yr�1 in Paul et al., 2002 ).In temperate regions, the C accumulation rate of surface (0–30 cm)soils after afforestation or secondar y successions after old agricultural fields and pastures ranges from �0.85 to 0.60 Mg C ha�1 yr�1

(Six et al., 2002; Hooker and Compton, 2003; Paul et al., 2003; Peichl and Arain, 2006; Ussiri et al., 2006; Alberti et al., 2008 ). In other cli- matic regions, soil C accumulation rates (usually 0–30 cm) range from very low values to 0.54 Mg C ha�1 yr�1 in tropical areas (Giar-dina et al., 2004; Silver et al., 2004; Lima et al., 2006 ) and from �0.40to 0.86 Mg C ha�1 yr�1 in boreal regions (Tremblay et al., 2006; Oui- met et al., 2007 ). Thus the soil C accumulation rate observed in the present study was high compared to previously measured values.

Post and Kwon (2000) noted that the following factors and processes are important in increasing SOC stocks: elevating the input rates of organic matter, changing the decompo sability of organic matter to increase light fraction SOC content, enhancing physical protection, and direct and indirect C transportation into deeper soil. In our study, BL had extremely low soil nutrient and C contents due to severe soil and water loss (Table 1). Successful restoration often requires bold and innovative management to disrupt the feedbacks that lead to long-term, sustained degradation and to mitigate the constraints imposed by abiotic conditions in the degraded system (McVicar et al., 2010 ). At our sites, this involved implementi ng aggressive artificial restoration measures including planting of P. massoniana and shrubs, combined with the addition of organic matter and fertiliser to restore soil fertility. P. massoniana is a good tree species for afforestati on on eroded soils due toits drought resistance and ability to grow on nutrient poor soils,especially in association with ectomycor rhizal fungi (Xie et al.,2008), and wide adaptabi lity to difficult environments. Also, ahigher C/N ratio in P. massoniana litter compared to secondary forest litter has been observed (Yang et al., 2005b ). Supply of organic matter with a higher C/N ratio might mitigate the intensive decom- position that occurs in secondary forest, which may lead to increased SOC stocks after afforestati on. Further, total root biomass in PM was slightly greater than that in SF (Table 5), which might decrease soil erosion due to binding of soil particles and enhancin gsoil porosity and water infiltration capability (De Baets et al., 2006;Yun et al., 2006 ). All of these factors may have contributed to the relatively high soil C accumulation rate observed in this study.

4.2. C accumulation in deep soil

The importance of soil C storage in deeper layers has attracted the attention of many researchers as high proportions of total Cstored within the soil profile may be found in subsoil horizons despite low C concentr ations (Batjes, 1996; Jobbagy and Jackson,2000; Salomé et al., 2010; Rumpel and Kögel-Knabner, 2011 ). A re- cent study suggests that in the northern circumpolar permafrost region, at least 61% of the total soil C is stored below 30 cm depth

(Tarnocai et al., 2009 ). Therefore, subsoil C may be even more important in terms of a source or sink for CO2 than topsoil C. The P. massoniana plantation in our study accumulate d SOC at a rate of 74.6 g m�2 yr�1 in the upper soil (0–20 cm), but accumulation in the deeper layers (20–100 cm) was also significant(30.4 g m�2 yr�1) (R2 = 0.964, p < 0.001), and about 9.6% of SOC accumulation occurred at the 60–100 depth. Various mechanism sfor increased SOC stocks in deep layers following afforestation have been discussed. Root biomass accumulation is an important factor in C sequestra tion following afforestation (Ussiri et al.,2006). The plantation in this study had only been established for 24 years, but potentially large changes in the quantity and quality of C inputs to the soil had occurred. The data for the entire soil profile suggest that C accumulate d throughout the entire 1 m depth,and the source of this C does appear to be directly derived from root C inputs as the actual and average depths of the root systems of P. massoniana are 121 cm and 60 cm respectively (Editorial Com- mittee of Flora of China, 1978 ). The distribution of plant roots can directly affect vertical distribut ion of SOC (Jobbagy and Jackson,2000), due to the large quantity of root excretion and dead roots,which could supply abundant C to the soil by microbial conversion.Previous studies have shown that 30–40% of the total input of SOC was derived from root excretion and inputs from dead roots (Leeand Pankhurst, 1992 ). Despite the importance of roots as a subsoil C source, root C flux to soil is poorly understood mainly due touncertainties associate d with the measure ment of total root C input, in particular from root exudation and root cell sloughing. Inaddition to the direct input of C from roots at depth, C deeper inthe soil profile may come from the vertical transport of dissolved organic C (Kalbitz et al., 2000; Schwendenmann and Veldkamp,2005). The process of dissolved organic C movement and retention within the mineral soil was found to be responsible for 20% of the total mineral soil C stock to 1 m depth in a Californian forest soil (Sanderm an and Amundson, 2008 ).

Although a large proportion of the C accumulation occurred inthe top 20 cm of soil in the present study (Table 3), our cumulative data for the 1-m depth suggest that the soil C pools deeper in the soil may reflect a positive response to land use change. This trend is consistent with other studies, which have shown that land use changes affect C stocks down to 1 m (Guo and Gifford, 2002; Vel- dkamp et al., 2003 ). However, it cannot be estimate d whether such C sequestration has already reached the maximum level orwhether it is continuing. To address these critical questions , there is a clear need to carry out long-term studies on the effects of afforestation on C storage in severely eroded areas. Such studies should include examina tion of deep soil layers to better understa ndundergro und C processes.

4.3. C accumulation in light fraction organic matter

Land use change can affect the distribution of different soil Cfractions (Yang et al., 2009 ). As a main component of soil organic C, the light fraction organic C pool is mostly derived from


freshly-added organic materials (e.g., plant and animal residues)that remain un-decomp osed over a short period of time (Muelleret al., 1998 ). Light fraction organic C is more labile than gross organic C, and is a sensitive indicator of changes in SOC induced byland managemen t and environmental stresses (Malhi et al.,2003). In our study, a significant change in LFOC resulting from afforestation was observed. The LFOC storage in the 0–60 cm depth ranged from 0.05 to 5.86 Mg ha�1 (a 117-fold difference) among the three land uses, while SOC storage ranged from 8.1 to59.6 Mg ha�1 (a 7-fold difference). This difference demonstrates the desirability of understanding the impacts of the LFOC pool onsoil quality under different land uses. For this study, it was hypoth- esized that the recovery of bare land through afforestation would alter the size of soil labile C pools. As hypothes ized, the amount of LFOC in the 0–60 cm depth in PM was significantly higher than in BL (Table 4). Also, a significantly higher LFOC stock was found inthe topsoil (0–5 cm) in PM than in SF. In addition to differences inmicroclimat ic conditions, explanation s given for the difference between PM and SF in LFOC have included differences in the ground vegetation cover, and quantity and quality of organic matter inputs to soils (Xie et al., 2008 ). Furthermore, slow litter decay rates in the 24-year-old P. massonian a plantatio n, because of high C/N ratio,would also contribute to the replenishme nt of the LFOC.

5. Conclusions

Our results reveal that significant and rapid above and below ground C accumulation occurs during afforestati on of bare land with P. massoniana . We also found deep soil layers (20–100 cm)can accumulate SOC significantly (30.4 g m�2 yr�1) after afforestation. The increase in the LFOC pool following afforestation of the bare land was found to be more pronounced than the increase inthe SOC pool. The LFOC in soil is a relatively sensitive indicator of future change in SOC for restoration of degraded soils. However ,because plots were localized in the same stand, the experiment was not properly replicated in space, and differences between the PM and two reference types of land (BL and SF) should not be assigned to differences in land use without caution. Also, the present study is a case study at a point in time 24 years following planting, and future studies incorporating spatially replicated observations in other ecosystems are required to further support these findings.

Acknowled gements

We are grateful to Mr. Davis M.R. from New Zealand Forest Re- search Institute Limited (Scion) and two anonymous reviewers for their constructive comments and suggestions . The Research was funded by Program for Changjiang Scholars and Innovative Re- search Team in University (No. IRT0960), Key Program of National Natural Science Foundation of China (No. 31130013), National Key Basic Research Program of China (No. 2012CB72220 3), Key Project of Chinese Ministry of Education and Fujian Province Education Department (No. 211083 and JA10063), Natural Science Founda- tion of Fujian Province (2009J01122).

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