effects of land-cover type and topography on soil organic carbon storage on northern loess plateau,...

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This article was downloaded by: [Moskow State Univ Bibliote] On: 09 October 2013, At: 11:01 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Acta Agriculturae Scandinavica, Section B - Soil & Plant Science Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/sagb20 Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China Xiangwei Han a b , Atsushi Tsunekawa a , Mitsuru Tsubo a & Shiqing Li b a Arid Land Research Center , Tottori University , Hamasaka 1390, 680-0001, Tottori, Japan b State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences , Northwest A&F University , Yangling, Shaanxi, 712100, China Published online: 10 Jul 2009. To cite this article: Xiangwei Han , Atsushi Tsunekawa , Mitsuru Tsubo & Shiqing Li (2010) Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China, Acta Agriculturae Scandinavica, Section B - Soil & Plant Science, 60:4, 326-334, DOI: 10.1080/09064710902988672 To link to this article: http://dx.doi.org/10.1080/09064710902988672 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

This article was downloaded by: [Moskow State Univ Bibliote]On: 09 October 2013, At: 11:01Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Acta Agriculturae Scandinavica, Section B - Soil &Plant SciencePublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/sagb20

Effects of land-cover type and topography on soilorganic carbon storage on Northern Loess Plateau,ChinaXiangwei Han a b , Atsushi Tsunekawa a , Mitsuru Tsubo a & Shiqing Li ba Arid Land Research Center , Tottori University , Hamasaka 1390, 680-0001, Tottori,Japanb State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau,Institute of Soil and Water Conservation, Chinese Academy of Sciences , Northwest A&FUniversity , Yangling, Shaanxi, 712100, ChinaPublished online: 10 Jul 2009.

To cite this article: Xiangwei Han , Atsushi Tsunekawa , Mitsuru Tsubo & Shiqing Li (2010) Effects of land-cover type andtopography on soil organic carbon storage on Northern Loess Plateau, China, Acta Agriculturae Scandinavica, Section B -Soil & Plant Science, 60:4, 326-334, DOI: 10.1080/09064710902988672

To link to this article: http://dx.doi.org/10.1080/09064710902988672

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

ORIGINAL ARTICLE

Effects of land-cover type and topography on soil organic carbonstorage on Northern Loess Plateau, China

XIANGWEI HAN1,2, ATSUSHI TSUNEKAWA1, MITSURU TSUBO1 & SHIQING LI2

1Arid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680-0001, Japan, 2State Key Laboratory of Soil

Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences,

Northwest A&F University, Yangling, Shaanxi 712100, China

AbstractChanges in land cover from cropland to conservation can sequester carbon in soil. On the Loess Plateau of China, vast areasof sloping cropland were converted into forest and grassland to control soil erosion. The northern plateau is atopographically heterogeneous, semi-arid region. A good understanding of the change of soil organic carbon (SOC)storage on the plateau in the process of land-cover change is important for assessing environmental changes and planningfuture land cover. We selected four land-cover types (cropland, planted grassland, abandoned orchard, and secondarygrassland), and two vegetation covers (Stipa bungeana and Caragana korshinskii) on shady and sunny slopes, to analyse theeffects of land cover and slope aspect on SOC storage. Soil C in the top 100 cm was significantly (PB0.05) greater inartificial grassland (2.49 kg m�2) and secondary grassland (2.98 kg m�2) than in cropland (1.69 kg m�2). The SOC poolin the surface soil and throughout the 1-m profile followed the order secondary grassland�artificial grassland � abandonedorchards � cropland. Sequestration extended to deep soil (80�100 cm). Slope aspect affected SOC concentration: winderosion of the shady slope marginally reduced surface SOC relative to the sunny slope. In deep soil, responses of SOCconcentration to slope aspect differed between vegetation covers: under C. korshinskii, SOC concentration was significantlygreater on the shady slope (P B 0.05), but no difference was found under S. bungeana.

Keywords: Grain-for-Green Policy, land-cover change, slope aspect, soil erosion, soil organic carbon pool.

Introduction

The Loess Plateau of China is the site of intensive

study because of its serious soil erosion and typical

land-cover changes resulting from the ‘Grain-for-

Green’ Policy (GFGP), which was introduced to

control erosion. The plateau covers 624 000 km2

(Shi & Shao, 2000). It has a long history of agriculture,

and used to be an important grain source. Because of

over-cultivation, it now suffers the most serious soil

erosion in the world (Shi & Shao, 2000): erosion in

most areas measures 5000�10 000 Mg km�2 per year

(Chen et al., 2007b). Soil erosion is extremely serious

on slopes, and long-term erosion has formed gullies all

over the plateau. The ecosystem has deteriorated

seriously too (Wang et al., 2009). To control soil

erosion and improve the vegetation cover, the Chinese

Government initiated GFGP in 1999. Under GFGP,

cropland with a slope greater than 258 is converted

into grassland or forest. About 5 330 000 ha of crop-

land will have been re-greened by 2010 (Peng et al.,

2006).

Land-cover change is recognized widely as a cause

of SOC pool changes (Lal, 2003a; Mensah et al.,

2003; Degryze et al., 2004). The conversion of forest

and grassland into cropland decreases the SOC pool

(Wang et al., 2001; Bonino, 2006; Zhang et al., 2007).

Conversely, the restoration of grassland (Lemaire

et al., 2005; Breuer et al., 2006), scrubland (Gong

et al., 2006), or forest (Bonino, 2006) increases

the SOC pool. The resultant land cover following

restoration is important to the rate of SOC sequestra-

tion. Gong et al. (2006) found significant differences

in soil organic matter (OM) content among waste-

land, cropland, abandoned land, artificial grassland,

shrubland, and woodland. Robles & Burke (1997)

Correspondence: Xiangwei Han, Arid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680-0001, Japan. Tel: �81-857-23-34111.

Fax: �81-857-29-6199. E-mail: [email protected]

Acta Agriculturae Scandinavica Section B � Soil and Plant Science, 2010; 60: 326�334

(Received 13 March 2009; accepted 22 April 2009)

ISSN 0906-4710 print/ISSN 1651-1913 online # 2010 Taylor & Francis

DOI: 10.1080/09064710902988672

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Page 3: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

found different responses of soil OM between

legumes and grass.

Topography also influences SOC sequestration

(Norton et al., 2003; Rhoton et al., 2006; Wei et al.,

2006), primarily through influencing the production

of biomass (Tatsuhara & Kurashige, 2001), and

secondarily through its effect on soil water and

temperature and thus on decomposition (Fekedulegn

et al., 2003). Tan et al. (2004) found site variables

(soil taxon, texture, drainage class, slope gradient and

elevation) contributed to more than 50% SOC varia-

tion in cropland and grassland. Liu et al. (2007) found

that slope angle and aspect had more important

effects than did land-cover type on soil OM, clay

content, and nutrient concentration.

SOC concentration is greatest in the surface soil

and decreases with depth. The soil depth to which

SOC is influenced by land-cover change varies

among studies: Degryze et al. (2004) reported that

the change of SOC with depth was restricted to the

surface soil (0�7 cm) in forest, yet Omonode & Vyn

(2006) found a considerable amount of SOC seques-

tered in deeper soil (0�100 cm).

Some studies on the plateau have examined the

effects of land-cover change on the SOC pool (Liu,

S.Z. et al., 2005; Peng et al., 2006; Chen et al.,

2007a), but most examined a limited soil depth (e.g.,

40 cm). Plateau-wide studies used a low spatial

resolution to explain micro-topographic effects.

Most site-based studies were carried out in the middle

to southern part of the plateau, which has higher

precipitation and favors forestry. In contrast, the

northern plateau, at its juncture with the Maowusu

Sandy Grassland, is dry, and the typical vegetation is

grass. The region suffers from both wind and water

erosion (Zhang, 1999), and risks becoming desert.

The ecology is very fragile. Thus, land restoration in

this region is urgent. GFGP was well implemented

there, and most of the slopes were converted into

grassland. This presents the opportunity to study

SOC changes as a result of land-cover change.

However, little information is available on the effects

of land-cover change and topography on SOC on the

Northern Loess Plateau. We conducted a study (1) to

evaluate the effect of land-cover conservation on SOC

sequestration, and (2) to reveal the effect of slope on

SOC concentration since the implementation of

GFGP.

Materials and methods

General description of study area

Soil samples were obtained from a small catchm-

ent, the Liudaogou catchment, in Shenmu County,

Shaanxi Province, China (38847? N, 110822? E). The

elevation ranges from 1094 m to 1274 m. The region

has a semi-arid climate with an average annual

precipitation of 437.4 mm (1957�1989; minimum

108.6 mm in 1965; maximum 819.1 mm in 1967)

(Academia Sinica & Ministry of Water Resources,

1993), 77% of which falls during June through

September. The mean annual temperature is 8.4 8C,

and the mean minimum temperature ranges from �9.7 8C in January to 23.7 8C in July. The annual

cumulative temperature above 10 8C is 3228 8C. The

mean frost-free season is 169 days. The average

annual potential evaporation is 785.4 mm, and the

mean desiccation degree is 1.8 (Li et al., 2007). The

catchment lies in a transition region between the

Loess Plateau and the Maowusu Sandy Grassland,

and thus represents an ecotone in which wind and

water erosion are both significant factors (Zheng et al.,

2006).

The natural vegetation of the study site is shrub�grassland. However, because of excessive cutting,

overgrazing, and serious soil erosion, the vegetation

has degraded. Since the GFGP was adopted, more

than half of the sloping croplands in the study area

have been returned to shrubland or grassland. The

main shrubs planted for recovery are Salix psammo-

phila and Artemisia ordosica on sandy soil, and

Caragana korshinskii on loam and clay soils. Some

of the cropland was planted with alfalfa (Medicago

sativa), and some with apricot (Armeniaca vulgaris).

Years later, the alfalfa grassland degraded, and then

underwent natural succession to secondary grassland

dominated by Stipa bungeana. Because of the water

deficit in the region, the apricot trees didn’t grow

well and were later abandoned. At present, the main

land-cover types are cropland, abandoned land,

grassland, and shrubland. Lowland croplands are

occupied by irrigated maize and vegetables. Sloping

croplands are planted to rainfed potato and beans.

Most of the sloping croplands receive manure and

nitrogen fertilizer, but restored croplands have

received nothing.

The loess extends to a depth of more than 30 m (Li

et al., 2007). Erosion by water and wind has

complicated the soils of the study catchment. The

northwest side of the catchment is covered by wind-

accumulated sand. A channel check dam holds

deposited soil. The southeast side is sandy loess.

Our study was conducted on the sandy loess (Table I).

Soil sampling and laboratory analysis

Site for evaluating effects of land-cover type. Soil samples

were collected from four main restored land-cover

types in the study area: artificial grassland with

planted alfalfa (AGL) for about 8 years; secondary

grassland, used to be planted with alfalfa for almost

Soil organic carbon of different land-covers 327

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Page 4: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

10 years, then underwent natural succession to S.

bungeana (SGL) more than 7 years ago; and orchards,

planted almond 8 years ago and covered by some

weeds (ABO). To illustrate the change of soil organic

carbon, samples were also collected from cropland

(CRL), which was continuously tilled and planted

with potato or soybeans for more than 25 years;

currently the area is planted with soybeans.

Different soil types are usually quite different in soil

organic carbon concentrations. For the purposes of

comparison, we collected samples from sites with

homogeneous soil conditions in early September

2007. Before land restoration, the sample sites were

cultivated in the same way. Five replicated represen-

tative samples were obtained for each land-cover type.

Site for evaluating the effects of slope. We chose a pair of

slopes facing each other, sunny (slope aspect was

1958, with an azimuth of 08 for north) and shady

(slope aspect was 158). On each slope, we collected

soil samples from two belts, one covered by

C. korshinskii and the other by natural grassland

dominated by S. bungeana. The last two land cover

types were sites for evaluating the effects of slope

(Table 1). The length of the shady slope was about

150 m. Along it, we selected seven sites from each

belt. The length of the sunny slope was about 90 m.

We selected six sites from the C. korshinskii belt and

five from the S. bungeana belt. All sites were spaced

about 20 m apart.

Soil sampling and laboratory analysis. At each sam-

pling site, we took three cores to a depth of 100 cm

at 20-cm intervals using a 20-cm by 5-cm soil auger,

and bulked the samples. We selected 20 sites for

comparison of the land-cover effect and 25 sites for

comparison of the topography effect.

We carefully examined the samples, and removed

roots, leaves, and other unwanted material. All

samples were air-dried and ground to pass through

0.25-mm screens before laboratory analysis. After

digestion in glass cuvettes at 185�190 8C (oil-bath),

the SOC was determined by the K2CrO7 titration

method (Nelson & Sommers, 1982). Soil bulk

density was determined using the core samples.

Calculations and statistical analysis

The SOC pool (DSOC, kg C m�2), expressed as

mass per unit area to a fixed depth, was calculated

from SOC concentration (SOC, g kg�1 soil) and

bulk density (r, g cm�3) as shown in Equation (1):

DSOCi�100�SOCi �ri � di (1)

Table

I.S

elec

ted

pro

per

ties

of

the

surf

ace

soils

of

dif

fere

nt

lan

d-c

ove

rty

pes

.

Soil

part

icle

com

posi

tion

(%)

Lan

d-c

over

typ

eT

ota

lN

(gkg�

1)

Inorg

an

icN

(mg

kg�

1)

Ava

ilable

P(m

gkg�

1)

Ava

ilable

K(m

gkg�

1)

B0.0

02

mm

0.0

02�0

.05

mm

0.0

5�2

mm

CR

L0.2

713.8

5.7

115.3

7.6

76.4

13.5

AG

L0.3

69.7

4.8

130.5

9.1

78.1

9.4

AB

O0.3

112.1

3.9

110.9

8.0

70.1

18.9

SG

L0.3

78.2

2.1

105.0

8.3

83.0

5.5

SB

0.3

37.3

5n

a127.5

8.1

74.9

13.3

CK

0.3

88.7

3n

a120.8

8.4

73.5

15.1

na�

not

ava

ilable

.C

RL�

crop

lan

d;

AG

L�

art

ific

ial

gra

ssla

nd

cover

edby

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alf

a;

AB

O�

aban

don

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cot

orc

hard

cove

red

wit

hA

rmen

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seco

nd

ary

art

ific

ial

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cover

ed

pre

dom

inan

tly

by

Stipa

bunge

ana;

SB�

natu

ral

gra

ssla

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cover

edb

yS

tipa

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ana;

CK�

natu

ral

shru

bla

nd

pla

nte

dw

ith

Cara

gana

kor

shin

skii.

328 X. Han et al.

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Page 5: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

where d is the soil depth (cm), subscript i is the

position of the soil layer, and 100 is used for unit

conversion.

To analyse the effect of land cover on SOC

sequestration, we compared soil data obtained from

the four land-cover types. We used one-way analysis

of variance (ANOVA) with land-cover type or soil

layer as the main factor to test the significance of

differences between SOC densities in or among soil

layers. When the analysis indicated a significant

difference (PB0.05), we used the least significant

difference (LSD) procedure to compare means. We

also tested the effects of topography of the paired

sunny and shady slopes in the C. korshinskii and

S. bungeana plots on SOC within each soil layer by

ANOVA. Significant differences were determined at

a�0.05. All statistical analyses were done using the

SPSS software (SPSS, 2006; Version 15.0).

Results

Spatial distribution of SOC and soil bulk density

SOC concentration decreased with increasing soil

depth in all four land-cover types and under both

vegetation covers (Table II). The mean concentra-

tions in the surface soil (0�20 cm) were all signifi-

cantly greater than in the deeper layers (PB0.01).

The SOC concentrations at 20�40 cm were all

approximately 30 to 45% less than in the surface

soil. There was no statistical difference among

concentrations in the lower layers, except in SGL

and S. bungeana, where the concentrations at 20�40 cm were significantly higher than those deeper in

the soil (PB0.05, PB0.01). SOC concentrations

within the same soil layer varied widely: the coeffi-

cients of variation (CVs) ranged from 12.2% to

41.3% (Table II). The top three layers, where the

plant roots concentrated, had higher variation. The

highest concentration in the surface soil appeared in

SGL. Caragana korshinskii had the highest concen-

trations of the deepest four layers. Concentrations

were lowest in CRL throughout the profile.

Soil bulk densities of the four land-cover types

varied both horizontally and vertically (Table II).

Unlike previous studies (Chen et al., 2007a; Omo-

node & Vyn, 2006), we found no statistical difference

among land-cover types or soil depths. CRL had the

highest mean value (1.42 g cm�3), and ABO the

lowest (1.32 g cm�3). Yet both of these land-cover

types tended to have higher bulk density in the second

layer (20�40 cm) than in the other layers, perhaps as a

result of tillage. The range of bulk density was smaller

than those of SOC concentration.

SOC pools of different land-cover types

Land-cover type and soil depth had significant effects

on SOC pool (Figure 1). SOC densities of SGL were

greatest, and were significantly greater than in CRL

in all but the deepest soil layer (PB0.01), followed by

those in AGL, which were significantly greater in the

0�20- and 60�100-cm layers than in CRL (PB0.05).

SOC pool in ABO was numerically but not statisti-

cally greater. SOC pools were significantly greater in

the 0�20-cm layer than in the deeper layers (PB

0.01). The patterns of SOC pool change down the

soil profile were inconsistent among land-cover

types: in CRL and AGL, SOC pool decreased to

a constant minimum at 20�100 cm, but in ABO and

SGL the SOC pool at 20�40 cm was significantly

higher than in the deeper three layers, which were

statistically indistinguishable.

The mean SOC pool throughout the soil profile

increased in the order CRLBABOBAGLBSGL

(Table III). The surface soil layer made a large

contribution to the C pool (�1/3), but the deeper

soil nevertheless held a considerable amount of

C (�2/3). Significant differences in SOC pool

among land-cover types were found both in surface

and deep soils.

Organic carbon density at different slope aspects

The surface SOC concentration fluctuated widely

along the slope (Figure 2). The lowest SOC con-

centration always appeared in the midslope. It tended

to be higher on the sunny slope at 0, 60, and 100 m,

and higher on the shady slope at 80 m.

Slope aspect had different effects on concentra-

tions between vegetation covers (Figure 3). Concen-

trations in the surface soil under both covers were

slightly higher on the sunny slope than on the shady

slope, but differences were not significant. Concen-

trations in the deeper layers under C. korshinskii were

significantly greater on the shady slope (PB0.05),

but there was no significant difference under

S. bungeana.

Discussion

SOC is balanced by gains and losses of organic C.

The gains are determined mainly by the amount and

types of plant and animal residues, while the losses

are regulated by the oxidation of existing soil OM, by

erosion, and, generally, also by leaching as dissolved

organic carbon, which could be ignored in our study

area owing to the low precipitation. Factors that

regulate both influence the SOC pool.

Soil organic carbon of different land-covers 329

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Page 6: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

Our results show that land-cover change had

significant effects on SOC. This is consistent with

results from previous studies under different climate

conditions (Degryze et al., 2004; Breuer et al., 2006;

Omonode & Vyn, 2006; Chen et al., 2007a). Crop-

land has relatively low SOC, because crop residues

are usually removed from the field during harvest,

intensive management breaks down soil aggregates

and increases SOC decomposition, and activities

such as tillage and harvesting destroy the surface-soil

structure, thus exacerbating erosion (Gregorich &

Anderson, 1985; Gregorich et al., 1998). Our results

show that when croplands were abandoned or

converted into grassland, SOC pool to a 1-m depth

increased by 21% and by 47% to 76%, respectively.

This increase could be explained by an increase in

the input of residues, a decrease in decomposition,

or a decrease in soil erosion because of vegetation

protection.

The increments of SOC after restoration appear to

have differed among land-cover types. The greatest

SOC pool was found in SGL, followed by AGL,

ABO, and CRL; this was consistent with previous

results (Peng et al., 2006; Li et al., 2007). Restored

land planted with grass (AGL, SGL) had signifi-

cantly greater SOC than did CRL, but the aban-

doned orchard (ABO) did not have significantly

greater SOC than did CRL. Alfalfa, planted in AGL

and the early SGL, produced as much as 400 t dry

hay ha�1 (Jiang et al., 2007) and considerable root

biomass (Cheng et al., 2007). High productivity

indicates high C gains. Fruit trees in ABO have less

Table II. Soil organic carbon (SOC) concentration and bulk density (r) at different intervals under four land-cover types and two natural

vegetation covers.

SOC (g kg�1)

Land-cover type Depth (cm) Number of samples Mean SD CV (%) r (g cm�3)

CRL 0�20 5 2.05** 0.66 32.4 1.35

20�40 5 1.22 0.33 26.7 1.51

40�60 5 1.06 0.33 31.6 1.45

60�80 5 0.82 0.12 15.0 1.38

80�100 5 0.85 0.22 26.0 1.39

AGL 0�20 5 3.38** 0.73 21.7 1.35

20�40 5 1.82 0.52 28.8 1.24

40�60 5 1.44 0.60 41.3 1.40

60�80 5 1.24 0.39 31.5 1.37

80�100 5 1.36 0.30 22.4 1.42

ABO 0�20 5 2.78** 0.52 18.8 1.33

20�40 5 1.52 0.53 34.5 1.43

40�60 5 1.35 0.50 37.4 1.26

60�80 5 1.04 0.18 17.4 1.28

80�100 5 1.06 0.19 18.4 1.30

SGL 0�20 5 3.83** 0.64 16.7 1.42

20�40 5 2.24* 0.80 35.5 1.41

40�60 5 1.73 0.28 16.3 1.46

60�80 5 1.43 0.27 18.8 1.42

80�100 5 1.42 0.29 20.5 1.30

SB 0�20 12 3.15** 0.59 18.7 na

20�40 12 1.90** 0.45 24.0 na

40�60 12 1.41 0.35 24.7 na

60�80 12 1.29 0.29 22.8 na

80�100 12 1.22 0.25 20.5 na

CK 0�20 13 3.32** 0.90 27.2 na

20�40 13 2.32 0.35 15.3 na

40�60 13 2.16 0.26 12.2 na

60�80 13 1.95 0.34 17.2 na

80�100 13 1.97 0.51 26.1 na

*,**Difference significant at PB0.05 or PB0.01 [one-way analysis of variance (ANOVA) and least significant difference (LSD)]; SD�standard deviation. na�not available. CRL�cropland; AGL�artificial grassland covered by alfalfa; ABO�abandoned apricot orchard

covered with Armeniaca vulgaris; SGL�secondary artificial grassland covered predominantly by Stipa bungeana; SB�natural grassland

covered by Stipa bungeana; CK�natural shrubland planted with Caragana korshinskii. The values for SB and CK are the average of the

sunny and shady slopes.

330 X. Han et al.

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Page 7: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

residual input than do grass, but they take up more

soil water and restrict weed growth, thus limiting C

gains. The difference in SOC between AGL and

SGL implies an effect of restoration time (Li et al.,

2007). Jiang et al. (2006) reported that SOC under

alfalfa increased over 20 years and then decreased as

a result of reduced production because of depletion

of soil water by old alfalfa stands. The age of

grasslands in our study was within the increasing

period, and the alfalfa was replaced by secondary

grasses, which need less water, thereby maintaining

high production. The levels of SOC increment were

somewhat different from those in other studies

(Gong et al., 2006; Omonode & Vyn, 2006; Chen

et al., 2007a; Li et al., 2007). This difference can be

explained by differences in soil, climate, and time of

restoration. The greater SOC in AGL and SGL than

in ABO indicates that restoration to grassland helped

to sequester C. Restoration time is also an important

factor influencing C-sequestration rate.

The highest SOC concentration and C sequestra-

tion were found in the surface soil. SOC at 0�20 cm

accounted for more than 30% (Table III) of the SOC

to 1 m. This is consistent with results in the

literature (Mensah et al., 2003; Degryze et al.,

2004; Chen et al., 2007a). We found C sequestration

to a depth of 100 cm in AGL and SGL. This result

disagrees with some studies (Mensah et al., 2003;

Degryze et al., 2004; Chen et al., 2007a), but

supports the findings by Lemaire et al. (2005) that

grassland soils can store significant amounts of SOC

at greater depths. Omonode & Vyn (2006) similarly

found 71% of SOC sequestered in the 20�100-cm

layer, comparable to our two-thirds. The SOC

Soil depth (cm)80-10060-8040-6020-400-20

SO

C in

eac

h so

il la

yer

( kg

Cm

-2)

1.20

1.00

0.80

0.60

0.40

0.20

0.00

SGL

AGL

ABO

CRL

Land-cover type

a

ab

bc

c

a

ab ab

b

aab

abb

aab

bcc

aab

abb

A

A

A

A

BB

B B

BB

BB

B

B

BC

BC

C

C

C

C

Figure 1. Soil organic carbon (SOC) pool in each soil layer. Error bars represent standard error. Within a land-use type, bars with a

different uppercase letter differ significantly among soil layers [PB0.05; one-way analysis of variance (ANOVA) and least significant

difference test (LSD)]. Within a soil layer, bars with a different lowercase letter differ significantly among land-cover types. CRL�cropland;

AGL�artificial grassland covered by alfalfa; ABO�abandoned apricot orchard covered with Armeniaca vulgaris; SGL�secondary artificial

grassland covered predominantly by Stipa bungeana; SB�natural grassland covered by Stipa bungeana; CK�natural shrubland planted to

Caragana korshinskii.

Table III. SOC pools (kg C m�2) in four land cover types measured at 0�100 cm (DSOC100), 0�20 cm (DSOC20), and 20�80 cm (DSOC80)

DSOC100

DSOC20 DSOC80

Land cover type Mean (kg C m�2) Mean (kg C m�2) DSOC20/DSOC100 (%) Mean (kg C m�2) DSOC80/DSOC100 (%)

CRL 1.69 a 0.56a 33.1 1.14a 67.4

AGL 2.49bc 0.91bc 36.5 1.58a 63.4

ABO 2.05ab 0.74ab 36.1 1.31ab 63.9

SGL 2.98 c 1.08 c 36.2 1.90 b 63.7

DSOC20/DSOC100�DSOC at 0�20 cm/DSOC at 0�100 cm. DSOC80/DSOC100�DSOC at 20�80 cm/DSOC at 0�100 cm. Values labeled with the

same letter do not differ significantly (p�0.05, LSD). CRL�cropland; AGL�artificial grassland covered by alfalfa; ABO�abandoned

apricot orchard covered with Armeniaca vulgaris; SGL�secondary artificial grassland covered predominantly by Stipa bungeana.

Soil organic carbon of different land-covers 331

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Page 8: Effects of land-cover type and topography on soil organic carbon storage on Northern Loess Plateau, China

concentration in deep soil was significantly lower in

cropland than in grassland. Crops did not grow well

in CRL, the roots were shallow, and crop residues

are always removed in the study area. Because of the

limited precipitation, little SOC can leach down.

This led to a low SOC level in deep soil. In contrast,

in AGL and SGL, roots of alfalfa could reach deeper

than 500 cm (Cheng et al., 2007), and the above-

and below-ground biomass was eventually trans-

formed into SOC.

The pattern of SOC distribution along the slope is

a result of severe soil erosion. Erosion by water and

re-deposition change the SOC distribution (Gregor-

ich & Anderson, 1985; Gregorich et al., 1998; Lal,

2003b; Liu, S.Z. et al., 2005), resulting in an increase

in SOC from the top to the bottom (Norton et al.,

2003). However, wind erosion removes surface soil

particles from exposed areas and deposits them in

protected areas, regardless of gradient position,

fuzzing up the trend of SOC distribution (Gregorich

et al., 1998). The effect of erosion on SOC is affected

by slope angle (Gregorich et al., 1998). Our shady

slope had a greater angle, so erosion and accumula-

tion would be more serious there (Rhoton et al.,

2006), perhaps explaining the increasing trend.

Slope aspect is another factor associated with SOC

quantity. Usually, soil on shady slopes contains more

SOC than does soil on sunny slopes on the Loess

Plateau (Lian et al., 2006). Our result under C.

korshinskii in deep soil is consistent with this conclu-

sion. The reason is that a shady slope usually has

a lower temperature and a higher water content

(Brubaker et al., 1993; Zhang et al., 2007). For a

plant such as C. korshinskii with a deep root system,

high water content favors fine root growth in deep

soil in our dry study area (Cheng et al., 2007),

resulting in more OM below the ground. A high

water content and low temperature also restrain soil

OM decomposition, decreasing SOC loss (Fissore

et al., 2008). In contrast, for a plant with shallow

roots (Li et al., 2005), namely S. bungeana, no

difference was found. The trend in the surface soil

was different from that in the deep soil: SOC

concentration was numerically (although not signifi-

cantly) greater on the sunny slope, perhaps on

account of wind erosion. The northwest wind pre-

vails in the catchment (Zhang, 1999; Kimura et al.,

2006), thus targeting the shady slope at our study

site, and removes surface particles, especially plant

residues and fine particles, reducing SOC on the

shady slope.

The SOC level in the study area is very low. The

average SOC pool to 1 m was 2.31 kg C m�2, which

was less than one-fourth of the average SOC pool in

China (9.1 kg C m�2, Wang et al., 2000; 10.53 kg C

m�2, Xie et al., 2004; 9.6 kg C m�2, Yu et al., 2007).

The mean 0�20-cm surface SOC pool (0.83 kg C

m�2) was much lower than that of the Loess Plateau

(2.46 kg C m�2, Xu et al., 2003). The low SOC level

is attributable to the low plant production due to

water deficit, low temperatures, and serious water

and wind erosion.

Acknowledgements

This research was supported by the Japanese Society

for Promotion of Science (JSPS) Core University

Programme and the Global Centre of Excellence

(COE) Programme of the Ministry of Education,

Culture, Sports, Science and Technology of Japan.

The authors thank Prof. Ming’an Shao and Dr Jun

Fan of the Institute of Soil and Water Conservation,

Chinese Academy of Sciences, for their support.

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