Catena 109 (2013) 110–117
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Catena
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Soil organic and inorganic carbon contents under various land uses
across a transect of continental steppes in Inner Mongolia
Zhi-Ping Wang a, b,⁎, Xing-Guo Han a, c, Scott X. Chang b, Bin Wang a, Qiang Yu c, Long-Yu Hou a, Ling-Hao Li a
a
b
c
State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Nanxincun 20, Xiangshan, Beijing 100093, China
Department of Renewable Resources, University of Alberta, Edmonton T6G 2E3, Alberta, Canada
State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history:
Received 9 July 2012
Received in revised form 8 April 2013
Accepted 19 April 2013
Keywords:
Spatial distribution
Soil layer
Particle-size fraction
Temperate grassland
Land cultivation
Northern China
a b s t r a c t
A huge amount of soil organic and inorganic carbon (SOC and SIC) is stored in soils and those stored carbon (C)
plays an important role in affecting the global climate change. Knowledge on the spatial distribution of and land
management effects on soil C in the Eurasian steppes is essential for understanding its impact on the global
climate change. We investigated both the distribution of SOC and SIC contents and the effect of land cultivation
on SOC content across a transect of continental steppes in Inner Mongolia, including the western plateau and
the eastern plain. There were weak increasing trends in SOC and SIC from west to east across the transect. In
general, SOC decreased but SIC increased with increasing soil depth. Both SOC and SIC were markedly different
among various land uses in the Xilin River basin, approximately located in the center of the studied continental
steppes. Grasslands and meadows together accounted for 93.9 and 98.1%, respectively, of the total SOC and SIC
storages in the basin, whereas wetlands played an insignificant role in soil C storage due to both their small area
and sandy soil texture. Land cultivation caused losses of 7.7 and 18.1% of SOC in the 0–10 cm soil layer on the
western plateau and the eastern plain, respectively. The loss of SOC mainly occurred in the coarse-size fraction
(>0.25 mm) in the surface 0–10 cm soil layer. Sandy soil texture, low mean annual temperature and infrequent
land cultivation somewhat limited the loss of SOC in the continental steppes.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The global air temperature is projected to rise due to increasing
concentrations of carbon (C) dioxide (CO2) and other greenhouse
gasses in the atmosphere (IPCC, 2007). Finding biotic and abiotic
approaches to mitigate such projected change in greenhouse gas
concentrations in the atmosphere is an urgent matter. The global soil
C storage, estimated at about 2500 Pg (1 Pg = 1015 g), which is 3.3
times the size of the atmospheric C pool and 4.5 times the size of the
biotic C pool (Lal, 2004), is the largest terrestrial C pool (Batjes,
1996), and thus is a key component of the global C cycle. A small
change in the soil C pool may result in large changes in greenhouse
gas fluxes between the soil and the atmosphere. The role of soils in
the global C cycle remains uncertain as mechanisms controlling soil C
storage are still poorly understood.
Land management practices have considerable impact on soil C storage (Arevalo et al., 2009; Houghton et al., 1999; Lal, 2004; Schuman et
al., 2002). For instance, land cultivation tends to accelerate soil organic
C (SOC) decomposition; nearly all loss of SOC occurred within 20 years
after cultivation in some studies, most of which occurred within the
⁎ Corresponding author at: Institute of Botany, Chinese Academy of Sciences, Xiangshan
20, Beijing, China. Tel./fax: +86 10 6283 6635(o).
E-mail addresses: [email protected], [email protected] (Z.-P. Wang).
0341-8162/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catena.2013.04.008
initial 5 years (Davidson and Ackerman, 1993). Steppe soils have a
substantial potential to sequester atmospheric C. Improved land
management practices have been suggested to be a viable approach
for reducing C release from the soil and sequestering C into the soil
(Houghton et al., 1999; Lal, 2004). This is a recognized cost-effective
and environmental friendly strategy for mitigating the increased
atmospheric CO2 concentrations. Thus, the effect of land management
practices on soil C needs to be evaluated and better understood.
Besides SOC, many soils contain a different C type: soil inorganic C
(SIC). The SIC comprises approximately a third of the global C pool in
soils (Hirmas et al., 2010), but factors affecting the dynamics of SIC is
poorly understood (Drees et al., 2001; Wu et al., 2009). Most of SIC is
found in arid and semi-arid regions and accumulates as carbonate
minerals, predominantly calcite (Drees et al., 2001). In China, a few
studies have been conducted to date to investigate SIC distribution
and storage in different ecosystems. For instance, the SIC content
following the conversion of cropland to forest was examined on the
central Loess Plateau of China which has a semi-arid continental
climate (Chang et al., 2012). Desert soils are a large reservoir of SIC
in northwestern China due to carbonates contained in the parent geological material and water deficit (Feng et al., 2001). Total SIC storage
was estimated to be around 55 Pg C in China using the second national soil survey data (Mi et al., 2008; Wu et al., 2009). Thus, changes in
SIC storage in various land uses and the response of SIC to land use
management need to be better understood.
Z.-P. Wang et al. / Catena 109 (2013) 110–117
Providing accurate estimates of soil C storage in China is both essential for appraising the global C inventory and critical for mitigating the
rising greenhouse gas concentrations in the atmosphere. Steppes
account for 41.7% of China's terrestrial land area and are mainly distributed in arid and semi-arid regions (NSBC, 2002), of which the steppes
in Inner Mongolia are an important component (Zhang et al., 1997).
Previously, a Northeast China Transect (NECT) was widely used as a
platform for studying the effect of global climate change on plants
and soils (Ni and Wang, 2004). Precipitation is the primary driving
force for changes in soil C contents along the NECT, while land use is
the second most important driver (Ni and Wang, 2004; Zhang et al.,
1997). Climate and soil texture exert dominant controls on soil organic
matter content in temperate grasslands in Inner Mongolia (Evans et al.,
2011). However, factors affecting soil C storage across the continental
steppes in Inner Mongolia have rarely been studied experimentally
and this should thus be a priority area for additional research.
The overall objective of this study was to examine the distribution
in SOC and SIC and to evaluate the effect of land cultivation on SOC
across the continental steppes in Inner Mongolia. Specifically, our
study was focused on three questions: i) How did both SOC and SIC
contents distribute across the continental steppes? ii) How did both
SOC and SIC contents differ under various land uses? and iii) How
did land cultivation affect SOC content in the continental steppes?
This study provides a better understanding of the roles of sites and
land cultivation in soil C in the Eurasian steppes.
2. Materials and methods
2.1. Site description
The transect used in this study (Fig. 1) was laid out within the
larger NECT (Ni and Wang, 2004; Zhang et al., 1997) and was similar
to the one studied by Cheng et al. (2009). Briefly, our transect spans
111
from 111.9 to 123.3°E and from 41.8 to 45.1°N. The percent of land
under cultivation and human population density largely increased
from west to east across the continental steppes in Inner Mongolia
(IMSB, 2012). Accordingly, the transect is divided into two distinct
sections: the western grassland-dominated plateau (8 sites) and the
eastern cropland-dominated plain (4 sites). In the continental
steppes of Inner Mongolia, plant growth extends usually from late
April to early October while soils are mainly Kastanozems based on
the FAO system of soil classification (IUSS Working Group WRB,
2006). A detailed description on the continental steppes of Inner
Mongolia can be found in Chen and Wang (2000).
The Xilin River basin (115°32′–117°12′ E, 43°26′–44°39′ N;
902–1506 m above sea level; with a total area of 10,786 km2) is approximately located in the center of the transect. A basin may be used
as a land unit to survey the distribution of soil C (Díaz-Hernández et
al., 2003); therefore, the Xilin River basin was selected as a case study
to understand the distribution of soil C in various land uses. The Xilin
River basin has a mean annual temperature and precipitation of approximately 0.6 °C and 350 mm, respectively.
Land uses in the Xilin River basin include grasslands, meadows,
wetlands, croplands, and sandlands that constitute 76.1, 15.1, 0.3,
1.4, and 7.1% of the total land base, respectively (Fig. 1). Grasslands,
the most extensive landscape, may be roughly classified into typical
and desert steppes in the southern and northern portions of the
basin, respectively. Plant species are dominantly Leymus chinense,
Stipa grandis, Stipa baicalensis, Filifolium sibiricum, and Agvopyron
cristatum in typical steppes and Stipa breviflora, Artemisia pectinata,
and Cleistogenes songorica in desert steppes. Most meadows were
fenced in the growing season and harvested for hay in the autumn.
Wet meadows are usually scattered in low-lying areas or near the
Xilin River. Wetlands are distributed along the river banks. The
Xilinhot Reservoir near Xilinhot city blocks the flow of water and induces seasonal wetlands in the lower reaches. As a result, wetlands
B
C
N
Altitude (m)
1500
1200
900
600
300
0
111
113
115
117
119
Longitude(oE)
121
123
46
45
44 o N)
(
43
de
42
41 atitu
L
125
Sampling sites
A
D
China
Grassland
Meadow
Cropland
Wetland
....... Sandland
0 10 20 Kilometers
Grasslands
Xilin River basin
0 200 400 Kilometers
Fig. 1. The geographic distribution of grasslands in China (A), the 12 soil sampling sites across continental steppes in Inner Mongolia (B), and a total of 183 sites (C) were sampled in
various land uses in the Xilin River basin (D).
112
Z.-P. Wang et al. / Catena 109 (2013) 110–117
may be classified into perennial and seasonal types. Croplands were
derived from cultivation of grasslands in the 1960–70s. Croplands
were usually planted with spring wheat, rape, and barley in a rotation
system. Sandlands were wind-derived in the drought desertification
period of the late Pleistocene and presently exist as sand dunes.
Sandlands are classified into shrubby and herbaceous types, mainly
covered by Artemisia arenaria, Prunus padus, and Ulmus macrocarpa.
Ltd, Shangyu, Zhejiang, China). The coarse-size fraction was gently
backwashed off the 0.25 mm sieve into a beaker. Water plus soil
that went through the 0.25-mm sieve was poured onto a 0.053-mm
sieve and the sieving procedure was repeated. The medium- and
fine-size fractions were washed into other beakers. All the fractions
were oven-dried at 50 °C, weighed, and ball-milled to fine powder
for analyzing SOC content.
2.2. Soil sampling
2.4. Soil C analysis
Field soil samplings were undertaken along the transect in July–
August 2011 and in the Xilin River basin in summer 2005 (Fig. 1).
The 12 sites approximately evenly distributed along the transect
were randomly selected and sampled. Their latitudes, longitudes,
and altitudes were recorded using a GPS unit (eTrex, Garmin). The
number of soil sampling sites was determined based on the area
and potential C density of each land use in the Xilin River basin. A
total of 183 sites were sampled in the basin, of which 80, 38, 40, 12,
and 13 sites were located in grasslands, meadows, wetlands, croplands, and sandlands, respectively (Fig. 1). The overall soil sampling
was representative for the transect and the basin.
Most of soil samples were collected using a stainless steel corer
(3.5 cm diameter) but a few through digging soil pits. Surface soils
were generally more susceptible to land cultivation and changes in
environmental factors, whereas underlying soils were much less affected by these factors. Accordingly, soils were sampled with uneven
depth intervals. Each site was sampled in three locations about 100 m
apart. Upland soils in each location were randomly collected within
an area with about 20 m in diameter and then mixed to form a composite sample for each layer. The 0–30 and 30–100 cm depths are referred to as the upper and lower layers, respectively. For each
location, six points were sampled for the 0–10 and 10–30 cm layers,
three of which were further sampled for the 30–60 and 60–100 cm
layers. For wetlands, however, the sampling distances among locations or points were adjusted according to the specific landforms.
All soil samples were minimally processed in the field and then
taken to the laboratory. Gravels generally accounted for a small fraction and were eliminated from the soil. In addition, visible plant materials were removed from the soil. To minimize land disturbance,
only 1 soil profile was dug to determine bulk density (BD) at each
site. The BD was determined using a stainless steel cylinder (5 cm
in diameter and 5 cm in height). Soils for BD determination were
oven-dried at 105 °C to a constant weight. Soils for SOC and SIC measurements were air-dried and sieved through a 2 mm sieve, and then
ground to fine powder (mesh number 100) using a pestle and mortar.
Soil C contents were analyzed following standard procedures. The
SOC was determined using a digestion method (Liu, 1996). First, 5 mL
0.8 mol L −1 K2Cr2O7 and 5 mL concentrated H2SO4 were added into
weighed soils and then boiled at 170–180 °C for 5 min, and then
the remaining K2Cr2O7 was titrated using 0.2 mol L −1 FeSO4. The
SIC was measured by manometric collection of CO2 evolved after
HCl addition into soils (Liu, 1996). Here, the SIC was assumed to
exist in the form of CaCO3 in soils. A factor of 0.12, the mole fraction
of C in CaCO3, was used to convert the calculated carbonate minerals
to SIC content.
2.3. Determination of soil texture and particle-size fractionation
Soil particles were classified into sand (2.0–0.05 mm), silt
(0.05–0.002 mm), and clay (b0.002 mm) based on the United States
Department of Agriculture system. To determine soil particle-size
composition, an air-dried soil sample of about 0.8 g that had passed
through a 2 mm sieve was put into a 100 mL beaker, to which 3 mL
0.5 mol L −1 sodium hexametaphosphate and 50 mL deionized
water were subsequently added. Then, the beaker was heated on a
hot plate and kept gently boiling for 1 h. The soil in the beaker was
cooled and then analyzed using a laser particle-size analyzer
(Mastersizer 2000, Malvern).
Soil particle-size fractionation was carried out by the wet-sieving
method according to Jastrow (1996) and McLauchlan and Hobbie
(2004). Coarse- (> 0.25 mm), medium- (0.25–0.053 mm), and fine(b0.053 mm) size fractions were separated using a series of sieves
(0.25- and 0.053-mm mesh size). Briefly, 50 g of air-dried soil was
submerged in deionized water on top of a 0.25-mm sieve at room
temperature, and then sieved for 10 min by moving the sieve up
and down at a rate of 30 repetitions min −1 using a shaker (Jinwei
2.5. Calculations and statistical analysis
Mean annual temperature (MAT) and precipitation (MAP) across
the transect were calculated using an interpolated method detailed
in Cheng et al. (2009). Mean annual meteorological values during
1971–2000 and geographical parameters in 7 locations (China
Meteorological Data Sharing Service System, http://cdc.cma.gov.cn/)
close to the sampling sites across the transect were used to obtain
the regression equations:
2
MAT ð CÞ ¼ 135:2936–0:7503EL–0:7987NL–0:01044A; R ¼ 0:96
2
MAP ðmmÞ ¼ −2523:75 þ 15:5625EL þ 23:3877NL–0:0141A; R ¼ 0:91
where EL is east longitude, NL is north latitude, and A is altitude. The
MAT and MAP in the 12 sampling sites (Fig. 1) were calculated using
those regression equations.
Soil C density (SCD) was calculated using layer interval (Di) (cm),
BD (Bi) (g cm −3) and mean SOC/SIC content (SCi) (g kg −1):
n
SCD ¼ ∑ Di Bi SC i
i¼1
where n is the number of soil layers, i is one of the 0–10, 10–30,
30–60, and 60–100 cm soil layers. An area-weighed approach was
used to calculate total SOC and SIC storages in the Xilin River basin.
Statistical analysis was performed using the Statistical Analysis
System program (SAS Institute, 1999). Duncan's multiple range test
was employed to compare the means of BD, SOC, or SIC among soil
layers, land uses, or the sampling sites. Different letters indicated
significant differences in each group of treatments. Statistical significance was set at P b 0.05.
3. Results
3.1. Environmental factors and soil C across continental steppes in Inner
Mongolia
The MAT was lower in the middle section than in the western or the
eastern sections, while the MAP linearly increased from west to east
across the continental steppes (Figs. 1 and 2A). The BD and soil texture
were different among some of the 12 sampling sites but lacked systematic changes across the transect of the continental steppes (Fig. 2B, C).
The SOC and SIC contents showed weak increasing trends from west
to east across the continental steppes (Fig. 2D, E).
Percentage of particle-size fractions (%)
Soil layer
(cm)
Particle-size fraction
(mm)
0–10
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
10–30
SOC content in particle-size fractions (g kg−1)
0–10
10–30
Site (Longitude, °E)a
111.89
112.71
113.55
114.90
115.83
116.67
117.68
118.36
120.33
120.96
122.06
123.25
31.3 B c
36.6 B efg
32.2 B a
32.8 B ef
51.2 A bcd
16.0 C bc
9.8 B c
5.8 B fg
21.3 A bc
5.8 B de
6.5 B d
23.5 A abc
43.5 AB b
43.7 AB ef
12.7 C bcd
53.3 A b
39.0 B de
7.7 C c
3.2 B d
3.5 B gh
13.8 A de
2.8 B fg
3.6 B e
15.3 A de
58.9 A a
34.9 B fg
6.2 C d
63.2 A a
29.5 B e
7.3 C b
2.2 DE d
3.8 C gh
15.8 A de
1.8 E g
3.3 CD e
14.3 B de
45.7 A b
44.0 A ef
10.3 B cd
46.6 A bc
41.2 A cd
12.2 B bc
7.7 CD cd
9.4 C cde
23.5 A bc
4.2 D efg
5.5 D d
19.9 B bcd
28.8 BCD c
59.8 A bc
11.4 D bcd
33.3 BC ef
44.5 AB cd
22.2 CD bc
9.6 C c
11.8 C bc
32.0 A a
9.4 C bc
9.5 C b
23.8 B abc
26.1 BC c
55.0 A de
18.9 C bc
28.5 B f
53.5 A bc
18.0 C bc
16.7 C b
13.5 C b
31.8 A a
8.2 D cd
8.1 D bc
25.7 B ab
29.8 C c
47.3 B cd
22.9 CD ab
27.3 C f
58.2 A b
14.4 D bc
12.4 C bc
11.3 CD bcd
30.6 A a
7.7 E cd
8.9 DE b
26.2 B a
42.4 A b
45.6 A def
12.0 B bcd
41.9 A cd
50.3 A bcd
7.8 B c
9.7 C c
8.5 C def
30.9 A a
4.8 D ef
6.9 CD cd
25.4 B ab
52.6
31.5
15.8
37.7
40.9
21.4
24.3
18.4
25.1
15.4
13.0
18.9
42.3 B b
42.4 B ef
15.3 C bcd
32.4 B ef
52.7 A bc
14.9 C bc
10.0 B c
7.6 BC ef
18.5 A cd
2.9 C fg
3.1 C e
11.8 B e
11.7 B d
70.5 A a
17.8 B bc
8.4 B g
80.7 A a
10.9 B bc
7.4 B cd
2.0 C h
12.0 A e
3.4 C efg
1.4 C f
10.7 AB e
13.9 DE d
69.1 A ab
17.0 D bcd
5.4 E g
58.5 B b
36.2 C a
22.3 A a
6.1 CD fg
11.8 BC e
11.8 BC b
3.4 D e
15.6 AB de
A ab
BC g
D bcd
B de
B cde
CD b
AB a
Ca
Ab
Ca
Ca
BC cd
Z.-P. Wang et al. / Catena 109 (2013) 110–117
When those variables were calculated for the western plateau (left
of the dashed line, Fig. 2) and the eastern plain (right of the dashed
line, Fig. 2), differences between the two regions were readily noticeable. On average, the MAT was 5.2 and 2.2 °C, while the MAP was 407
and 274 mm, on the western plateau and the eastern plain, respectively (Fig. 2A). The BD in the 0–10, 10–30, 30–60, and 60–100 cm
soil layers were 1.51, 1.48, 1.53, and 1.46 g cm −3, respectively, on
the western plateau and 1.49, 1.54, 1.59, and 1.72 g cm −3, respectively, on the eastern plain (Fig. 2B). Soils consisted roughly of 60%
sand, 36% silt, and 4% clay in both the 0–10 and 10–30 cm soil layers,
similar between the western plateau and the eastern plain (Fig. 2C).
The SOC in the four soil layers were 14.1, 9.3, 7.3, and 3.6 g kg −1, respectively, on the western plateau, and 15.2, 8.8, 6.5, and 4.1 g kg −1,
respectively, on the eastern plain (Fig. 2D). Similarly, the SIC were 2.0,
Fig. 2. Spatial distributions of mean annual temperature and precipitation (A), bulk soil
density (B), soil texture (C), soil organic carbon (D), and soil inorganic carbon
(E) across continental steppes in Inner Mongolia. Soil textural compositions are classified into sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (b0.002 mm). The X axis
indicates the longitudes of the 12 sampling sites with the same sequence in Fig. 1.
Trend lines are shown for the MAP (solid line), BD, SOC and SIC in the 0–10 cm
(solid line), 10–30 cm (dashed line), 30–60 cm (dot line), and 60–100 cm (dot and
dashed line) soil layers, and the percentage of sand (solid line for the 0–10 cm and
dashed line for the 10–30 cm). The percentages of silt and clay as well as all standard
deviations (SD) are omitted for the purpose of clarity.
Table 1
Percentage of particle-size fractions and their soil organic carbon (SOC) contents across the continental steppes of Inner Mongolia.
Means with different upper- and lower-case letters identify significant differences (P b 0.05) among the percentages or the SOC contents in particle-size fractions of two soil layers in each site and among the 12 sampling sites in each
particle-size fraction, respectively, analyzed using Duncan's multiple range test.
a
The sites are expressed using the longitudes of the 12 sampling sites with the same sequence in Fig. 1.
113
114
Z.-P. Wang et al. / Catena 109 (2013) 110–117
4.0, 6.5, and 8.6 g kg −1, respectively, on the western plateau, and 6.4,
8.6, 9.6, and 10.7 g kg −1, respectively, on the eastern plain (Fig. 2E).
The SOC in particle-size fractions were different (P b 0.05) among
the 12 sampling sites but also lacked systematic changes across the
continental steppes in Inner Mongolia (Table 1). On average, the
coarse-, medium-, and fine-size fractions accounted for approximately 35, 50, and 15% of the total soil mass, respectively, in the 0–30 cm
soil layers. The SOC contents were generally the highest in the fine
fraction, followed by the coarse fraction (Table 1).
3.2. Soil C in various land uses in the Xilin River basin
The BD slightly increased with increasing soil depth, averaging
1.45, 1.16, 1.21, 1.31, and 1.35 g cm −3 in grasslands, meadows,
wetlands, croplands, and sandlands, respectively, in the 0–10 cm
soil layer (Table 2). The SOC contents significantly decreased with
increasing soil depth. Meadows had the highest SOC content of
33 g kg −1 in the 0–10 cm layer, whereas sandlands had the lowest
(Table 2). In contrast, the SIC was negligible in sandlands but
increased with increasing soil depth in the other land uses. The SIC
were usually lower than the SOC in almost all soil layers (Table 2).
Generally speaking, SOC dominated in the upper soil layer while SIC
in the lower soil layer (Fig. 3). Average SOC densities were 4.3, 8.0, 4.3,
4.6, and 2.8 kg C m −2 in grasslands, meadows, wetlands, croplands,
and sandlands, respectively, in the upper layer, and 4.2, 7.6, 5.8, 5.8,
and 2.9 kg C m −2, respectively, in the lower layer. Correspondingly,
SIC densities in those land uses were 0.61, 1.79, 0.79, 1.11, and
0.02 kg C m−2, respectively, in the upper layer and were 3.39, 7.01,
2.05, 3.66, and 0.06 kg C m−2, respectively, in the lower layer.
Meadows had the highest SOC density of 15.7 kg C m −2 in the
0–100 cm depth, followed by about 10 kg C m−2 in wetlands and
croplands, 8.5 kg C m −2 in grasslands, and 5.6 kg C m −2 in sandlands.
Meadows also had the highest SIC density of 8.8 kg C m−2, whereas
sandlands had almost no SIC, indicating that SIC was easily leached in
sandlands while perhaps accumulated in meadows.
Based on the total area of 10,786 km 2 (Fig. 1) and land use ratios
(Fig. 4A) and average C densities (Fig. 4B), total soil C storages were
Table 2
Soil characteristics in various land uses in the Xilin River basin, Inner Mongolia.
Land use
Number of
sampling sites
(n)
Soil layer
(cm)
BD
(g cm−3)
SOC
(g kg−1)
SIC
(g kg−1)
Grassland
80
80
80
76
38
38
38
38
40
40
40
40
12
12
12
12
13
13
13
13
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
1.45 c
1.49 b
1.54 a
1.56 a
1.16 c
1.32 b
1.42 a
1.43 a
1.21 b
1.33 ab
1.43 a
1.45 a
1.31 a
1.4 a
1.43 a
1.47 a
1.35 b
1.64 a
1.66 a
1.64 a
12.2
8.6
5.1
3.1
32.7
17.5
9.9
6.3
18.9
12.3
7.2
5.9
13.0
11.4
8.5
4.4
16.7
4.8
2.3
2.6
0.42
1.92
3.50
3.43
1.77
5.76
7.24
6.59
1.82
1.98
1.88
2.41
2.78
3.26
3.95
3.84
0.06
0.04
0.04
0.06
Meadow
Wetland
Cropland
Sandland
a
b
c
d
a
b
c
c
a
b
b
b
a
a
ab
b
a
b
b
b
c
b
a
a
b
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
The mean value is shown for each determination. Means with different lower-case letters identify significant differences (P b 0.05) between the soil layers in each land use,
analyzed using Duncan's multiple range test. Abbreviations are bulk density (BD), soil
organic carbon (SOC), and soil inorganic carbon (SIC).
Fig. 3. The SOC and SIC densities were determined separately in two distinct soil layers:
the upper (0–30 cm) layer (A) and the lower (30–100 cm) layer (B) of various land
uses in the Xilin River basin. Means with different upper- and lower-case letters indicate significant differences (P b 0.05) among land uses for SOC and SIC, respectively, in
the upper or lower soil layers, analyzed using Duncan's multiple range test. The value is
mean ± standard deviation. The numbers of sampling sites (n) are the same as those
for various land uses in Table 2.
estimated to be 101.8, 39.9, 0.6, 2.3, and 4.2 Tg C in grasslands,
meadows, wetlands, croplands, and sandlands, respectively, in the
0–100 cm depth (Fig. 4C). Total SOC and SIC storages were 104.4
and 47.3 Tg C, most of which were distributed in grasslands and
meadows, in the Xilin River basin.
3.3. Effect of land cultivation on SOC in the continental steppes
The effect of land cultivation on SOC was examined by comparing
SOC contents in paired grassland–cropland sites (Table 3). On the
western plateau, a comparison in SOC between paired grassland–cropland sites was provided in Wang et al. (2008). On average, land cultivation decreased SOC content by 7.7, 16, −2.4, and 13.9% in the 0–10,
10–30, 30–60, and 60–100 cm soil layers, respectively. On the eastern
plain, the croplands were usually maize fields converted from grasslands around the 1970–1980s. Average SOC were 15.2, 8.8, 6.5, and
4.1 g kg −1 in the 0–10, 10–30, 30–60, and 60–100 cm soil layers, respectively, in grasslands, and 12.0, 11.8, 8.6, and 5.3 g kg−1, respectively, in croplands. The SOC decreased on one site, increased on another
site, and decreased in surface soils of the other two sites. As a result,
land cultivation decreased SOC content by 18.1, −28.6 (a negative
value indicates an increase), −42.1, and −95.5% in the 0–10, 10–30,
30–60, and 60–100 cm soil layers, respectively, on the eastern plain.
Thus, conversion of grasslands to croplands caused more SOC loss in
the 0–10 cm soil layer on the eastern plain than on the western
plateau.
Land cultivation largely did not change the distribution of soil
particle-size fractions (Table 4). The medium-sized fraction dominated
in all paired grassland–cropland sites. On average, coarse-, medium-,
and fine-size fractions accounted for approximately 30%, 55%, and
15%, respectively, of the total soil mass in the 0–30 cm soil. In all
particle-size fractions, however, the SOC contents were not different
(P > 0.05) between the 0–10 and 10–30 cm soil layers (Table 4). The
SOC content in the coarse-size fraction of the 0–10 cm soil layer was
higher in grasslands than in croplands. Accordingly, the SOC content
in the coarse-size fraction of the surface soils was most affected by
land use change whilst the fine fraction the least.
Z.-P. Wang et al. / Catena 109 (2013) 110–117
115
4. Discussion
4.1. Soil C across the continental steppes
The SOC showed weak increasing trends from west to east across
the transect of the continental steppes (Fig. 2D). There was a significant negative correlation between the SOC in the 0–10 cm soil layer
and the MAT (R 2 = 0.37, P b 0.05, n = 12). Although the MAP
showed an increasing trend from west to east across the continental
steppes, there was no significant correlation between the SOC in the
0–10 cm soil layer and the MAP (R 2 = 0.09, P > 0.05, n = 12),
indicating that the MAP was not a major driving factor affecting the
distribution of SOC across the continental steppes in Inner Mongolia.
It is well known that SOC content is controlled by the production
and decomposition of organic matter in an ecosystem. Temperature
and precipitation, two major determinants of microbial processes,
are usualy identified as the primary factors controlling the dynamics
of soil organic matter. It is established that SOC content is usually affected by soil texture, climate, and land management practices. The
relative importance of those factors would be site specific. Using a
modeling approach, Evans et al. (2011) found that SOC increased
with increasing precipitation and decreased with increasing temperature across the NECT in Inner Mongolia. This may be explained by
the greater response of SOC production than decomposition to precipitation and the greater response of decomposition than production
to temperature. Our results are consistent with the effect of temperature but inconsistent with the effect of precipitation reported in Evans
et al. (2011), partly because the two transects studied were not identical. Our results indicate that MAT and MAP might be partly responsible for driving the spatial distribution of SOC across the continental
steppes in Inner Mongolia (Fig. 2). Soil texture may be a major factor
determining SOC content and storage in the continental steppes.
Interestingly, SIC also showed increasing trends across the continental steppes from west to east (Fig. 2E). The SIC comprises
lithogenic and pedogenic components, with the former inherited
from parent material while the latter formed through the dissolution
Fig. 4. The SOC and SIC storages in the Xilin River basin, Inner Mongolia. Land use ratios
(A) and mean carbon densities in the 0–100 cm soil depth (B) were used for estimating
carbon storages in the 0–100 cm soil depth (C). Means with different upper- and lowercase letters indicate significant differences (P b 0.05) among land uses for SOC and SIC
densities, respectively, analyzed using Duncan's multiple range test. Carbon density is
mean ± standard deviation. The numbers of sampling sites for carbon densities (n) are the
same as those in various land uses in Table 2.
Table 3
Effects of land cultivation on soil organic carbon (SOC) in the western grassland-dominated plateau and the eastern cropland-dominated plain.
Soil layer
(cm)
Western plateau
0–10
10–30
30–60
60–100
Eastern plain
Site 1
Change in SOC (%)
12.7
27.8
−8.3
29.2
BD
(g cm−3)
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
0–10
10–30
30–60
60–100
Site 2
SOC
(g kg−1)
Grassland
1.44
20.4 a
1.46
11.1 bc
1.58
7.8 cd
1.67
2.0 e
Cropland
1.68
11.3 bc
1.51
12.2 b
1.56
10.7 bc
1.77
5.0 de
Change in SOC (%)
44.6
−9.9
−37.2
−150
Site 3
−15.4
−13.2
−2.3
−5.3
BD
(g cm−3)
Site 4
24.6
16.8
−2.9
−21.9
SOC
(g kg−1)
BD
(g cm−3)
Average
8.7
32.6
4.1
53.7
SOC
(g kg−1)
BD
(g cm−3)
7.7
16
−2.4
13.9
SOC
(g kg−1)
BD
(g cm−3)
SOC
(g kg−1)
1.5
1.61
1.66
1.71
5.5
2.8
3.4
3.6
a
b
ab
ab
1.69
1.77
1.94
1.89
8.3
5.8
2.0
0.3
ab
bc
cd
d
1.34
1.31
1.17
1.61
26.7
15.4
12.9
10.4
a
bc
bc
c
1.49
1.54
1.59
1.72
15.2
8.8
6.5
4.1
1.45
1.63
1.58
1.61
3.1
2.7
2.3
3.0
b
b
b
b
1.16
1.36
1.82
1.67
10.9
9.2
4.7
1.0
a
ab
bcd
cd
1.24
1.21
1.27
1.22
22.6
23.0
16.6
12.0
a
a
b
bc
1.38
1.43
1.56
1.57
12.0
11.8
8.6
5.3
43.6
3.6
32.4
16.7
−31.3
−58.6
−135
−233.3
15.4
−49.4
−28.7
−15.4
18.1
−28.6
−42.1
−95.5
The 4 paired sites were independent of each other. A detailed description of the 4 paired sites on the western plateau can be found in Wang et al. (2008), while the 4 paired sites on
the eastern plain are consistent with the 4 eastern sites in Fig. 1 and Table 1, the longitudes (°E) are 120.33 for site 4, 120.96 for site 3, 122.06 for site 2, and 123.25 for site 1. The
mean value is shown for each determination (n = 3). For SOC, means with different lower-case letters identify significant differences (P b 0.05) between soil layers of land uses in
each site, analyzed using Duncan's multiple range tests. Abbreviations: BD, bulk density; SOC, soil organic carbon. % change in SOC = 100 × (SOC in grassland − SOC in
cropland) / SOC in grassland. The original data on the western plateau (Wang et al., 2008) are not listed here but used to perform the calculations.
116
Z.-P. Wang et al. / Catena 109 (2013) 110–117
Table 4
Soil organic carbon (SOC) held in particle-size fractions between grasslands and croplands on the western plateau in Inner Mongolia.
Soil layer Particle-size fraction Site 1
(cm)
(mm)
Grassland
Percentage of particle-size fractions (%)
0–10
10–30
SOC content in particle-size fractions (g kg−1)
0–10
10–30
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
2–0.25
0.25–0.053
b0.053
37.5 b
51.3 a
11.2 c
33.1 b
56.4 a
10.4 c
6.5 b
4.5 b
20.1 a
3.5 b
3.9 b
21.9 a
Site 2
Site 3
Site 4
Cropland
Grassland
Cropland
Grassland
Cropland Grassland
33.1 b
59.5 a
7.4 c
30.0 b
62.8 a
7.2 c
5.3 b
5.4 b
18 a
4.5 b
5.2 b
20.7 a
38.5
55.4
6.1
41.9
50.4
7.7
7.6
4.7
22.6
3.3
4.2
23.4
34.4
58.6
6.9
35.7
56.9
7.4
3.9
5.3
26.5
3.3
5.2
23.1
12.2
60.3
27.4
12.7
60.6
26.7
39.1
18.8
33.5
27.9
14.6
31.4
15.3
65.9
18.8
8.2
66.1
25.7
28.2
15.3
27.3
26.6
14.1
30.2
b
a
c
a
a
b
b
b
a
b
b
a
b
a
c
b
a
c
c
c
a
c
c
b
b
a
b
b
a
b
a
d
b
c
d
bc
b
a
b
b
a
b
a
b
a
a
b
a
16.2
63.9
19.9
20.6
66.1
13.3
30.6
20.4
30.4
31.8
19.4
33.9
b
a
b
b
a
b
a
a
a
a
a
a
Cropland
ND
ND
The details of the 4 paired sites can be found in Wang et al. (2008). ND is Not Determined due to the loss of soil samples. Means with different lower-case letters identify significant
differences (P b 0.05) among grassland and cropland, soil layers and particle-sizes in the percent of particle-size fractions or the SOC content in particle-size fractions of two soil
layers in each site, analyzed using Duncan's multiple range tests.
and precipitation of carbonates (Wu et al., 2009). Thus, SIC content is
mainly determined by soil parent material and climate. At the country
level, the SIC content in each soil layer is positively related with MAT
but negatively related with MAP (Mi et al., 2008). In this study,
however, there is a positive relationship between the SIC in the
0–10 cm soil and MAP (R 2 = 0.24, P b 0.05, n = 12) but not with
MAT (R 2 = 0.09, P > 0.05, n = 12) (Fig. 2A, E). On the other hand,
the continental steppes were dominated by the sand fraction
(Fig. 2C), which is mainly composed of silicate with little SIC. Thus,
the SIC content is mainly related to soil parent material rather than
climate across the continental steppes in Inner Mongolia.
In Inner Mongolia, however, other factors also potentially affected
soil C storage across the continental steppes. Grazing tends to
increase the occurrence of dust storms in the arid and semi-arid
regions. For instance, heavy grazing resulted in soil desertification
and a considerable loss of SOC in the semiarid Horqin sandy grassland
(Su et al., 2005). In recent decades, wind erosion and dust storms
have been frequent and severe in northern China (Feng et al., 2001;
Hoffmann et al., 2008; Normile, 2007; Wang et al., 2003, 2006).
Wind erosion and dust storms cause gains, losses, or relocation of
soils and considerably affect the soil constituents. This might also be
a potential factor affecting changes in soil C across the continental
steppes. In this study, we did not investigate the effect of grazing
and dust storms on soil C and this aspect deserves further research
in the future.
2003). The croplands converted from grasslands or meadows in the
past several decades had intermediate SIC densities between grasslands and meadows (Fig. 4B). As expected, sandlands had almost no
SIC since sandy soils are mainly composed of silicate. It is also possible
that sandy soils allowed the SIC to be leached out of the soil profile
quickly.
The total soil C storage was dominated by the SOC in the upper
layer but by the SIC in the lower layer (Table 2, Fig. 3). On average,
the SOC and SIC in the 0–30 cm layer accounted for about 50 and
15% of the totals in 1 m depth, respectively (Fig. 3), which is consistent with Batjes (2002), who found that about 44% of soil C in the
1 m depth was held in the top 30 cm soil in central and eastern
Europe. Average SOC and SIC densities were 9.4 and 4.4 kg C m −2, respectively, in the 0–100 cm soil depth in the Xilin River basin. The
values are lower than those in previous studies that also determined
soil C in the arid and semiarid grasslands of China. For instance,
average SOC density was approximately 20.6 kg C m −2 in the top
75 cm soil in the Qinghai–Tibetan Plateau (Wang et al., 2002) and
15 kg C m −2 to the 1 m depth in northwestern China (Wang et al.,
2003). Grassland soils had 15–20 kg C m −2 of SIC in Inner Mongolia
(Mi et al., 2008) and 13.6 kg C m −2 in northwestern China (Wu et
al., 2009).
4.2. Marked differences in soil C storage in various land uses in the Xilin
River basin
Land cultivation affected SOC storage and CO2 fluxes in terrestrial
ecosystems (Chen et al., 2006; Houghton et al., 1999; Lal, 2004; Wu et
al., 2009). Through a meta-analysis, Guo and Gifford (2002) found
that soil C storage declined by 59% at a global scale when land was
converted from pasture to cropland. The SOC decreased by 25, 39,
and 55% after alpine grasslands had been cultivated for 8, 16, and
41 years, respectively (Wu and Tiessen, 2002). However, the continental steppes in Inner Mongolia had slight to moderate loss of SOC
caused by cultivation (Table 3). Specifically, on the western plateau
cultivation caused a 7.7% loss of SOC in the 0–10 cm layer over
about a 35-year period, which was consistent with Celik (2005) and
Evrendilek et al. (2004) on a Mediterranean plateau. On the eastern
plain, cultivation caused a 18.1% loss of SOC in the 0–10 cm soil
layer over about a 20-year period, which was within the SOC loss
range in China (10–40%) caused by cultivation (Wu et al., 2003).
It is assumed that soil conditions were the same between
croplands and grasslands just before grasslands were converted to
croplands and the difference in SOC contents between paired grassland
and cropland sites explains the effect of cultivation (Table 3). Land cultivation breaks up soil aggregates and accelerates SOC decomposition.
As a result, the SOC contents decreased in the coarse-size fraction in
The SOC contents are usually higher in grasslands than in croplands. In the Xilin River basin, however, the SOC densities were slightly lower in grasslands than in croplands (Figs. 3A, D and 4B).
Grasslands are the most extensive landscape with a high portion of
desert soils. On the other hand, croplands were usually converted
from the most fertile grasslands. This could be why the SOC densities
in grasslands were slightly lower than that in croplands in the Xilin
River basin.
Grasslands played a dominant role in total SOC and SIC storages in
the Xilin River basin, contributing 68.8 and 67.6% of the totals, respectively (Fig. 4C), largely due to their extensive land area. Meadows
cover 15.1% of the basin but contributed 25.1 and 30.5% to the total
SOC and SIC, respectively. Therefore, meadows are an important
land use for total C storage in the basin. Wetlands cover approximately 0.4% of the basin and contributed about 0.4% of the total SOC
storage (Fig. 4). This is because wetlands are considerably barren in
the study area when compared with those in other areas such as in
Britain (Garnett et al., 2001) and southwestern China (Wang et al.,
4.3. Cultivation caused a slight loss of SOC on the western plateau but a
moderate loss on the eastern plain
Z.-P. Wang et al. / Catena 109 (2013) 110–117
the 0–10 cm soil layer of croplands as compared with grasslands
(Table 4). The coarse-size fraction represents an active SOC pool
while fine-size fraction is a passive SOC pool, and thus the former has
a faster turnover rate than the latter (Arevalo et al., 2012). This
suggests that in the continental steppes in Inner Mongolia land cultivation decreased the SOC contents mainly via the loss of SOC in the
coarse-size fraction of the surface soil.
Irrigation and fertilization were more common for croplands on
the eastern plain than on the western plateau. Environmental factors
have less influence on lower soil layers. A slight difference in the MAT
between the western plateau and the eastern plain might not cause a
significant temperature effect on the SOC in the lower layer. Furthermore, SOC has a turnover cycle of decadal or less in the upper layer
but in the order of hundreds to thousands of years or more in the
lower layer (Schimel et al., 1994). The SOC with more recalcitrant
components is usually held in the lower layer. All these enhanced
the SOC accumulation and/or decreased the SOC decomposition in
the lower layer of croplands on the eastern plain. In addition,
when grassland soils were barren with low SOC contents of about
0.4–0.7% in the lower layer, a slight change in the SOC content may
cause significant changes in the SOC loss rates (Table 3). Improved
land management practices are essential approaches for reducing
soil C loss and enhancing soil C sequestration. However, the effect of
land management practices on SOC contents has not been sufficiently
evaluated in the continental steppes of Inner Mongolia and thus
requires further research.
Acknowledgments
We acknowledge financial support provided by the Key Project of
National Natural Science Foundation of China (30830026), the National Basic Research Program of China (2010CB833502) and the general project of the National Natural Science Foundation of China
(30970518). Funding from the Natural Science and Engineering
Council of Canada (NSERC) to SXC provided partial support for this
work. We thank Yu-Zhang Dong, Yu Zhang, Zhan-Jun Liu, Jian-Jun
Chen, Guo-Liang Zhang, and Linghe Li for assistance with the field
samplings and Jianqiu Zheng and Jing Wang for assistance with the
laboratory analysis.
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