Soil & Tillage Research 105 (2009) 21–26

Soil organic carbon changes in particle-size fractions following cultivationof Black soils in China

Aizhen Liang a, Xueming Yang b, Xiaoping Zhang a,*, Neil McLaughlin c, Yan Shen a, Wenfeng Li a

a Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, Chinab Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow N0R1G0, Canadac Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa K1A0C6, Canada

A R T I C L E I N F O

Article history:

Received 14 September 2008

Received in revised form 2 May 2009

Accepted 7 May 2009

Keywords:

China Black soils

Organic carbon

Restoring potential

Soil texture

A B S T R A C T

Soil texture can be an important control on soil organic carbon (SOC) retention and dynamics. The

(clay + silt)-sized SOC pool (SOC < 20 mm) in non-cultivated or grassland soils has been proposed to

reach an equilibrium or maximum level named protective capacity. Proper knowledge of SOC in this size

fraction in non-cultivated and cultivated Black soils is important to evaluate management-induced

changes in SOC in NE China. Twenty-seven paired soil samples (non-cultivated vs. cultivated) were

collected in the Black soil zone in Heilongjiang and Jilin provinces. Bulk soil was dispersed in water with

an ultrasonic probe and then soil size fractions were collected using the pipette technique for SOC

analyses. Soil organic carbon in bulk soil and size fractions was measured by dry combustion. Average

content of SOC < 20 mm was 23.2 g C kg�1 at the 0–30 cm depth for the non-cultivated soils, accounting

for 75.1% of the total SOC at the same depth. There was significant positive relationship between soil clay

plus silt content and SOC < 20 mm in non-cultivated soils. Accordingly, a model of the maximum

SOC < 20 mm in 0–30 cm depth of non-cultivated Black soils was developed: y = 0.36x where y is the

maximum SOC < 20 mm pool (g C kg�1) and x is the percentage of clay + silt (<20 mm) content. The

average content of SOC < 20 mm was 18.7 g C kg�1 at 0–30 cm depth for cultivated soils, accounting for

81.5% of total SOC. This average value of SOC was 4.4 g C kg�1 less than the maximum value

(23.1 g C kg�1) and accounted for 55.0% of the difference of SOC between non-cultivated and cultivated

Black soils. Cultivation resulted in 45.0% loss of sand-sized (>20 mm) SOC concentration relative to

SOC < 20 mm. This result indicates that SOC < 20 mm and sand-sized SOC both play important roles in

SOC dynamics resulting from management practices. This model can be applied to calculate the actual

potential to restore SOC for cultivated Black soils under conservation tillage in NE China.

� 2009 Elsevier B.V. All rights reserved.

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Soil & Tillage Research

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1. Introduction

Quantifying the potential of cropland soils to restore ante-cedent soil organic carbon (SOC) will help to evaluate thecontribution of cropland soils as a C source or sink to the globalC balance. However, there are many uncertainties in SOC dynamicsof the soil system (Li, 2002), and these are probably some of themost limiting factors for correctly determining the potential of soilC sequestration (Smith, 2004). Some researchers found thatparticulate organic C (53–2000 mm) is more sensitive to manage-ment change than total SOC (Ellert and Gregorich, 1995; Chanet al., 2002; Six et al., 2002a), and others noted particulate organicC changes were limited before SOC in fine particles reached

* Corresponding author at: 3195 Weishan Road, Gaoxin District, Changchun, Jilin

Province 130012, China. Tel.: +86 431 85542234; fax: +86 431 85542298.

E-mail addresses: aizhenliang@hotmail.com (A. Liang),

zhangxiaoping@neigae.ac.cn (X. Zhang).

0167-1987/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2009.05.002

saturation (Hassink, 1997; Carter et al., 2003). Hassink (1997)proposed that clay and silt (particles <20 mm) in soil physicallyretain SOC, and named this protective ability a ‘‘capacity factor’’ orthe maximum SOC that could be stored in that combined sizefraction. Furthermore, this ‘‘(clay + silt)-sized SOC’’ (SOC < 20 mm,for simplicity) could be modeled as a function of total clay + silt insoils as given in Eq. (1).

SOC<20 mm ¼ 4:09þ 0:37ðclayþ siltÞ (1)

in which SOC < 20 mm and clay + silt are in g kg�1 soil. In NEChina, the SOC < 20 mm in some cultivated Black soils wasestimated as approximately 75% of total SOC (Zhao et al., 1993),but there is no information on how these values depart from ‘‘themaximum’’ observed prior to cultivation. Estimating this differ-ence would be valuable for understanding both the potential of Csequestration in cultivated Black soils in NE China and the effect ofcropping management on C dynamics. Accordingly, the objectivesof this study were (1) to determine the maximum amount of

A. Liang et al. / Soil & Tillage Research 105 (2009) 21–2622

SOC < 20 mm pool in non-cultivated Black soils and (2) to comparethe difference of SOC < 20 mm between cultivated and non-cultivated Black soils in NE China.

2. Materials and methods

2.1. Study site

The study site was the bow-shaped Black soils (Udolls, US SoilTaxonomy) zone in Heilongjiang and Jilin provinces of NE China(Fig. 1). The topography in Black soil region is characterized byundulating plateau with slopes of 1–58.

The Black soil zone is located in the temperate zone with acontinental monsoon climate. The mean annual temperaturevaries between 0.5 8C and 6.0 8C, and the mean annual precipita-tion varies between 500 mm and 600 mm, with more than 80%occurring in June to September. The landscape was grassland in itsnatural state and steppe-meadow grasses are the predominantnative species. Conversion to cultivated crops started early in thelast century, and mass cultivation occurred in the 1940s and 1950s.Traditional cropping practices are continuous soybean in north,corn–soybean rotation in center, and continuous corn in south ofthe Black soil zone. Typical field management practices includeremoving all aboveground plant biomass after harvest, incorpor-ating stubble and main roots into soils with two or three hand-hoeing operations, and forming ridges. More details about studysite were given by Liang et al. (2009).

2.2. Soil sampling

In 2004 and 2005, 27 pairs of cultivated/non-cultivated neighborsites were sampled. In each paired site, the cultivated soil wasapproximately 50–100 m distant from its non-cultivated counter-part, which was never used for crop production. Several criteriawere used to confirm sample sites as non-cultivated soils,including consultations with older residents in the local villages,observing vegetation and soil vertical profiles, and comparing

Fig. 1. Distribution of sampling sites in

results from the laboratory chemical and physical analysis withpublished results.

The land was converted from natural grassland to cultivatedcrops less than 100 years ago, and the elders in the local villagesknew both where the soil had never been disturbed by cultivationand the reasons why. For some sites, the reasons are based on localprotection policy, such as retaining non-cultivated land to providebiomass for use as fuel for heating and cooking in houses, orestablishing ecological sites where removal of natural vegetation isprohibited. Also, position on the landscape resulted in some sitesnot being cultivated, such as a non-cultivated site in a smalltriangular zone on the edge of a deep natural waterway eroded inthe landscape (Fig. 2).

Natural vegetations on Black soils are steppe-meadow speciesand most are the herbage community including Bupleurum

scorzonerifolium, Sanguisorba officinalis, Potentilla chinensis, Salix

rosmarinifolia, Platycodon grandiflorus (Fu, 1995). Presence of thesespecies is a good indicator of a non-cultivated site.

As a final check profiles of non-cultivated soils in the Black soilzone were examined, and laboratory chemical and physical analyseswere compared to published data on non-cultivated Black soils.

Soil core samples were taken to a depth of 30 cm using a manualsoil probe with 2.64 cm internal diameter (Jia et al., 1995) whichallowed separation of each soil core into 4 segments including 0–5 cm, 5–10 cm, 10–20 cm, and 20–30 cm. The diameter of thecutting edge of the probe was 4 mm less than the inside diameterof the barrel which eliminated core compaction resulting fromfriction between the core and barrel. Five to seven sub-sampleswere taken at each site and combined into a single compositesample. The soil samples were gently broken to pass a 7 mm sieveand air-dried. Visibly identifiable crop residues were manuallyremoved and discarded.

2.3. Soil physical analysis

The bulk density of soil samples was calculated using the innerdiameter of the core sampler cutting edge, segment depth and

Jilin and Heilongjiang provinces.

Fig. 2. An example of a particular non-cultivated site in a triangular zone on the

edge of a deep natural waterway eroded in the landscape in Heilongjiang province.

A 1.2-m wide trail ran north toward the waterway, and then turned west parallel to

the waterway. The triangular zone on the outside of the turn in the trail was a

‘‘waste’’ zone too short for cropped ridges.

A. Liang et al. / Soil & Tillage Research 105 (2009) 21–26 23

oven-dried soil weight (105 8C). Soil water suspensions (soil:-water = 1:5) were prepared, allowed to settle for one half hour, andsoil pH was measured using an acidity meter (pHS-3B, Leici,Shanghai). Samples were dispersed in 0.1 M NaOH (for acidic soils)or 0.1 M Sodium oxalate (for neutral soils), organic matter wasoxidized with H2O2, and soil texture determined by the pipettemethod (Soil Science Society of China, 2000).

2.4. Soil organic carbon analysis

Twenty-five grams of air-dried soil were placed in a 250 mlbeaker and 125 ml distilled water was added. The soil suspensionwas mixed and allowed to settle over night. The suspensions weredispersed by treating with an ultrasonic probe (JY92, Xinzhi,Ningbo, China) for 10 min at 24 kHz (Hassink, 1997; Oorts et al.,2005; Zhao et al., 2006). The ultrasonic energy needed for completedispersion was 480 J ml�1 (Schmidt et al., 1999a,b); suspensiontemperature was kept <32 8C by using a 50% duty cycle (1 s on/1 soff) and by placing the beaker in a running tap water bath. Thedispersed soil suspension was transferred to a 1 l glass cylinder,and the cylinder was capped and shaken end over end tothoroughly homogenize the soil water suspension. Silt plus clay(<20 mm) and clay (<2 mm) fractions were collected by siphoningthe suspension at the appropriate depth and time based on Stokes’law (Soil Science Society of China, 2000), oven-dried at 60 8C, and

Table 1Selected soil properties in a depth of 0–30 cm in non-cultivated and cultivated soils.

Soil depth (cm) Clay (%) Silt (%) Sand (%)

Non-cultivated soil

0–5 35.5 � 1.44a 29.2 � 0.85 35.3 � 1.46

5–10 36.2 � 1.17 27.4 � 0.84 36.4 � 1.27

10–20 37.1 � 1.19 27.1 � 0.77 35.8 � 1.44

20–30 36.1 � 1.24 27.0 � 0.81 36.9 � 1.24

Weighted mean 36.4 27.5 36.2

Cultivated soil

0–5 31.8 � 1.41 30.2 � 0.83 38.0 � 1.49

5–10 32.1 � 1.56 30.2 � 1.07 37.7 � 1.41

10–20 35.7 � 1.56 28.8 � 0.95 35.5 � 1.44

20–30 36.3 � 1.64 29.7 � 1.30 33.9 � 1.33

Weighted mean 34.7 29.6 35.8

a Means plus or minus standard error.

ground for SOC analyses. All soil samples were free of carbonate,hence SOC content was assumed equal to total C. Soil organiccarbon in the soil particle fractions was determined using theFlashEA1112 elemental analyzer (ThermoFinnigan, Milan, Italy).

The ANOVA followed by the LSD test was used to examine theeffects of tillage on SOC in each size fraction. Pearson correlationwas used to evaluate the relationship between the percentages ofsoil particles and both total SOC and C fractions. These procedureswere performed using SAS (SAS, 2004). Statistical significance wasdetermined at the P < 0.05 level except if indicated differently.

3. Results

3.1. Selected properties of non-cultivated and cultivated soils

Both non-cultivated and cultivated Black soils in our study weretypically clay loams, and soil texture varied little with land use anddepth (Table 1). Both non-cultivated and cultivated soils wereneutral or slightly acidic with average pH of 7.0 and 6.5,respectively (Table 1). Soil bulk densities at 0–5 cm, 5–10 cmand 10–20 cm depths were lower for the non-cultivated soils thancorresponding depths in the cultivated soils, and almost identicalfor both soils at the 20–30 cm depth (Table 1). Bulk densityincreased with increasing depth for non-cultivated soils, but notmuch for cultivated soils (Table 1).

Average SOC contents in bulk soils for 0–30 cm depth werelower in cultivated (22.9 g C kg�1) than in non-cultivated soils(30.9 g C kg�1). Although SOC contents decreased with soil depthfor both land uses, the decrease was more pronounced in non-cultivated sites (Table 1).

3.2. The model of the maximum SOC < 20 mm

Unless otherwise stated, SOC contents of the various sizefractions are reported here as g SOC kg�1 of bulk soil. Size fractionsused here are clay <2 mm, (clay + silt) <20 mm, and sand>20 mm.Sand-sized SOC was calculated by subtracting SOC < 20 mm fromtotal SOC. The SOC < 20 mm obtained here includes water-solublecarbon, which has been shown by an independent work (Fang,2005) to account for only 0.15–0.20% of total SOC. The averageSOC < 20 mm was 23.2 g C kg�1 in a 0–30 cm depth of non-cultivated soils (Table 2), accounting for 75.1% of total SOC. Clay-sized SOC was 8.96 g C kg�1, accounting for 38.6% of SOC < 20 mm.Subsurface soil (10–30 cm) had lower SOC than the surface layer(0–10 cm depth), which did not occur for the SOC < 20 mm (Fig. 3).The amount of sand-sized SOC accounted for 24.9% of the total SOCfor the non-cultivated soil, and decreased substantially with depth.For cultivated soils, the average content of SOC < 20 mm was18.7 g C kg�1 in 0–30 cm depth (Table 2), accounting for 81.5% of

pH Bulk density (g cm�3) Total C (g kg�1)

6.91 � 0.15 0.94 � 0.04 44.2 � 2.21

6.90 � 0.16 1.08 � 0.04 34.3 � 2.33

7.00 � 0.16 1.08 � 0.03 30.0 � 1.85

7.09 � 0.16 1.17 � 0.03 24.5 � 1.87

7.0 1.09 31.8

6.37 � 0.16 1.04 � 0.02 24.2 � 1.39

6.36 � 0.16 1.19 � 0.03 24.1 � 1.40

6.52 � 0.14 1.20 � 0.03 23.1 � 1.64

6.71 � 0.13 1.18 � 0.03 21.4 � 1.92

6.53 1.17 22.9

Table 2Clay + silt contents in soils, and SOC < 20 mm in non-cultivated and cultivated Black

soils (0–30 cm depth).

Sample ID (clay + silt) contents

in non-cultivated

soils (%)

SOC < 20 mm in

non-cultivated

soils (g kg�1)

SOC < 20 mm in

cultivated soils

(g kg�1)

1 70.8 23.2 18.9

2 70.0 21.9 19.0

3 67.1 20.8 16.9

4 71.4 25.4 18.3

5 71.8 21.5 17.1

6 69.1 28.5 23.0

7 68.1 21.1 17.9

8 79.3 29.2 21.5

9 63.6 26.4 25.6

10 64.3 21.7 15.5

11 71.6 26.5 26.0

12 67.3 28.0 27.8

13 71.9 30.7 25.9

14 60.0 29.7 18.0

15 61.6 29.4 24.6

16 57.1 25.7 22.7

17 58.7 22.0 21.7

18 58.2 27.8 21.7

19 58.2 15.9 11.9

20 60.6 29.2 21.0

21 53.8 16.0 13.3

22 57.1 14.5 12.9

23 61.6 26.2 13.4

24 52.5 12.9 8.4

25 60.2 18.3 12.7

26 58.3 16.8 12.7

27 59.7 18.3 15.2

Average 63.8 23.2 18.7

A. Liang et al. / Soil & Tillage Research 105 (2009) 21–2624

the total SOC while the amount of sand-sized SOC accounted foronly 18.5% of total SOC. The amounts of SOC < 20 mm and sand-sized SOC at 0–5 cm and 5–10 cm were both lower in cultivatedthan non-cultivated soils.

The clay plus silt content in non-cultivated soils ranged from51% to 83%. There were linear positive relationships between the

Fig. 3. Contents of total SOC and SOC in different particles. Clay-sized SOC (<2 mm) is a

SOC < 20 mm and clay-sized SOC pools do not add up to the total SOC (error bar is sta

clay plus silt content, and the SOC < 20 mm in all depths,0–5 cm, 5–10 cm, 10–20 cm and 20–30 cm (data not shown). Wedid an analysis of covariance with clay + silt content as thecovariate, and depth as a factor. The covariate (clay + siltcontent) was significant (P < 0.05) but depth was not significant(P < 0.05). Consequently, the data for the entire 0–30 cm depthwere pooled and a simple linear regression was done withSOC < 20 mm as the dependent variable and clay + silt content ofthe soil as the independent variable. The intercept for thisregression was not significant (P > 0.05) and so a reduced modelwithout intercept was used in the final regression model givenin Eq. (2).

y ¼ 0:36x ðr ¼ 0:510; P<0:01; n ¼ 27Þ (2)

where y = the maximum SOC < 20 mm (g kg�1 soil), x = thepercentage of clay plus silt particles (%).

Observed and predicted SOC < 20 mm are plotted in Fig. 4. TheSOC < 20 mm predicted with the model developed by Hassink(1997) is plotted on the same graph for comparison.

3.3. The theoretical potential to restore SOC for cultivated Black soils

We applied our model (y = 0.36x) to cultivated Black soils toestimate the potential restoring capacity of SOC < 20 mm(Table 3). Average estimated maximum of SOC < 20 mm for the27 cultivated sites was equal to 23.1 g kg�1 in the top 30 cm of soil,which was as expected nearly identical to the observed averagevalue (23.2 g kg�1) for the 27 non-cultivated soils. ObservedSOC < 20 mm (18.7 g kg�1) was 4.4 g kg�1 less than the maximumin cultivated soil, accounting for a 55.0% difference of total SOCbetween non-cultivated and cultivated soil. Therefore, averageestimated potential to restore SOC < 20 mm of the 27 cultivatedsites was 4.4 g kg�1 in the top 30 cm of soil. Sand-sized SOC was3.6 g kg�1 less in cultivated soil than the non-cultivated soil,accounting for 45.0% of the difference of total SOC between thesetwo soils.

lso included in the SOC < 20 mm, and consequently, the sand-sized (>20 mm) SOC,

ndard error, n = 27).

Fig. 4. Plot of observed SOC < 20 mm vs. percent clay + silt. Line a is the maximum

SOC < 20 mm predicted by Hassink’s model, line b is maximum SOC < 20 mm

predicted by our regression model: y = 0.36x (r = 0.510, n = 27), and lines c and d are

the 95% confidence intervals for our model.

Table 3Comparison of total SOC, SOC < 20 mm and sand-sized SOC in non-cultivated and

cultivated Black soils (0–30 cm).

Non-cultivated

soil (g kg�1)

Cultivated

soil (g kg�1)

Difference

(g kg�1)

SOC loss in

size fraction/

total SOC loss

Total SOC 30.9 22.9 8.0

SOC < 20 mm 23.1 18.7 4.4 55.0%

Sand-sized SOCa 7.8 4.2 3.6 45.0%

a Sand-sized SOC was calculated as the difference between total SOC and the

SOC < 20 mm.

A. Liang et al. / Soil & Tillage Research 105 (2009) 21–26 25

4. Discussion

The total SOC content in the non-cultivated soils was thehighest near the surface in the top 5 cm and steadily decreasedwith increasing depth, but in the cultivated soils, the soil organiccarbon was much lower and relatively constant in the 0–20 cmdepth (Table 1). Cultivation both mixed the soil in the plow layerand promoted mineralization of the SOC resulting in nearlyconstant SOC in the 0–20 cm depth (Liang et al., 2008).

The current study was based on an assumption that the amountof SOC < 20 mm had a limit and that C in non-cultivated Black soilswas saturated. The non-cultivated soils have not been disturbedover thousands of years, and consequently, it is reasonable toexpect that SOC has reached an equilibrium where annual input ofC from the natural vegetation biomass is equal to the annualoutput of C via mineralization or consumption by soil micro floraand fauna.

Hassink’s model was developed from published SOC data forsoils from around the world with a broad range of both texture andmineralogy while our model is based on a much smaller data setfrom the Black soils in NE China. The slopes for the two models arenearly identical, 0.36 g C kg�1 clay + silt for our model compared to0.37 g kg�1 clay + silt for the Hassink model. Our model does nothave an intercept while Hassink’s model has an intercept of 4.09.Different parameters are expected, because the maximum of SOCassociated with the fine size fraction is influenced by many factors,including clay mineralogy and climate (Li, 2001; Six et al., 2002b;Haider and Guggenberger, 2002). Clay minerals in soils used in thisstudy were dominated by 2:1 type minerals, and so we did nothave variations in clay mineralogy to influence SOC < 20 mm.Australian soils investigated in Hassink’s study (1997) hadsubstantially lower SOC, likely a result of high temperatures,low rainfall and limited input of plant residues, and were excluded

from his model. All of these factors provide an indication of thecomplexity of SOC < 20 mm.

Hassink’s model over-predicted SOC < 20 mm measured in ourstudy by an average 19.2% (Fig. 4). A model developed for the localdata generally fits the local data much better than a more generalmodel such as the Hassink model which is based on published datafor a wide range of soils around the world. Zinn et al. (2005) alsoproposed that simple linear functions relating SOC to clay + siltmay not be the best descriptors for every region and differentfunctions must be tested for each region.

The SOC capacity models of soils such as Eq. (2) and the Hassink(1997) model (Eq. (1)) provide a means to estimate SOC changeunder cultivation, and the mechanism and potential for soil Csequestration. The SOC < 20 mm pool increases with increasingclay + silt contents, which is the main mechanism of the texturalcontrol on SOC retention (Zinn et al., 2007). In a study applying thisfunction to Baijiang (Albolls, US Soil Taxonomy) cultivated soils inJilin Province, China, Zhao et al. (2006) found that the actualcontent of SOC < 20 mm was greater than the maximum calculatedfrom Hassink’s model.

The agricultural use of Black soils led to great SOC loss fromsand-sized C. This result was different from the finding that thechange of sand-sized C was limited when SOC in fine particlesreached saturation (Hassink, 1997; Carter et al., 2003). AlthoughSOC < 20 mm was believed to be relatively stable and played animportant role in maintaining C levels, the share of 55.0% total SOCloss from this fraction in cultivated Black soils suggests thatattention needs to be paid to the fate of C both in fine and coarseparticles when studying effect of agriculture on C dynamics.However, Jolivet et al. (2003) found that SOC < 20 mm decreasedby only 20% after 30 years of cultivation, so long-term Caccumulation was attributed mainly to the clay + silt fraction,which protected complex molecules supplied by decomposition ofsand-sized organic matter. The soil’s capacity to accumulate newlyadded carbon was determined by the degree of saturation ofSOC < 20 mm (Hassink, 1995; Hassink and Whitmore, 1997;Jolivet et al., 2003). Six et al. (2002b) suggested that with increasedC input, the increase in SOC content was smaller when theprotective capacity approached saturation.

Our present studies are focused on the potential of conserva-tion tillage as a management practice to prevent soil erosion andto restore SOC in the Black soils in NE China. The model will play akey role in understanding SOC dynamics in these studies.Certainly, clay + silt pool is not homogeneous and C in eachparticle size is probably retained by different mechanisms and hasdifferent C:N ratios (Zinn et al., 2007). While the clay + silt poolconcept appears useful, it is a simplification and clearly, furtherstudy is needed.

5. Conclusion

Paired soil samples were obtained from 27 non-cultivated andadjacent cultivated soil sites in the Black soil zone in NE China, andSOC determinations were made in different size fractions. In non-cultivated Black soils, significant positive relations were foundbetween clay + silt contents and both total SOC and SOC < 20 mm.Based on those relations and on the assumption that SOC < 20 mmwas saturated, a model of the maximum SOC < 20 mm in non-cultivated Black soils was developed. In cultivated Black soils themean difference between present SOC < 20 mm and estimatedpotential maxima was 4.4 g C kg�1, or 55.0% of total SOC differencebetween non-cultivated and cultivated soils. The other 45.0% oftotal C lost was from the sand fraction. The model provides a tool toestimate the potential of cultivated Black soils restoringSOC < 20 mm, but more work is needed to validate and refinethis model.

A. Liang et al. / Soil & Tillage Research 105 (2009) 21–2626

Acknowledgements

This research was supported by the projects from NationalNatural Science Foundation of China (40801071), NationalScientific and Technical Supporting Programs (2006BAD15B01),Program for Advanced Science Field (KZCX3-SW-NA3-31) andDoctoral Research Foundation (O8H2041) in Northeast Institute ofGeography and Agroecology, Chinese Academy of Sciences.

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