soil organic carbon stock and distribution in cultivated land converted to grassland in a...
TRANSCRIPT
Soil Organic Carbon Stock and Distribution in Cultivated LandConverted to Grassland in a Subtropical Region of China
J. H. Zhang • F. C. Li • Y. Wang • D. H. Xiong
Received: 8 May 2013 / Accepted: 30 September 2013 / Published online: 13 October 2013
� Springer Science+Business Media New York 2013
Abstract Land-use change from one type to another affects
soil carbon (C) stocks which is associated with fluxes of CO2
to the atmosphere. The 10-years converted land selected from
previously cultivated land in hilly areas of Sichuan, China
was studied to understand the effects of land-use conversion
on soil organic casrbon (SOC) sequestration under landscape
position influences in a subtropical region of China. The SOC
concentrations of the surface soil were greater (P \ 0.001)
for converted soils than those for cultivated soils but lower
(P \ 0.001) than those for original uncultivated soils. The
SOC inventories (1.90–1.95 kg m-2) in the 0–15 cm surface
soils were similar among upper, middle, and lower slope
positions on the converted land, while the SOC inventories
(1.41–1.65 kg m-2) in this soil layer tended to increase from
upper to lower slope positions on the cultivated slope. On the
whole, SOC inventories in this soil layer significantly
increased following the conversion from cultivated land to
grassland (P \ 0.001). In the upper slope positions, converted
soils (especially in 0–5 cm surface soil) exhibited a higher
C/N ratio than cultivated soils (P = 0.012), implying that
strong SOC sequestration characteristics exist in upper slope
areas where severe soil erosion occurred before land con-
version. It is suggested that landscape position impacts on the
SOC spatial distribution become insignificant after the con-
version of cultivated land to grassland, which is conducive to
the immobilization of organic C. We speculate that the
conversion of cultivated land to grassland would markedly
increase SOC stocks in soil and would especially improve the
potential for SOC sequestration in the surface soil over a
moderate period of time (10 years).
Keywords Soil organic carbon � Cultivated soil �Land conversion � Landscape position � Soil erosion �Sloping field
Introduction
During the conversion of natural ecosystems to agricultural
systems, with an increase in tillage intensity, the soil organic
carbon (SOC) pool is depleted, and therefore, the CO2 flux to
the atmosphere increases (Lal et al. 1998). The depletion of
SOC caused by the conversion of native grassland to culti-
vated fields is both extensive and well documented (McGill
et al. 1988; Davidson and Ackerman 1993; Kern and John-
son 1993; Monreal and Janzen 1993; Mikhailova et al. 2000;
Saviozzi et al. 2001; Guo and Gifford 2002; Zinn et al.
2005). Previous studies demonstrated that SOM decompo-
sition increased through physical disturbance by tillage, as a
result of macroaggregate disruption and the exposure of
previously protected soil to microbial processes (Cambard-
ella and Elliott 1992; Tisdall 1996; Ayoubi et al. 2012).
These studies emphasized the depletion of SOC pools fol-
lowing agricultural use, i.e., the conversion of forestland or
grassland into cultivated land. On the other hand, some
studies have addressed the reverse processes, i.e., the effects
of the conversion of cultivated land into grassland on SOC
stocks. In their review of C changes from 23 different
studies, Conant et al. (2001) concluded that SOC increased
in all but one (98 %) of the studies on cultivation-to-pasture
conversions, with a mean annual increase in SOC stocks of
J. H. Zhang � F. C. Li � Y. Wang � D. H. Xiong
Key Laboratory of Mountain Surface Processes and Ecological
Regulation, CAS, Chengdu 610041, People’s Republic of China
J. H. Zhang (&) � F. C. Li � Y. Wang � D. H. Xiong
Institute of Mountain Hazards and Environment, Chinese
Academy of Sciences and Ministry of Water Conservancy,
P O Box 417, Chengdu 610041, People’s Republic of China
e-mail: [email protected]
123
Environmental Management (2014) 53:274–283
DOI 10.1007/s00267-013-0181-y
nearly 5 % and a [3 % annual increase in SOC content.
However, the increment of SOC stocks varies with different
climatic conditions. For example, the increased SOC stocks
in Australia caused by a conversion from cultivation to
pasture would be well below the values measured in other
cooler, wetter environments. Therefore, it was noted that
there is a need for region-specific data regarding the vege-
tation communities and the full range of land-uses in any
given environment before accurate and reliable predictions
of SOC changes can be made (Wilson et al. 2011). In sub-
tropical regions such as the Sichuan Basin in China, rela-
tively few studies have contributed to an understanding of
SOC changes from the impacts of converting cultivated land
into grassland. In particular, there are scarce data on SOC
stock changes under the influence of landscape positions
after a similar land-use conversion.
After the 1998 Yangtze River Floods, the central gov-
ernment of China initiated the ‘‘ecological reconstruction
in the Upper Yangtze River Basin’’ and enacted the ‘‘grain-
for-green’’ policy, where cultivated fields with steep slopes
or those over 25� were converted into forestland and
grassland in 1999. This project was intended to convert
14.67 million ha of cultivated land into forestland and
grassland across China (The National Forestry Adminis-
tration of China 1999), which was one of the largest eco-
logical reconstructions worldwide to date. Over the last
decade, the question as to whether SOC stocks increased in
converted land remains an issue that must be addressed
because SOC is associated with CO2 flux to the atmosphere
and is therefore relevant to global climate change.
The distribution of SOC can be linked to a number of
factors at the landscape scale, such as the climate, topog-
raphy, geological processes, parent material, and vegeta-
tion (e.g., Zhang et al. 2010). However, given that some
factors are similar at different locations, other aspects such
as topography may contribute to spatial variations in the
distribution of SOC. Most studies to date involve the
estimation and mapping of SOC stocks and SOC autocor-
relation, and only a few have addressed the relationship
between landscape positions and the spatial distribution of
SOC stocks (Hancock et al. 2010; Zhao et al. 2012). Some
studies on cultivated soils at the field scale have revealed
that SOC stocks in footslope and toe slope positions are
greater than those in shoulder slope and mid-slope posi-
tions (Pierson and Mulla 1990; Papiernik et al. 2007).
Small-scale agricultural area studies show that soils at
lower slope positions have high SOC contents compared to
soils in upper slope positions (Zhang et al. 2006, 2008).
However, little is known of the SOC dynamics on the slope
after the conversion of cultivated land to grassland.
Previous studies on SOC changes were conducted by
considering only the land-use conversion or topographic
impacts. By combining these two aspects, this study
examined landscape position impacts on SOC sequestration
under land-use conversion to grassland from cultivated
land. In view of the geomorphological features of steep
slopes in this region, where soil property variations may
occur over a short distance in the line of the slope, SOC
changes were therefore examined on a hillslope scale (5 m
intervals). Our objectives for this study were (1) to estimate
SOC stocks and to quantitatively assess changes in SOC
stocks after the conversion from agricultural to grassland
ecosystems and (2) to examine landscape position effects
on SOC stocks and dynamics under such a conversion.
Materials and Methods
Study Site
This study was carried out in the central part of the Sichuan
Basin, southwestern China (30�0402800–30�3900000N and
104�1103400–104�5303600E; Fig. 1). The climate at the study
site is in the subtropical humid zone, which is characterized
by four distinct seasons, namely spring, summer, autumn
and winter. The mean annual temperature is 17.4 �C,
ranging from a high of 38.7 �C to a low of -5.4 �C, and the
[10 �C accumulative temperature reaches 5,421 �C per
year. The mean annual precipitation is 872 mm, 90 % of
which occurs between May and October. The maximum
monthly rainfall over the course of the year occurs in July,
which makes up an average of 30 % of the annual rainfall,
and the minimum occurs in February, with \1 % of the
annual rainfall. Sunshine averages 1,241 h annually, with
mean annual solar radiation of 90 kc cm-2. The elevation
ranges from 400 to 587 masl, indicating the geomorpho-
logical characteristic of hilly areas. Local farmers have
dissected long hillslopes into short slopes to minimize soil
loss by water and to facilitate field management operations.
Accordingly, the current hillslopes mostly have a length of
10–25 m, with common slope steepness from 10 to 35 %.
In 1999, steep slopes used for agriculture were required to
be converted to forest or grassland by the Chinese
government.
The soils were derived from sedimentary rocks from the
Jurassic Age and were classified as Regosols by FAO soil
taxonomy (FAO 1988), with strong physical weathering
but weak chemical weathering. In cultivated soils, the
dominant crops were wheat (Triticum aestivum L.), corn
(Zea mays L.), sweet potato (Ipomoea batatas (L.) Lam),
peanut (Arachis hypogaea L.), and rape (Brassica napus
L.). The farmers had a uniform crop rotation system on the
cultivated lands, which typically consisted of wheat, corn
and sweet potatoes, allowing for the collection of soil from
three crops to assess SOC changes. Inorganic nitrogen
fertilizer was generally applied to cultivated lands at a rate
Environmental Management (2014) 53:274–283 275
123
of 330 kg N ha-1 year-1. After the conversion of culti-
vated land to uncultivated land, the slopes were covered
with dominant native grass species, including cogongrass
(Imperata koenigii (Retz.) Beauv.), hairy tare (Vicia hirs-
uta (Linn.) S. F. Gray), Chinese mugwort (Artemisia argyi
Levl. et Van), hairy finger (Digitaria sanguinalis (L.)
Scop.), and others.
Soil Sampling and Analysis
Soil samples were collected from three different land types,
including converted land (previously cultivated on steep
slopes but converted to forest or grassland), cultivated land,
and original uncultivated (undisturbed) land. The three
different land-use types were located within 1 km of each
other. Two and four slopes were selected to collect soil
samples for the converted land and cultivated land,
respectively, and an ancient tomb area was considered the
original uncultivated land. The coordinates and elevation
of each sampling point were measured using a survey-
grade Differential Global Positioning System (DGPS). Soil
samples were collected at 5 m intervals along a transect of
the converted and cultivated slopes with slope lengths of 19
and 21.5 m, respectively. There were 10 and 20 soil profile
samples for converted land and cultivated land, respec-
tively. Soil sampling for SOC, total N, physical, and
chemical determinations was carried out using an 8 cm
diameter hand operated core sampler, and soil was col-
lected down to the bedrock, with soil depths of 25–33 cm
for converted soils and 32–46 cm for cultivated soils. At
each sampling point of soil profiles, three soil cores were
collected within an 80-cm range on the contour, and each
core was segmented into subsample sections at 5 cm depth
increments from the soil surface to the bedrock, and they
were combined across subsamples by depth for each
sampling point. Soil bulk densities (kg m-3) were deter-
mined for each segment using the oven-dried (at 105 �C for
24 h) weight and sample volume (Liu 1996). Measured soil
bulk densities could be used for direct calculations of SOC
inventories because little gravel was found in the soils
derived from mudstone. Soil thickness and bulk density
were used to calculate SOC inventories as the product of
the concentration, soil bulk density, and soil thickness.
For the original uncultivated land, soil samples were
taken from ancient tomb areas where the natural land has
not been converted for agricultural use. Four replicates of
soil profile samples were randomly taken from a relatively
level plot, with the same sampling and analysis methods as
described above. The sampling depth to the bedrock was
39 cm for original uncultivated soils (Table 1). The soil
properties for original uncultivated soils were not thought
to be influenced by landscape positions that were pre-
sumably caused by non-erosional effects.
Soil samples were air-dried, crushed, and passed
through a 2 mm-mesh sieve to remove coarse fragments.
Composite soil samples for each 5-cm depth were passed
through a 0.25 mm-mesh sieve and analyzed for SOC
concentrations. The SOC concentration was determined by
wet oxidation with K2Cr2O7, and the measurement of total
nitrogen (TN) followed the classical Kjeldahl digestion
method (Liu 1996). Soil particle-size fractions were ana-
lyzed using the Mastersizer 2000 laser diffraction particle
Fig. 1 A map showing the
location of the study area in the
Sichuan Basin of southwestern
China
276 Environmental Management (2014) 53:274–283
123
size distribution analyzer. Soil pH was determined using a
digital pH meter with a glass electrode by mixing 10 ml of
soil sample with 20 ml of deionized water.
Statistical Analysis
Simple linear regression was used to test correlations
between the SOC concentration/inventory and slope land-
scape elements (the significance of the regression at
P \ 0.05). An analysis of variance was performed to detect
the significance of differences between different land-uses
and between different positions in the landscape using post
hoc Fisher LSD analysis (P \ 0.05). All statistical analyses
were conducted on the basis of the original data.
Results
Characteristics of Soil Depth and SOC Depth
Distribution at Different Landscape Positions
On cultivated slopes, there was an increasing trend in the soil
depth from the upper to lower slope positions. The soil depth
on the converted land exhibited a similar distribution along
the slope transects (Table 1). When compared with the
original uncultivated soils, soil depths for both cultivated
and converted soils decreased in the upper slope positions,
and slightly decreased in the middle positions. However,
both cultivated and converted soils had the same soil layer
thickness as the original uncultivated soils at the lower slope
positions. Overall, it was determined that SOC concentra-
tions in the soil profile decreased with soil depth. The largest
change in SOC concentrations was observed at the 0–5 cm
depth in converted soils, with a mean increase of 59 % (from
a mean of 8.14 g kg-1 for cultivated soils to 12.95 g kg-1
for converted soils), while SOC concentrations in layers
below the 10 cm depth remained constant for converted
soils (Fig. 2). As a result, SOC concentrations increased
only in the surface soil within a period of 10 years after the
conversion of cultivated soils to grassland. The total N depth
distribution in the soil profile followed a remarkably similar
pattern with the SOC depth distribution (Fig. 3).
The ratio of SOC concentrations in the 0–10 cm soil layer
to those in the 10–20 cm layer showed a small change
ranging from 1.4 to 1.2 among different slope positions on
the cultivated land (Fig. 4). However, this ratio varied with
slope positions on the converted land, ranging from 2.7, 2.2,
to 1.8 in the upper to lower slope positions, and it increased
by a mean of 70 % compared to that of the cultivated land
(Fig. 4). In the upper slope positions, the SOC concentration
ratio (0–10/10–20 cm) on the converted land reached or
slightly exceeded that of the original uncultivated soils.
Whereas the lowest ratio was found in the lower slopeTa
ble
1L
and
scap
eel
emen
tsan
dsu
rfac
e(0
–1
5cm
)so
ilp
rop
erti
esal
on
gth
eh
ills
lop
etr
anse
ctin
the
Sic
hu
anB
asin
of
sou
thw
este
rnC
hin
a(±
stan
dar
dd
evia
tio
n)
Item
Co
nv
erte
dla
nd
Cu
ltiv
ated
lan
dO
rig
inal
un
cult
ivat
ed
lan
d
Slo
pe
po
siti
on
Up
per
Mid
dle
Lo
wer
Mea
nU
pp
erM
idd
leL
ow
erM
ean
N/A
Nu
mb
ero
fsl
op
es2
41
Ele
vat
ion
(m)
41
0–
41
94
24
–4
32
42
1
Slo
pe
len
gth
(m)
0–
55
–1
51
5–
19
0–
55
–1
51
5–
21
.52
0
Slo
pe
stee
pn
ess
(mm
-1)
0.2
8±
0.0
80
.34
±0
.07
0.2
7±
0.0
40
.30
±0
.06
0.2
1±
0.0
70
.21
±0
.01
0.1
4±
0.0
40
.19
±0
.04
0.0
4±
0.0
1
So
ild
epth
(cm
)2
8.7
5±
8.8
42
4.5
0±
6.3
63
2.5
0±
3.5
42
8.5
8±
6.2
53
1.8
0±
10
.92
46
.00
±8
.75
41
.80
±5
.81
39
.87
±8
.49
39
.0±
1.4
1
pH
8.5
3±
0.0
98
.51
±0
.05
8.3
5±
0.0
98
.46
±0
.08
8.1
6±
0.2
98
.32
±0
.29
8.4
2±
0.3
18
.30
±0
.30
7.9
2±
0.0
4
0.0
2–
0.0
02
mm
par
ticl
e
size
frac
tio
n(%
)
57
.36
±4
.68
57
.55
±2
.98
55
.45
±1
.32
56
.79
±2
.99
50
.53
±4
.01
48
.64
±5
.40
48
.16
±5
.42
49
.11
±4
.94
54
.52
±3
.61
\0
.00
2m
mp
arti
cle
size
frac
tio
n(%
)
13
.44
±1
.18
12
.89
±0
.09
12
.40
±0
.22
12
.91
±0
.50
11
.29
±1
.03
11
.37
±1
.39
11
.43
±1
.45
11
.36
±1
.29
18
.11
±1
.69
Bu
lkd
ensi
ty(g
cm-
3)
1.4
9±
0.0
11
.51
±0
.03
1.4
4±
01
.48
±0
.01
1.3
5±
0.0
31
.38
±0
.05
1.3
5±
0.0
41
.36
±0
.04
1.1
2±
0.0
2
Environmental Management (2014) 53:274–283 277
123
positions of the converted land and was markedly lower than
on the original uncultivated land (Fig. 4).
Stocks and Distribution of SOC along the Hillslope
Transect
SOC inventories in the 0–15 cm surface soil had an
increasing trend from upper to lower slope positions on the
cultivated slope but were similar among the three slope
positions on the converted land (Fig. 5a). The SOC
inventories for this soil layer (0–15 cm) significantly
increased (P \ 0.001) after the conversion from cultivated
land to grassland, with a mean increment of 25 % com-
pared with those of the cultivated soils. However, large
0
5
10
15
20
25
30
35
40
45
Soi
l dep
th (
cm)
TN concentration (g kg-1)
Cultivated land
Converted land
Uncultivated land
(c)
0
5
10
15
20
25
30
35
40
45
Soi
l dep
th (
cm)
TN concentration (g kg-1)
Cultivated land
Converted land
Uncultivated land
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5 2 2.5 3
0 0.5 1 1.5 2 2.5 3
0 0.5 1 1.5 2 2.5 3
Soi
l dep
th (
cm)
TN concentration (g kg-1)
Cultivated land
Converted land
Uncultivated land
(a)
(b)
Fig. 3 Distribution of soil total nitrogen in converted land, cultivated
land, and original uncultivated land at different landscape positions,
a upper slope, b middle slope, and c lower slope. The original
uncultivated land did not have a representative at each slope position,
so the data from this site are repeated in panels a, b, and c
0
5
10
15
20
25
30
35
40
45
Soi
l dep
th (
cm)
SOC concentration (g kg-1)
Cultivated land
Converted land
Uncultivated land
(c)
0
5
10
15
20
25
30
35
40
45
Soi
l dep
th (
cm)
SOC concentration (g kg -1)
Cultivated land
Converted land
Uncultivated land
0
5
10
15
20
25
30
35
40
45
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Soi
l dep
th (
cm)
SOC concentration (g kg -1)
Cultivated land
Converted land
Uncultivated land
(a)
(b)
Fig. 2 Soil organic carbon profile distribution of the converted land,
cultivated land, and original uncultivated land at different landscape
positions for the a upper slope, b middle slope, and c lower slope. The
original uncultivated land did not have a representative at each slope
position, so the data from this site are repeated in panels a, b, and c
278 Environmental Management (2014) 53:274–283
123
SOC inventory gaps in this soil layer were still observed
between converted soils and original uncultivated soils
(33 % less than the latter).
For the 15–30 cm subsoil layers, SOC inventories were
significantly lower in the upper and/or middle slope posi-
tions than in the lower slope positions on the converted
land (P = 0.025, 0.023, respectively, for upper and middle
slopes) and cultivated land (P = 0.004 only for upper
slopes) (Fig. 5b). Although SOC inventories for the
0–15 cm layer notably increased compared to the culti-
vated land, those of the 15–30 cm layer in the converted
land were significantly low in the upper and middle slope
positions (P = 0.006 and 0.009, respectively). In lower
slope positions, SOC inventories in the 15–30 cm layer of
the converted land still did not exceed those of the culti-
vated land (P = 0.404), suggesting little improvement in
the potential for SOC sequestration in the subsoil of the
converted land.
C/N Ratio
In the near surface soil (0–5 cm), C/N ratios for cultivated
soils were slightly lower in upper slope positions than in
middle and lower slope positions (but with an insignificant
difference of P = 0.333), whereas those for converted soils
were found to be significantly higher in upper slope posi-
tions than in middle and lower slope positions (P = 0.002)
(Fig. 6a). In contrast, no significant differences in the C/N
ratio were observed among different slope positions in the
5–30 cm soil layer for both the cultivated and converted
soils (P = 0.056 and 0.240, respectively) (Fig. 6b). Con-
verted soils exhibited a high C/N ratio in the upper slope
positions compared to the cultivated soils, especially for
the near surface soil (0–5 cm) (P = 0.012 and 0.050,
respectively, for the 0–5 and 5–30 cm soil layers).
0
0.5
1
1.5
2
2.5
3
3.5
Upper Middle Lower
SO
C r
atio
of 0
-10
cm to
10-
20 c
m
Slope landscape position
Cultivated land
Converted land
Uncultivated land
Fig. 4 Soil organic carbon concentration ratios of the 0–10 cm to
10–20 cm soil layers
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
SO
C in
vent
ory
(kg
m-2
)
Slope landscape position
Cultivated landConverted landUncultivated land
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Upper Middle Lower
Upper Middle Lower
SO
C in
vent
ory
(kg
m-2
)
Slope landscape position
Cultivated landConverted landUncultivated land
(a)
(b)
Fig. 5 Soil organic carbon inventories in the 0–15 cm surface soil
layer (a) and the 15–30 cm subsoil layer (b)
0
2
4
6
8
10
12
14
16
Upper Middle Lower
C/N
rat
io
Slope landcape position
Cultivated landConverted land
0
2
4
6
8
10
12
14
16
Upper Middle Lower
C/N
rat
io
Slope landcape position
Cultivated landConverted land
(a)
(b)
aa
aa a
b
aa
a
aa
b
Fig. 6 C/N ratios in the 0–5 cm surface soil layer (a) and the
5–30 cm subsoil layer (b). The same letter adjacent to the error bars
indicates no significant difference between the two types of land-uses
based on Fisher’s LSD (0.05). Error bars represent the standard errors
of the means
Environmental Management (2014) 53:274–283 279
123
Relationship Between SOC Concentrations
and Landscape Elements
Slope length and steepness are important parameters of
sloping fields that can reflect geomorphological features.
As a result, the relationship between SOC distribution and
these two parameters was analyzed (Table 2). For the
cultivated land, SOC concentrations in the 0–5 cm surface
soil were significantly and positively correlated with slope
length (R2 = 0.33, P \ 0.01). Meanwhile, positive corre-
lations between the SOC inventory of the soil profile and
the slope length were also highly significant (R2 = 0.38,
P \ 0.01) for the cultivated land (Table 2). For the con-
verted land, however, no significant correlations were
found between the SOC concentration in the surface soil
(0–5 cm depth) and the slope length (R2 = 0.0001,
P [ 0.10). In addition, there were no significant correla-
tions between the SOC inventory of the soil profile and the
slope length (R2 = 0.04, P [ 0.10; see Table 2). Signifi-
cant (or highly significant) and negative correlations were
also found between the SOC concentration/inventory and
the slope steepness for the cultivated land (Table 2). A
correlation analysis showed that the slope position impacts
on SOC concentrations/inventories were scarcely observed
in converted soils, whereas SOC concentrations/inventories
were strongly influenced by slope positions in cultivated
soils. Furthermore, the lack of a correlation of this type of
converted land was attributed to relatively consistent SOC
concentrations/inventories at different slope positions.
Discussion
Inherent SOC Concentrations
In the upper and middle slope positions, the SOC con-
centrations in the soil layers below the 0–10 cm depth were
somewhat lower on the converted land than on the culti-
vated land (Fig. 2). This trend could mostly be explained
by severe soil degradation, which occurred in the upper and
middle slope positions of the converted land before the
land conversion from cultivated land to grassland was
performed. Previous studies have demonstrated that intense
tillage causes soil losses in upper slope positions (Lobb
et al. 1995; Zhang et al. 2008), and serious water erosion
occurs at the middle slope positions, resulting in SOC
redistribution within the slope landscape (Govers et al.
1996; Zhang et al. 2006; Debasish-Saha and Sharma 2011).
Soil accumulation is normally present at the toe slope
positions. The steep slope of the converted land (Table 1)
contributed to strong soil redistribution by water and tillage
erosion before land conversion (Zhang et al. 2012). Severe
soil degradation had essentially occurred over the entire
soil profile given that soil degradation emerged in the lower
parts of the soil profile because soil erosion starts at the soil
surface and develops down the profile. In the context of this
study, it was therefore speculated that converted soils
originally had lower SOC concentrations than cultivated
soils because of the steeper slopes of the former. Despite
the difference in SOC depth distribution between cultivated
and converted land, it was evident that the converted land
had greatly improved SOC sequestration potential in the
surface soil within a 10-year period after land conversion,
as stated in the preceding text.
Land-Use Conversion
In regions such as the Sichuan Basin, the soils are char-
acterized by a thin soil layer (generally 30–50 cm deep),
beneath which rock stratum immediately emerges, irre-
spective of land-use types. The SOC inventories in the
surface layer (0–15 cm) of converted soils increased sub-
stantially, but those in the subsoil layer (15–30 cm) did not
rise in comparison with the cultivated soils. This finding
indicated that SOC restoration occurred only in the surface
soil over a moderate period of time after the conversion of
cultivated soils to uncultivated soils. Our findings were
similar to the depth responses in other studies, which
showed that over time, a high SOC accumulation occurs in
the soil surface in combination with losses at depths below
the soil surface (Steinbeiss et al. 2008; O’Brien et al.
2010). The increased SOC in the surface layer of converted
soils is largely ascribed to much greater biomass prolifer-
ation near the surface in perennial grasslands than in cul-
tivated lands (Slobodian et al. 2002). Previous studies
suggested that plant inputs at depths for converted land are
greatly reduced as a result of a lack of tillage mixture,
while fresh crop residues are continuously delivered to
deeper layers because of tillage (Yang et al. 2008; O’Brien
et al. 2010). At our site, however, lower SOC inventories
with depth for converted soils than for cultivated soils may
be partly attributed to previously severe soil degradation
that occurred before land conversion, as stated in the pre-
ceding text. The SOC depth distribution showed that the
Table 2 Correlations between SOC concentration/inventory and
slope landscape elements
SOC Soil
depth
Converted land
(n = 10)
Cultivated land
(n = 20)
Slope
length
Slope
gradient
Slope
length
Slope
gradient
Concentration 0–5 cm -0.01 -0.29 0.57** -0.46*
Inventory Profile -0.21 0.20 0.62** -0.58**
**, * Represent significance at P \ 0.01 and \0.05 levels,
respectively
280 Environmental Management (2014) 53:274–283
123
upper 30 cm soil layers contained approximately 91 and
77 % (based on the data of Fig. 2) of the SOC stock in the
soil profile for converted and cultivated soils, respectively.
This finding suggested that the soil depth (0–30 cm) played
a crucial role in the SOC sequestration of this region.
Consequently, although the increase in SOC stocks was
present in the surface soil after the conversion of cultivated
soils to grassland soils, such effects on SOC sequestration
in this region were profound, and therefore, there is a need
to improve the SOC stocks in the surface soil. A recent
study from northwest New South Wales, Australia has also
obtained a similar result, which indicated that land-use
conversion effects on the SOC pool were restricted to a
great extent to near surface soil layers (Wilson et al. 2011).
Yimer et al. (2006) reported that approximately 45 % of
the SOC stock was located in the top 30 cm of the soil
profile. Another study based on a meta-analysis for a few
parts of the world indicated that 64 % of sequestered SOC
remained in the upper 50-cm soil layer in most cases
(Conant et al. 2001). The SOC depth distribution for our
study was apparently different from those studies, with the
former showing a higher SOC ratio of the 0–30 cm soil
layer to the whole soil layer. This may be due to a shallow
soil profile (up to a maximum of 50 cm deep) and origi-
nally low SOC content in subsoils of our study area.
C/N Ratio Effects
In the 0–5 cm layer near the surface soil, the C/N ratios of
the converted soils were higher than those of the cultivated
soils. Previous studies demonstrated that a wide C/N ratio
or low N content results in the slow decomposition of soil
organic matter because the organisms need N to decom-
pose residues (Alexander 1991). In cultivated soils, nitro-
gen fertilizer applications provide additional N for
organisms during the residue decomposition process,
resulting in a rapid and more thorough decomposition of
organic matter. Conversely, the large C/N ratio in con-
verted soils creates a favorable soil environment for the
immobilization of organic matter as a result of less soil N.
The highest C/N ratio was present in the upper slope
positions of the converted land where soil erosion was most
severe on the whole slope before land conversion. In
contrast, lower C/N ratios were found in cultivated soils in
the upper slope positions. This trend was ascribed to the
fact that high plant inputs into surface soils of the con-
verted land provided fresh organic matter with a large
proportion of labile C (Gregorich et al. 1996; O’Brien et al.
2010). A similar result was also reported by Karlen et al.
(2008), who indicated that soils in upper landscape posi-
tions had lower C/N ratios in cropland. High C/N ratios in
the upper slope positions may be associated with the
greatest risk of N saturation leading to nitrate leaching on
the hilltop where there was a high rate of N fertilizer
application (330 kg N ha-1 year-1), which was supported
by Schipper et al. (2004). The hilltop soils were most
susceptible to leaching because intense tillage caused
downslope soil movement, which resulted in severe soil
losses and thin soil layers with a coarse texture (Papiernik
et al. 2007; Zhang et al. 2008).
Landscape Position Impacts
In this region, tillage erosion is the dominant soil redis-
tribution process because of short and steep slopes, leading
to the most severe soil degradation occurring in upper slope
positions (Zhang et al. 2006, 2012). However, our SOC
stock data showed that after the conversion of cultivated
soils to grassland soils, SOC inventories in the 0–30 cm
soil layer in the upper slope positions with severe soil
erosion became similar to those in the lower slope positions
with soil deposition. This finding showed that the land-
scape position impacts on SOC spatial distribution became
insignificant following the conversion of cultivated land to
grassland. The high rate of SOC sequestration could be
ascribed to low original SOC contents at erosional sites. A
similar result was reported by Mann (1986), suggesting that
the initial SOC stock exerted a strong impact on SOC
changes in the surface soil. This change was associated
with robust soil conservation at previously erosional sites
after the conversion of cultivated soils. Under cultivated
conditions, soil redistribution occurred over a short dis-
tance along the slope, mainly as a result of tillage erosion.
Hence, in the study area with short slopes, erosion due to
tillage that may be a dominant process of soil redistribution
caused the progressive downslope movement of soil. This
resulted in severe soil and SOC losses from the upper slope
positions and accumulations of soil and SOC in the lower
slope positions (Zhang et al. 2006, 2008). In our study, the
SOC concentrations in the soil layers below the 15 cm
depth in cultivated land appeared to have an increasing
trend from the upper to lower slope positions. This sug-
gested that the differences in SOC concentrations between
the profiles of cultivated soils and original uncultivated
soils gradually became smaller along the slope transect.
This trend remained in converted soils even after 10 years
of land conversion. Under cultivated conditions, this pat-
tern of SOC distribution along the slope transect was in
agreement with previous studies, which suggested that soils
on footslope and toe slope positions contain larger stocks
of organic C than soils in shoulder or midslope positions
(Pierson and Mulla 1990; Malo et al. 2005; di Folco and
Kirkpatrick 2011; Schwanghart and Jarmer 2011). These
findings indicated that SOC restoration of the surface soil
was easier than that of the subsoil, primarily because of
biomass inputs near the soil surface. This implied that a
Environmental Management (2014) 53:274–283 281
123
much longer time would be required for the subsoil to
reach the same rate of SOC stocks in the surface soil if the
restoration is possible.
Conclusions
Ten years after the conversion of cultivated land to grass-
land, SOC inventories in the 0–15 cm surface soils
increased significantly. For the converted land, C/N ratios
in the near surface soil (0–5 cm) were found to be signif-
icantly higher in the upper slope positions than in the
middle and lower slope positions, whereas C/N ratios in the
5–30 cm soil layer were similar among different slope
positions. When compared with cultivated soils, converted
soils exhibited a large C/N ratio in the upper slope posi-
tions, particularly in the near surface soil (0–5 cm). This
trend provides strong SOC sequestration characteristics in
upper slope areas where severe soil erosion occurred before
land conversion. Few landscape position impacts occurred
on SOC concentrations/inventories in the converted soils,
whereas SOC concentrations/inventories were strongly
influenced by landscape positions in cultivated soils. It is
suggested that the landscape position impacts on the SOC
spatial distribution become insignificant after the conver-
sion of cultivated land to grassland, which plays a favor-
able effect on the immobilization of SOC. The conversion
of cultivated land to grassland would markedly increase
SOC stocks in soil and would especially improve the
potential for SOC surface soil sequestration over a mod-
erate period of time (10 years).
Acknowledgments This study was financially supported by the
National Natural Science Foundation of China (41271242) and the
135 Strategic Program of the Institute of Mountain Hazards and
Environment, CAS (SDS-135-1206).
References
Alexander M (1991) Introduction to soil microbiology, 2nd edn.
Krieger, Malabar
Ayoubi S, Karchegani PM, Mosaddeghi MR, Honarjoo N (2012) Soil
aggregation and organic carbon as affected by topography and
land use change in western Iran. Soil Till Res 121:18–26
Cambardella CA, Elliott ET (1992) Particulate organic matter
changes across a grassland cultivation sequence. Soil Sci Soc
Am J 56:777–783
Conant RT, Paustian K, Elliot ET (2001) Grassland management and
conversion into grassland: effects on soil carbon. Ecol Appl
11:343–355
Davidson EA, Ackerman IL (1993) Change in soil carbon inventories
following cultivation of previously untilled soils. Biogeochem-
istry 20:161–193
Di Folco M, Kirkpatrick JB (2011) Topographic variation in burning-
induced loss of carbon from organic soils in Tasmanian
moorlands. Catena 87:216–225
FAO (1988) Soil map of the world. Revised legend. World Soil
Resources Report 60, FAO, Rome
Govers G, Quine TA, Desmet PJJ, Walling DE (1996) The relative
contribution of soil tillage and overland flow erosion to soil
redistribution on agricultural land. Earth Surf Processes Landf
21:929–946
Gregorich EG, Monreal CM, Schnitzer M, Schulten H-R (1996)
Transformation of plant residues into soil organic matter:
chemical characterization of plant tissue, isolated soil fractions,
and whole soils. Soil Sci 161:680–693
Guo LB, Gifford RM (2002) Soil carbon stocks and land use change:
a meta analysis. Glob Chang Biol 8:345–360
Hancock GR, Murphy D, Evans KG (2010) Hillslope and catchment
scale soil organic carbon concentration: an assessment of the role
of geomorphology and soil erosion in an undisturbed environ-
ment. Geoderma 155:36–45
Karlen DL, Tomer MD, Neppel J, Cambardella CA (2008) A
preliminary watershed scale soil quality assessment in north
central Iowa, USA. Soil Till Res 99:291–299
Kern JS, Johnson MG (1993) Conservation tillage impacts on national
soil and atmospheric carbon levels. Soil Sci Soc Am J 53:
200–210
Lal R, Kimble J, Follett RF, Cole CV (1998) The potential for US
cropland to sequester carbon and mitigate the greenhouse effect.
Sleeping Bear, Ann Arbor, p 128
Liu GS (1996) Soil physical and chemical analysis & description of
soil profiles. Chinese Standard Press, Beijing (in Chinese)
Lobb DA, Kachanoski RG, Miller MH (1995) Tillage translocation
and tillage erosion on shoulder slope landscape positions
measured using 137Cs as a tracer. Can J Soil Sci 75:211–218
Malo DD, Schumacher TE, Doolittle JJ (2005) Long-term cultivation
impacts on selected soil properties in the northern Great Plains.
Soil Till Res 81:277–291
Mann LK (1986) Changes in soil carbon storage after cultivation. Soil
Sci 142:279–288
McGill WB, Dormaar JF, Reinl-Dwyer E (1988) New perspectives on
soil organic matter quality, quantity and dynamics on the
Canadian prairies. In: Land degradation and conservation tillage.
Proceedings of the 34th annual meeting of the Canadian society
of soil science/AIC, Calgary, 21–24 August, pp. 30–48
Mikhailova EA, Bryant RB, Vassenev II, Schwager SJ, Post CJ
(2000) Cultivation effects on soil carbon and nitrogen contents at
depth in the Russian Chernozem. Soil Sci Soc Am J 64:738–745
Monreal CM, Janzen HH (1993) Soil organic carbon dynamics after
eighty years of cropping a Dark Brown Chernozem. Can J Soil
Sci 73:133–136
O’Brien SL, Jastrow JD, Grimley D, Gonazalez-Meler MA (2010)
Moisture and vegetation controls on decadal-scale accrual of soil
organic carbon and total nitrogen in restored grasslands. Glob
Chang Biol 16:2573–2588
Papiernik SK, Lindstrom MJ, Schumacher TE, Schumacher JA, Malo
DD, Lobb DA (2007) Characterization of soil profiles in a
landscape affected by long-term tillage. Soil Till Res 93:
335–345
Pierson FB, Mulla DJ (1990) Aggregate stability in the Palous region
of Washington: effect of landscape position. Soil Sci Soc Am J
54:1407–1412
Saha D, Kukal SS, Sharma S (2011) Landuse impacts on SOC
fractions and aggregate stability in typic ustochrepts of North-
west India. Plant Soil 339:457–470
Saviozzi A, Levi-Minzi R, Cardelli R, Riffaldi R (2001) A
comparison of soil quality in adjacent cultivated, forest and
native grassland soils. Plant Soil 233:251–259
Schipper LA, Percival HJ, Sparling GP (2004) An approach for
estimating when soils will reach maximum nitrogen storage. Soil
Use Manag 20:281–286
282 Environmental Management (2014) 53:274–283
123
Schwanghart W, Jarmer T (2011) Linking spatial patterns of soil
organic carbon to topography—a case study from south-eastern
Spain. Geomorphology 126:252–263
Slobodian N, Van Rees K, Pennock D (2002) Cultivation-induced
effects on belowground biomass and organic carbon. Soil Sci
Soc Am J 66:924–930
Steinbeiss S, Bebler H, Engels C, Temperton VM, Buchmann N,
Roscher C, Kreutziger Y, Baade J, Habekost M, Gleixner G
(2008) Plant diversity positively affects short-term soil carbon
storage in experimental grasslands. Glob Chang Biol 14:
2937–2949
The National Forestry Administration of China (1999) Project plans
for conversion of farmland into forestland and grassland (in
Chinese)
Tisdall JM (1996) Formation of soil aggregates and accumulation of
soil organic matter. In: Carter MR, Stewart BA (eds) Structure
and organic matter storage in agricultural soils. Lewis, Boca
Raton, pp 57–96
Wilson BR, Koen TB, Barnes P, Ghosh S, King D (2011) Soil carbon
and related soil properties along a soil type and land-use
intensity gradient, New South Wales, Australia. Soil Use Manag
27:437–447
Yang XM, Drury CF, Reynolds WD, Tan CS (2008) Impacts of long-
term and recently imposed tillage practices on the vertical
distribution of soil organic carbon. Soil Till Res 100:120–124
Yimer F, Ledin S, Abdelkadir A (2006) Soil organic carbon and total
nitrogen stocks as affected by topographic aspect and vegetation
in the Bale Mountains, Ethiopia. Geoderma 135:335–344
Zhang JH, Quine TA, Ni SJ, Ge FL (2006) Stocks and dynamics of
SOC in relation to soil redistribution by water and tillage
erosion. Glob Chang Biol 12:1834–1841
Zhang JH, Nie XJ, Su ZA (2008) Soil profile properties in relation to
soil redistribution by intense tillage on a steep hillslope. Soil Sci
Soc Am J 72:1767–1773
Zhang K, Dang H, Tan S, Cheng X, Zhang Q (2010) Change in soil
organic carbon following the ‘Grain-for-Green’ programme in
China. Land Degrad Dev 21:13–23
Zhang JH, Ni SJ, Su ZA (2012) Dual roles of tillage erosion in lateral
SOC movement in the landscape. Eur J Soil Sci 63:165–176
Zhao X, Wu P, Gao X, Persaud N (2012) Soil quality indicators in
relation to land use and topography in a small catchment on the
loess plateau of china. Land Degrad Dev. doi:10.1002/ldr.2199
Zinn YL, Lal R, Resck DVS (2005) Changes in soil organic carbon
stocks under agriculture in Brazil. Soil Till Res 84:28–40
Environmental Management (2014) 53:274–283 283
123