change of soil organic carbon after cropland afforestation in ‘beijing-tianjin sandstorm source...

Chin. Geogra. Sci. 2014 Vol. 24 No. 4 pp. 461–470 Springer Science Press

doi: 10.1007/s11769-014-0701-6 www.springerlink.com/content/1002-0063

Received date: 2013-10-16; accepted date: 2014-02-21 Foundation item: Under the auspices of Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA05060600),

Knowledge Innovation Programs of Chinese Academy of Sciences (No. KSCX2-EW-J-5), National Key Technology Research and Development Program of China (No. 2011BAD31B02)

Corresponding author: ZHANG Wanjun. E-mail: [email protected] © Science Press, Northeast Institute of Geography and Agroecology, CAS and Springer-Verlag Berlin Heidelberg 2014

Change of Soil Organic Carbon after Cropland Afforestation in ′Beijing- Tianjin Sandstorm Source Control′ Program Area in China

ZENG Xinhua1, 2, ZHANG Wanjun1, LIU Xiuping1, CAO Jiansheng1, SHEN Huitao1, ZHAO Xin1, 2, ZHANG Nan-nan1, 2, BAI Yuru3, Yi Mei3

(1. Key Laboratory of Agricultural Water Resources, Hebei Key Laboratory of Agricultural Water-Saving, Center for Agricultural Re-sources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. Forestry Research Institute of Chifeng City, Chifeng 024006, China)

Abstract: Land use change is one of the major factors that affect soil organic carbon (SOC) variation and global carbon balance. How-

ever, the effects of land use change on SOC are always variable. In this study, using a series of paired-field experiments, we estimated

the effects of revegetation types and environmental conditions on SOC stock and vertical distribution after replacement of cropland with

poplar (Populus tomentosa) and korshinsk peashrub (Caragana korshinskii) in three climate regions (Chifeng City, Fengning City and

Datong City of the ′Beijing-Tianjin Sandstorm Source Control′ (BTSSC) program area. The results show that SOC sequestration rate

ranges from 0.15 Mg/(ha·yr) to 3.76 Mg/(ha·yr) in the soil layer of 0–100 cm in early stage after cropland afforestation in the BTSSC

program area. The SOC accumulation rates are the highest in Fengning for both the two vegetation types. Compared to C. korshinskii, P.

tomentosa has greater effects on SOC accumulation in the three climate regions, but significantly greater effect only appears in Datong.

The SOC density increases by 20%–111% and 15%–59% for P. tomentosa and 9%–63% and 0–73% for C. korshinskii in the 0–20 cm

and 20–100 cm soil layers, respectively. Our results indicate that cropland afforestation not only affects SOC stock in the topsoil, but

also has some effects on subsoil carbon. However, the effect of cropland afforestation on SOC accumulation varied with climate regions

and revegetation types. Considering the large area of revegetation and relatively high SOC accumulation rate, SOC sequestration in the

BTSSC program should contribute significantly to decrease the CO2 concentration in the atmosphere.

Keywords: soil organic carbon (SOC); cropland afforestation; soil profile; carbon sequestration; Beijing-Tianjin Sandstorm Source

Control program

Citation: Zeng Xinhua, Zhang Wanjun, Liu Xiuping, Cao Jiansheng, Shen Huitao, Zhao Xin, Zhang Nannan, Bai Yuru, Yi Mei, 2014.

Change of soil organic carbon after cropland afforestation in ′Beijing-Tianjin Sandstorm Source Control′ program area in China. Chi-

nese Geographical Science, 24(4): 461–470. doi: 10.1007/s11769-014-0701-6

1 Introduction

Pedosphere plays an important role in the global carbon cycle because of its large carbon stock, which stores nearly 1500 Pg of carbon (C) as soil organic carbon (SOC) in the first meter of depth (Jobbágy and Jackson,

2000; Hiederer and Kochy, 2011). This large pool of SOC can actively respond to environmental changes such as land use change (de Koning et al., 2003). Land use change is widely considered to be one of the major factors that affect SOC variation and global carbon bal-ance (IPCC, 2000; Chen et al., 2007; Lal, 2008). How-

462 Chinese Geographical Science 2014 Vol. 24 No. 4

ever, there is no consensus about the change of SOC after the transformation from cropland to forest. Some researchers insist that tree plantations store large amounts of carbon in their biomass, but decrease SOC due to the larger heterotrophic respiration (Guo and Gifford, 2002; Berthrong et al., 2009). In contrast, other studies show that soil can sequester large amounts of organic carbon with fairly high SOC accumulation rate during forest development (Morris et al., 2007; Zhang et al., 2010). These contradictory results might be ascribed to the effects of revegetation types, former land use pat-tern, stand age, tree density and environmental condi-tions, including temperature, precipitation, and soil at-tributes (Paul et al., 2002; Cerri et al., 2004; Eclesia et al., 2012).

Vegetation type is an important factor influencing SOC contents and stabilization (Wei et al., 2010; Eclesia et al., 2012). Jobbagy and Jackson (2000) found that vegetation type could change the rates of primary pro-ductivity, biomass allocation and vertical root distribu-tions, thus affect the nutrient cycling, organic matter fractions and carbon storage. Besides vegetation type, climate, soil attributes and stand age also affect SOC contents and stocks. Needelman et al. (1999) found that the SOC contents increased linearly with clay content in Illinois, USA. Amundson (2001) observed that SOC increased with increasing mean annual precipitation (MAP), but SOC mean residence time decreased with higher MAP and mean annual temperature. Guo and Gifford (2002) observed increases in SOC contents with increasing age in plantations. In addition, the interac-tions among soil texture, stand age and climate condi-tions might also determine regional patterns of SOC accumulation (Berthrong et al., 2012). However, few studies have analyzed the synthesized effects of land use change on the quantity, quality, and vertical distribution of SOC regionally, considering the effects of climate or vegetation type (Kirschbaum et al., 2008). The Bei-jing-Tianjin sandstorm source area contains a variety of climate regions and land use changes, and is a good place to study the effects of climate and vegetation type on the dynamic of SOC after afforestation.

The Beijing-Tianjin sandstorm source area is known as an area that is suffering from serious wind erosion because of the special topography, climate condition and underlying surface with abundant sands. Strong wind erosion causes significant declines in stocks of soil car-

bon and nutrients, resulting in soil fertility decline and soil degradation (Pimentel et al., 1995; Jacinthe and Lal, 2001). In order to prevent strong wind erosion and pro-tect the ecological security of Beijing, Tianjin and sur-rounding areas, the China′s central government launched an ecological program named ′Beijing-Tianjin Sandstorm Source Control′ (BTSSC) program in this

area. The program has harnessed a land area of 1.8 107

ha, including approximately 2.6 106 ha of cropland during the period 2000–2010. Although the effects on water conservation, wind shelter and sand fixation have been studied in this area (Deng, 2007; Luo, 2008; Yan, 2010), few studies have been focused on the dynamic of soil carbon storage. Additionally, the comparative study of the effect of carbon sequestration among different climate regions has not been reported in the previous studies.

Thus, the main objectives of this study were: 1) to es-timate the effects of afforestation on soil carbon seques-tration with different vegetation types, 2) to compare the effects of afforestation on SOC in different climate re-gions and 3) to detect the profile distribution of soil carbon after cropland afforestation in the BTSSC pro-gram area.

2 Materials and Methods

2.1 Study area All study sites (Chifeng City, Fengning City and Datong City) are located in the BTSSC program area, which are designed to represent the Otingdag desertification gov-ernance area, water source protected area of the Yanshan Mountain and the desertification governance area of farming-pastoral, respectively (Fig. 1). Chifeng is in the temperate semi-arid continental monsoon climate region with average annual temperature range of 0℃–7℃ and mean annual precipitation of 300–500 mm. The natural vegetation mainly consists of sandy shrubs and pasture. The soil is mainly Kastanozems, Aeolian soil and Brown calcic soil. Fengning is in the temperate semi- humid and semi-arid continental monsoon climate re-gion with average annual temperature range of 0.8℃–6.6℃ and mean annual precipitation of 450– 600 mm. The natural vegetation mainly consists of co-niferous trees, deciduous trees and grassy marshland. The soil is mainly Brown soil, Cinnamon soil and Kas-tanozems. Datong is in the semi-arid continental climate

ZENG Xinhua et al. Change of Soil Organic Carbon after Cropland Afforestation in ′Beijing-Tianjin Sandstorm Source… 463

Fig. 1 Location of study sites and distribution of sampling plots in study area

region with average annual temperature range of 7.7℃– 9.8℃ and mean annual precipitation of 380–430 mm, with 64% occurring between June and August. The natural vegetation are mainly Caragana korshinskii, Populus tomentosa, Stipa baicalensis and Betula platy-phylla. The soil is generally Kastanozems and Brown calcic soil.

2.2 Field investigation, soil sampling and labora-tory analysis At each site, three adjacent land use patterns (conven-tional cropland, P. tomentosa stand and C. Korshinskii stand) were selected in August 2011 (Fengning and Da-tong) and 2012 (Chifeng). The conventional croplands were used as control stands to evaluate the effect of af-forestation on SOC. To minimize the differences among the plantation and control stands, the control stands were within 200 m from P. tomentosa stands with simi-lar elevations and aspects, and had the same soil type and agricultural management as the sites before affor-ested with P. tomentosa and C. korshinskii. Agricultural management in this region has been changed since the 2000s, so the SOC contents in conventional croplands are considered to be essentially unchanged as well. In Fengning and Datong, the sites were historical cropland with long-term cultivation of wheat (Triticum aestivum) and maize (Zea mays). P. tomentosa and C. korshinskii

were established on cropland in 2002, 9 years prior to the present study. In Chifeng, the site was historical cropland dominated by Z. mays and millet (Setaria italic), and P. tomentosa and C. korshinskii were estab-lished on cropland in 2000 and 2003, 12 and 9 years prior to the present study, respectively. The trees in each site were planted directly following conventional tillage, and no management practices were carried out after af-forestation.

For each land use pattern, three plots (30 m × 30 m for P. tomentosa, 5 m × 5 m for C. korshinskii and crop-land) were established. At each plot in P. tomentosa stand and C. korshinskii stand, the height (H), diameter and canopy area (CA) of each tree were measured. The diameter parameters included diameter at breast height (DBRH, at 1.35 m) for P. tomentosa and diameter at basal height (DBAH, at 0.1 m) for C. korshinskii. Total cover-age and canopy density were also estimated (Table 1).

In each sampling plot, a pit of 1 m long × 1 m wide × 1 m deep was dug out at the central area. After exclud-ing recognizable soil surface litter, several stainless steel cutting rings (5.05 cm diameter by 5.0 cm height) were used to sample 100 cm3 of soil at each layer at depth intervals of 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm and 60–100 cm. Based on this method, a total 15 soil samples (5 soil layers × 3 plots) were collected at each afforestation stand and cropland. The soil samples were


Table 1 Characteristics of investigated afforested tree species and site conditions in ′Beijing-Tianjin Sandstorm Source Control′ (BTSSC) program in China

Chifeng Fengning Datong

Forest species P. tomentosa C. korshinskii P. tomentosa C. korshinskii P. tomentosa C. korshinskii

Location 42°22′N, 120°46′E 42°32′N, 120°45′E 41°10′N, 116°45′E 41°17′N, 116°52′E 39°44′N, 113°36′E 40°06′N, 113°37′E

Altitude (m) 408 417 600 614 1115 1115

Stand age (year) 12 9 9 9 9 9

Density (tree/ha) 400 1667 1000 1667 400 3333

Canopy density 0.20 0.60 0.50 0.85 0.35 0.50

Soil type Kastanozems Kastanozems Brown soil Brown soil Kastanozems Kastanozems

Tree height (m) 9.6 1.9 15.7 2.4 5.6 1.3

Diameter (m) 0.110 0.024 0.143 0.032 0.095 0.019

Canopy area (m) 1.6 × 1.8 1.4 × 1.1 1.9 × 2.1 1.6 × 1.3 1.7 × 2.2 1.2 × 0.8

Notes: Diameter for P. tomentosa is diameter at breast height (1.35 m), for C. korshinskii.is diameter at basal height (0.1 m)

scraped out and roots were manually removed. The soils were dried at 105℃ to constant weight to calculate bulk density. Other four representative soil profiles were lo-cated at the four corners of each plot for composite soil analysis. Soil samples were collected using 5 cm di-ameter tube auger within 0–10 cm, 10–20 cm, 20– 40 cm, 40–60 cm and 60–100 cm depths. Finally, a layer-by-layer mixing of the five soil layer samples from the four corners of each plot was done to build a com-posite soil sample. A relatively small fraction of the soil samples was dried at 105℃ to a constant weight and then used to determine soil water content. The remain-ing soil was air-dried, ground and passed through a 0.25 mm soil sieve prior to laboratory analysis. Then SOC was measured using the Walkley-Black method (Nelson and Sommers, 1982).

2.3 Data calculation and analysis Density of SOC was calculated as follows:

g1(1 ) /10

n

i i ii

SOCD V BD SOC T (1)

where SOCD is SOC density (Mg/ha), and Ti, BDi, SOCi and Vg are, soil thickness (cm), bulk density (g/cm3), soil organic carbon (g/kg) and volume percentage of the gravel, > 2 mm at the ith layer, respectively.

seq stand ckC SOCD SOCD (2)

where SOCDstand and SOCDck are SOC densities of the plantation stands and control stands, respectively (Mg/ha), and Cseq is the difference of SOC density be-tween plantation stand and control stand (Mg/ha).

The differences in SOC and soil bulk density between the afforestation stands and control stands were due to the years of afforestation development. The mean an-nual change in SOCD was calculated by dividing the total change in SOCD by the afforested stand age. All statistical analyses were performed using SPSS software (SPSS 16.0 for windows, SPSS Inc., Chicago, IL, USA). Before analysis, all variables were checked for normal distribution (Kolmogorov–Smirnov test) and homoge-neity (Levene test). Two-way ANOVA was used to test the effects of revegetation type, climate region and their interaction on SOCD change rates after afforestation. One-way ANOVA was used to test the significant dif-ferences in SOCD change rates between the two vegeta-tion types in each region and among the three climate regions for each species. One-way ANOVA with Dun-can′s multiple range test was also used to test the significant differences in SOC content among the affor-estation and control stands. The significant level was set at p < 0.05. All the graphs were plotted in SigmaPlot 11.0 and Origin 8.5 softwares.

3 Results

3.1 SOC content, density and soil bulk density along soil profile The average values of SOC content generally decreased with increasing soil depth across the three sites (Fig. 2). However, the distribution of SOC content along the soil profile varied with revegetation types and climate re-gions. The contents of SOC in the 0–10 and 10–20 cm soil layers under P. tomentosa were significantly higher


Fig. 2 SOC content monitored at 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm and 60–100 cm soil layers for P. tomentosa (POT) stands, C. korshinskii (CAK) stands and control (CK) stands in Chifeng, Fengning and Datong of BTSSC program in China

than those under C. korshinskii in Fengning and Datong (p < 0.05) but only slightly higher in Chifeng (p > 0.05). In Fengning and Datong, the change rate of SOC content under P. tomentosa varied from 0.14 g/(kg·yr) to 0.57 g/(kg·yr) and from 0.02 g/(kg·yr) to 0.59 g/(kg·yr) across the five soil layers respectively, which were broader than the variations under C. korshinskii (from 0.10 g/(kg·yr) to 0.39 g/(kg·yr) and from 0.00 g/(kg·yr) to 0.09 g/(kg·yr), respectively). But in Chifeng, the change rate of SOC content varied from 0.03 g/(kg·yr) to 0.16 g/(kg·yr) under P. tomentosa, more narrow than that under C. korshinskii (–0.02–0.25 g/(kg·yr)). The SOC content in the entire 0– 100 cm soil profile increased across all the afforestation stands, with higher increments in topsoil layers (0–20 cm) and lower increments in deeper soil layers (20–100 cm).

The distribution of SOC density along the soil profile also varied with revegetation types and climate regions (Fig. 3). In Fengning, the SOC densities in the five soil layers of the P. tomentosa and C. korshinskii stands were all significantly higher than the control stand (p < 0.05). The densities of SOC in the 0–10 cm and 10–20 cm soil layers under P. tomentosa were 13.21 Mg/ha and 11.26 Mg/ha, respectively, which were 33.24% and 20.68% higher than those under C. korshinskii (9.91 and 9.33 Mg/ha, respectively). However, the densities of SOC in the 20–40 cm and 40–60 cm soil layers under P. tomen-tosa (17.25 Mg/ha and 15.11 Mg/ha, respectively) were 11.49% and 26.30% lower than those under C. korshin-skii (19.23 Mg/ha and 19.08 Mg/ha, respectively). Simi-lar to Fengning, the SOC densities in the five soil layers

under P. tomentosa and C. korshinskii in Chifeng were all significantly higher than those under control stand (p < 0.05). In Datong, only the densities of SOC in the 0–10 and 60–100 cm soil layers under P. tomentosa were significantly higher than those under control stand (p < 0.05) (Fig. 3).

Soil bulk density increased with increasing soil depth in the afforestation and control stands. Regardless of climate regions, soil bulk density decreased in the over-all 0–100 cm soil profile after cropland afforestation, with higher decrements under P. tomentosa than under C. korshinskii. Meanwhile, the decrements of soil bulk density decreased with increasing soil depth, with larger decrement in the surface layer (0–20 cm) than in the deeper layer (20–100 cm) (Fig. 4).

3.2 Response of SOC density to revegetation type and climate condition The results of Two-way ANOVA revealed that revege-tation type, climate region and their interaction signifi-cantly affected the change rates of SOC density (p < 0.05), and climate region was the main factor influenc-ing SOC sequestration (Table 2).

The effects of climate condition on SOC accumula-tion varied with revegetation type (Fig. 5). For P. to-mentosa, the change rate of SOC density in Fengning (3.76 Mg/(ha·yr)) was significantly higher than that in Datong and Chifeng (1.47 Mg/(ha·yr) and 1.28 Mg/(ha·yr), respectively) (p < 0.05), while no significant difference was found between the latter two regions


Fig. 3 SOC density (SOCD) monitored at 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm and 60–100 cm soil layers for P. tomentosa (POT) stands, C. korshinskii (CAK) stands and control (CK) stands in Chifeng, Fengning and Datong of BTSSC program in China

Fig. 4 Soil bulk density monitored at 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm and 60–100 cm soil layers for P. tomentosa (POT) stands, C. korshinskii (CAK) stands and control (CK) stands in Chifeng, Fengning and Datong of BTSSC program in China

Table 2 Effects of climate region, revegetation type and their interaction (two-way ANOVA) on change rate of SOC density

Source of variation F value p

Climate region (C) 4.81 × 103 ***

Revegetation type (V) 181.85 ***

C × V 205.94 ***

Note: *** represents significant effect of p < 0.001

(p > 0.05). For C. korshinskii, the change rates of SOC density were in the order of Fengning > Chifeng > Da-tong (at the rates of 3.67 Mg/(ha·yr), 1.19 Mg/(ha·yr) and 0.15 Mg/(ha·yr), respectively), with marked differ-ences among the three regions (p < 0.05).

The effects of revegetation type on SOC sequestra-

tion also varied with climate condition (Figs. 5 and 6). In Fengning, the change rates of SOC density were nearly equal between the P. tomentosa stand and C. korshinskii stand, at average rates of 3.76 Mg/(ha·yr) and 3.67 Mg/(ha·yr) in the overall 0–100 cm soil profile, respectively. The SOC accumulation appeared in the whole 0–100 cm depth for both P. tomentosa and C. korshinskii (Fig. 6). Compared with C. korshinskii, P. tomentosa had a significant greater effect on SOC se-questration in the top 20 cm (p < 0.05), but had a weaker effect in the 20–60 cm layer. In Chifeng, there was no significant difference in the change rates of SOC density between P. tomentosa (1.28 Mg/(ha·yr)) and C. korshin-skii (1.19 Mg/(ha·yr)) in the overall 0–100 cm soil (p >


0.05), and SOC accumulation happened in all the five soil layers for both the two stands. However, the situa-tion in Datong was rather different. The change rate of SOC density under C. korshinskii (0.15 Mg/(ha·yr)) was significantly lower than that under P. tomentosa (1.47 Mg/(ha·yr)) (p < 0.05). The SOC accumulation under C. korshinskii happened in the top 40 cm, while that under P. tomentosa happened in the 0–20 cm and 40–100 cm layers (Fig. 6).

4 Discussion

4.1 Spatial variations of SOC sequestration The SOC distribution along the soil profile was found to be changed by land use conversion (Don et al., 2007; Franzluebbers and Stuedemann, 2009; Laganiere et al., 2010). In this study, SOC content decreased along the soil profile for both the two revegetation types, with higher SOC content in the 0–10 cm layer. However, the patterns of SOC variation along soil profile varied with revegetation types and climatic regions. Revegetation types and microclimates seriously affect root biomass, litter production and soil conditions, leading soil carbon sequestration spatially nonuniform (Wang et al., 2010). In Fengning, the increment of SOC density in the 0–20 cm layer under P. tomentosa was higher, while that in the 20–60 cm layer was lower than those under C. kor-shinskii. The higher increase of SOC density in the top-

soil under P. tomentosa may be associated with the higher carbon input. Due to the physiological difference of plant species, P. tomentosa has higher leaf biomass than C. korshinskii, and this contribute to higher above-ground litter production which favoring the topsoil car-bon sequestration. Meanwhile, the ratio of root to shoot and the portion of cumulative root below 20 cm depth is higher for shrubs than trees (Canadell et al., 1996;

Fig. 5 Average change rate of SOC density as influenced by different afforested types (P. tomentosa, POT; C. korshinskii, CAK) in different regions of BTSSC program in China. The error bars denote standard error of the mean (n = 9). Lowercase shows significantly different between species with the same region at 0.05 level, and uppercase shows significantly different among regions with the same species at 0.05 level

Fig. 6 Average change rate of SOC density in five different soil layers (0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm and 60–100 cm) as influenced by P. tomentosa and C. korshinskii in different regions of BTSSC program in China. The error bars denote standard error of the mean (n = 9)


Jackson et al., 1996), leading C. korshinskii allocate more photosynthetic production into deep soil layers than P. tomentosa, and fine root residue and rhizodeposition have been suggested to be the main sources of the subsoil car-bon accumulation (Rasse et al., 2005; 2006).

In Chifeng, the increments of SOC density in the top-soil (0–20 cm) under C. korshinskii were significantly higher than those under P. tomentosa (p < 0.05). It may due to the reason that C. korshinskii belongs to N-fixing species, which increases soil N by biological nitrogen fixation and then augments SOC in agricultural soils (Binkley et al., 2004). Additionally, the canopy of C. korshinskii is close to the soil surface which efficiently reduces the wind velocity near soils (Wei et al., 2009). This will increase the SOC sources and protect against soil losses by wind and water erosion. Meanwhile, dust from wind erosion which contains much more organic carbon than the soil left behind on the eroded land is usu-ally intercepted by the canopy and deposited in soils un-der the canopy (Young 1989), and this is another source of organic carbon in topsoil under C. korshinskii.

In Datong, due to the low annual precipitation (370 mm), large potential annual evaporation (1056 mm) and poor water retention capacity, soil moisture in this area was rather low which seriously limited C. korshinskii growth, because the growth of C. korshinskii under semiarid condition is seriously restricted by soil water content (Li et al., 2006; Zhang et al., 2006). A pheno-type variation investigation of C. korshinskii in North-west China and North China showed that the number of leaves was positively correlated with mean annual pre-cipitation (Song et al., 2005), suggesting that the growth of C. korshinskii in Datong was poorer than other re-gions with higher precipitation. A low amount of litter-fall production makes SOC increments rather low in topsoil under C. korshinskii. Though the C. korshinskii stand in Datong has a higher tree density than the other two regions, it has the lowest change rate of SOC den-sity in this area. Meanwhile, due to the weak effect of soil leaching, SOC increment was also low in the sub-soil. These contribute to the less SOC density accumula-tion in the overall 0–100 cm soil profile under C. kor-shinskii than under P. tomentosa.

4.2 Implication for SOC sequestration in BTSSC program Since the 1970s, a batch of important ecological envi-

ronment construction projects has been carried out in China, giving China the largest revegetation area around the world (Piao et al., 2009). The BTSSC program is one of the vital ecological projects, which plays an im-portant role in ecological environment improvement and carbon sequestration. The program has harnessed a land

area of 1.8 107 ha, of which 2.6 106 ha of cropland have been converted into tree plantation during 2000–2010. According to our study, cropland afforesta-tion significantly increased SOC in this area. However, the effect of SOC accumulation varied with climate re-gion and revegetation type. Climate region, revegetation type and their interaction significantly affected SOC accumulation in this area. Plant growth with SOC ac-cumulation rates followed the order of Fengning > Chifeng > Datong under C. korshinskii and Fengning > Datong > Chifeng under P. tomentosa in the 0–100 cm soil profile. This has important implications for land management in BTSSC program area. The serious wind-soil erosion as well as low nutrient and water availability at Datong would not favor C. korshinskii growth compared with other regions in the BTSSC pro-gram area. Our results further indicated that a combina-tion of little precipitation and serious soil degradation would impede SOC sequestration after cropland affore-station.

Based on different revegetation types and climate re-gions, we estimated that the SOC sequestration rate ranged from 0.15 Mg/(ha·yr) to 3.76 Mg/(ha·yr) in the 0–100 cm soil layer for early stage after cropland affor-estation in the BTSSC program area. The increased SOC in the study area agreed with many other studies con-ducted in other regions (Huntington, 1995; Wang et al., 2011; 2012). Huntington (1995) found that cropland afforestation significantly increased SOC in the south-eastern USA. Wang et al. (2011) found significant SOC accumulation of 0.96 Mg/(ha·yr) in the surface soil layer (0–20 cm) after conversion of cropland to larch planta-tions in the northeastern China. Wang et al. (2012) also reported the increase in SOC after cropland afforestation by 0.49 Mg/(ha·yr) and shrubland by 0.39 Mg/(ha·yr) in the hilly Loess Plateau, China. Considering the large area of revegetation in this region, the BTSSC program should have a strong positive contribution to slow the accumulation of CO2 in the atmosphere and maintain global carbon balance.

The results of our study showed the distribution and


change of SOC under two revegetation types with simi-lar age in three climate regions. Further studies should be conducted to reveal the changes in SOC and nutrients at different stand ages. This will provide dynamic traits of soil carbon and nutrients after land use change and help to predict future changes of soil carbon and nutri-ents. Therefore, considering the effect of carbon seques-tration influenced by land use conversion and vegetation growth, we recommend additional research to further evaluate land use pattern and stand age on soil organic carbon and nutrients.

5 Conclusions

By analyzing the data from two revegetation stands of three climate regions in the BTSSC program in China, we observed a certain amount of SOC accumulation and soil bulk density decline after cropland afforestation, and the effects varied with revegetation type and climate region. Significant SOC accumulations were found in Fengning under P. tomentosa and C. korshinskii. Soil carbon accumulation not only appeared in topsoil (0–20 cm), but also in subsoil (20–100 cm). Therefore, subsoil carbon should be taken into account when evaluating SOC change after cropland afforestation. C. korshinskii has a greater carbon sequestration effect on topsoil (0–20 cm) than P. tomentosa in serious wind erosion area. However, P. tomentosa has greater effect of carbon accumulation on topsoil (0–20 cm) but less effect on subsoil (20–100 cm) compared to C. korshinskii in rela-tively humid regions. Therefore, considering the interac-tion effect of revegetation type and climate region on SOC sequestration, different types of restoration vegeta-tion should be established in different climate regions in the BTSSC program area.

Acknowledgments

We thank Dr. Moiwo J Paul and Song Yigang for their help with language revision, and valuable comments on the manuscript. We also thank all who participated in field data collection and collation for this research.

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change of soil organic carbon after cropland afforestation in ‘beijing-tianjin sandstorm source...

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