Journal of Environmental Quality
TECHNICAL
REPORTS
TECHNICAL
REPORTS
PLANT AND ENVIRONMENT INTERACTIONS
Losses in Carbon and Nitrogen Stocks in Soil Particle-Size Fractions
along Cultivation Chronosequences in Inner Mongolian Grasslands
Nianpeng He,* Yunhai Zhang, Jingzhong Dai, Xingguo Han, and Guirui Yu
G
rasslands cover approximately 40% of the
world’s land area and store approximately one-third
of the total terrestrial carbon (C), of which more than
70% is stored in the top 100-cm soil layer (White et al., 2000).
Changes in land use can induce substantial changes in soil
organic carbon (SOC), which represents an important source
or sink of atmospheric CO2, although the processes and mechanisms involved in C cycles in soil are not completely understood
(Guo and Gifford, 2002; IPCC, 2007). In the past few decades,
grasslands around the world have received more attention for
their potential to act as C sinks in view of their substantial area
and immense SOC stock (Conant et al., 2001; Soussana et al.,
2004; Lal, 2009).
Temperate grasslands in northern China constitute an area
of approximately 110 × 106 ha, and a large number of native
grasslands have been converted into croplands; this occurred
especially in the 1960s and 1990s. Some studies have shown
that conversion from native grasslands to croplands can result in
a 20 to 70% net loss of SOC in surface soil, and the decrease
depends on grassland type, soil properties, and duration of
cultivation (White et al., 2000; Guo and Gifford, 2002; Wang et
al., 2008). When native grasslands are converted into croplands,
cultivation practices destroy native vegetation and soil structure,
decrease surface cover, and modify the biogeochemical cycles
in soil. This enhances the microbial decomposition of soil
organic matter (SOM) and wind and water erosion, which leads
to substantial SOM losses (Guo and Gifford, 2002; Liu et al.,
2004; Lal, 2009). On the other hand, whenever a change in
land use decreases SOC, the reverse change usually increases
SOC by C sequestration, and vice versa (Conant et al., 2001;
Guo and Gifford, 2002). Therefore, data from cultivation
chronosequences in northern China should be a good indicator
of not only the SOC as a source of atmospheric CO2 but also
of the potential of C sequestration in the region through the
conversion from croplands to grasslands.
Cultivation in semiarid grasslands induces large changes in soil
organic matter (SOM) stock. To better predict the effects of
cultivation on SOM pools, there is a need to identify the soil
fractions that are affected and the extent to which they are affected.
Using four cultivation chronosequences in Inner Mongolian
grasslands of northern China, we investigated the changes in
soil organic carbon (SOC) and total nitrogen (N) stocks in soil
particle-size fractions to identify the effect of cultivation on SOM
dynamics. The results showed that conversion of native grasslands
into croplands significantly decreased the SOC stocks (4.34–31.65
Mg C ha−1) and N (0.19–2.54 Mg N ha−1) in the 0- to 100-cm layer
after cultivation. Prominent changes were observed in the SOC and
N stocks in the 0- to 10-cm layer and were, on average, 6.56 Mg C
ha−1 (24.85%) and 0.63 Mg N ha−1 (23.48%), respectively. The effect
of cultivation on the SOC and N stocks in soil fractions was in the
order sand > silt > clay. The C and N stocks in the 0- to 10-cm soil
layer in the sand fraction in croplands decreased, on average, by 4.74
Mg C ha−1 (35.86%) and 0.48 Mg N ha−1 (41.30%), respectively,
compared with those in native grasslands. The declines in the silt and
clay fractions were small. Thus, sand fraction was a more important
contributor to C and N losses in soil after cultivation than silt or
clay fraction. Our findings indicate that the preliminary responses
of SOC and N to cultivation in a semiarid grassland area and have
significant implications for assessing the loss or gain of C and N
during grassland conversion.
N. He and G. Yu, Key Lab. of Ecosystem Network Observation and Modeling,
Institute of Geographic Sciences and Natural Resources Research, Chinese
Academy of Sciences (CAS), Beijing 100101, China; N. He and Y. Zhang, State Key
Lab. of Vegetation and Environmental Change, Institute of Botany, CAS, Beijing
100093, China; J. Dai, College of Ecology and Environmental Science, Inner
Mongolia Agricultural Univ., Hohhot, Inner Mongolia 010018, China; X. Han, State
Key Lab. of Forest and Soil Ecology, Institute of Applied Ecology, CAS, Shenyang,
Liaoning 110016, China. Assigned to Associate Editor Pierre Benoit.
Copyright © 2012 by the American Society of Agronomy, Crop Science Society
of America, and Soil Science Society of America. All rights reserved. No part of
this periodical may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording, or any information
storage and retrieval system, without permission in writing from the publisher.
J. Environ. Qual. 41
doi:10.2134/jeq2011.0258
Received 18 July 2011.
*Corresponding author (henp@igsnrr.ac.cn).
© ASA, CSSA, SSSA
5585 Guilford Rd., Madison, WI 53711 USA
Abbreviations: AC0, Site A native grassland; AC19, Site A 19-yr cultivation; AC49,
Site A 49-yr cultivation; BC0, Site B native grassland; BC19, Site B 19-yr cultivation;
BC39, Site B 39-yr cultivation; BD, bulk density; CC0, Site C native grassland; CC39,
Site C 39-yr cultivation; CC51, Site C 51-yr cultivation; SOC, soil organic carbon;
SOM, soil organic matter.
1507
Physical fractionation has been increasingly used to investigate
the turnover of specific SOM pools with functional significance,
especially when assessing the effect of land-use changes on SOM
stocks and turnover (Christensen, 2001). Recent evidence has
shown that particle-size fractions (sand, silt, and clay) can be used
as early indicators to evaluate the dynamics and turnover of SOM
under various land use practices (Amelung et al., 1998; Leifeld
and Kögel-Knabner, 2005; Olk and Gregorich, 2006; He et al.,
2009). Typical grasslands in Inner Mongolia, China, are sandy
soils and develop under a semiarid climate in the temperate zone
with an annual precipitation of 300 to 400 mm and usually high
wind, especially in spring and autumn (Hu et al., 2006, Kang
et al., 2007). Therefore, the practice of cultivation in the region
renders the soil more susceptible to wind erosion. The frequently
occurring dust storms since the end of the 20th century have been
considered a direct consequence of cultivation and degradation
of Inner Mongolian grasslands. An increasing demand for natural
resources and animal products to cope with sharply rising human
populations has further placed tremendous pressures on grassland
cultivation in the region. However, the effects of cultivation on
C and N distribution in particle-size fractions in sandy soils
have not been evaluated in the temperate grasslands of northern
China, although some similar studies have been conducted in the
clay soils of Europe and Northern America.
In this study, we investigated the C and N stocks in soil and
soil fractions from four cultivation chronosequences in the
Xilin River basin. The objectives of the study were to identify
the changes in SOC and N stocks in soil particle-size fractions
for the conversion from grassland to cropland. Understanding
the response of SOM fractions to reclamation will help us
assess C sources and sinks associated with perennial grassland
establishment and management in the region.
reclaimed in 1990 (AC19, 19-yr cultivation) and 1960 (AC49,
49-yr cultivation) (Fig. 1). Site B (43°35′ N,116°44′ E) included a
native grassland (BC0) that had been subjected to long-term free
grazing and exhibited light degradation. Croplands at Site B were
reclaimed in 1990 (BC19, 19-yr cultivation) and 1970 (BC39,
39-yr cultivation). Site C (43°31′ N, 116°57′ E) included a native
grassland (CC0) that was mowed annually in mid-August and had
a good grassland condition. Croplands at site C were reclaimed in
1970 (CC39, 39-yr cultivation) and 1958 (CC51, 51-yr cultivation).
Site D (43°29′ N, 116°49′ E) included a native grassland (DC0)
that was mowed annually in mid-August and had a good grassland
condition. Croplands at site D were reclaimed in 1990 (DC19,
19-yr cultivation) and 1970 (DC39, 39-yr cultivation).
At each site, the grasslands and croplands are contiguous
and were formerly floristically and topographically similar. The
cropping system commonly used in these croplands is wheat(Triticum spp.) or rapeseed (Brassica napus)-fallow rotation.
Wheat or rapeseed is usually sown in early May and harvested
in late August. In the fallow year, all the weeds produced are
incorporated into the soil as green manure though plowing (depth,
?20 cm). Fertilizers at 54 to 83 kg N ha−1 and 42 to 71 kg P ha−1
are applied once during sowing in each cropping year.
Field Sampling
In early May 2009, before tilling, we selected five random
sampling sites within each plot (>20 ha for each plot) to serve as
replicates. The sites were at least 15 m apart from each other and from
the boundary of the plots. At each sampling site, three soil cores (2 m
apart) were collected and combined from four layers at depths of 0 to
10, 10 to 30, 30 to 50, and 50 to 100 cm. A total of 20 soil samples
were taken from each experimental plot. The soil bulk density (BD)
was measured using soil cores at depths of 0 to 10, 10 to 30, 30 to
Materials and Methods
Study Area
The study was conducted in a typical steppe region on the
Mongolian plateau in northern China (43°33′N, 116°40′E).
The climate of this region is semiarid continental, and the mean
annual temperature and precipitation from 1982 to 2009 were
0.96°C and 334 mm, respectively. The soil is of the chestnut type
(i.e., calcic Kastanozem), which is equivalent to calcic-orthic
Aridisol in the US soil classification system. The dominant
vegetation is composed of grassland plant species [i.e., Leymus
chinensis (Trin) Tzvel, Stipa grandis P.A. Smirn., and Cleistogenes
squarrosa (Trin.) Keng] (Chen and Wang, 2000).
Site Selection
Experimental sites were located in the Xilin River basin, which
is one of the most representative areas in the Inner Mongolia
steppe region (Tong et al., 2004). In this region, native grasslands
are predominant, and croplands are mainly derived from
reclamation of grasslands in the 1960s and 1990s. Each cultivation
chronosequence comprises native grassland and croplands
derived in different reclamation periods. The four cultivation
chronosequences used in the study are described below.
Site A (43°33′ N, 116°40′ E) included a native grassland (AC0)
that had been subjected to long-term free grazing and exhibited
light degradation with respect to aboveground productivity
and plant community composition. Croplands at site A were
1508
Fig. 1. Location of study area and picture of crop-fallow system in
Inner Mongolian grasslands.
Journal of Environmental Quality
50, and 50 to 100 cm, with three replicates for each plot (Blake and
Hartage, 1986); this allowed us to estimate the masses of C and N.
Particle-Size Fractionation and Chemical Analysis
Bulk soil sieved through a 2.0-mm screen was fractionated into
sand (particle size, 50–2000 μm), silt (2–50 μm), and clay (<2 μm)
by using ultrasonic energy to disrupt soil aggregates following the
method of Roscoe et al. (2000) and Chen and Chiu (2003). In brief,
after manually removing visible root remnants, 50 g of soil (particle
size <2 mm) was dispersed in 250 mL of distilled water with a KS-600
probe-type ultrasonic cell disrupter system (Shanghai Precision &
Scientific Instrument Co. Ltd.) in continuous operation at 360 W
for 32 min. Sand (50–2000 μm) and coarse silt (20–50 μm) were
separated by wet sieving. To further separate fine silt (2–20 μm) and
clay (<2 μm), the samples were repeatedly centrifuged at 150 × g
for 5 min. Supernatants were collected and transferred into 250-mL
centrifuge bottles and centrifuged at 3900 × g for 30 min; the
precipitated fraction was referred to as clay. The sizes of the particles
recovered after ultrasonic dispersion, wet sieving, and centrifugation
varied from 96 to 99% of the initial soil dry mass (He et al., 2009);
soluble C and N, which were not recovered in fractionation, were
assigned to the clay pool (Zinn et al., 2005).
Bulk soil and soil fractions were dried at 50°C and ground for
chemical analysis. The organic C content (%) of the samples was
measured with a modified Mebius method (Nelson and Sommers,
1982). In brief, 0.5-g soil samples were digested with 5 mL of 1
mol L−1 K2Cr2O7 and 5 mL of concentrated H2SO4 at 180°C for 5
min, followed by titration of the digests with standardized FeSO4.
The total N content (%) of the samples was analyzed using the
modified Kjeldahl wet digestion procedure (Gallaher et al., 1976)
with the Kjeltec 2300 Analyzer Unit (FOSS).
Calculations and Data Analysis
Some literature has documented that the equivalent soil mass
method is more suitable than the fixed depth method to quantify
the changes in soil C stock in response to land use or management,
although soil samples are typically collected by the fixed depth
(Ellert and Bettany, 1995; Lee et al., 2009). Therefore, we selected
the equivalent soil mass method to calculate soil C and N stocks in
this study and obtained the correcting soil thickness depending on
soil BD following Ellert and Bettany (1995) and Lee et al. (2009).
Soil organic C (Mg C ha−1) and total N (Mg N ha−1) stocks were
calculated in a ground area up to soil depths of 100 cm as follows:
SOC = Σ[Di × Si × Bi × OMi]/100
TN = Σ[Di × Si × Bi × TNi]/100
where Di, Si, Bi, OMi, and TNi represent the correcting soil
thickness by the equivalent soil mass method (cm), crosssectional area (ha), BD (g cm–3), organic C content (%), and
total N content (%), respectively (i = 1, 2, 3, and 4, respectively).
Similarly, C and N stocks (Mg C ha−1 and Mg N ha−1,
respectively) in sand, silt, and clay were calculated as follows:
Cstorage(fractioni) = [Ccon.(fractioni) × F × D × S × B]/105
Nstorage(fractioni) = [Ncon.(fractioni) × F × D × S × B]/105
where Ccon.(fractioni) is the C content of the soil fraction (%);
Ncon.(fractioni) is the N content of soil fraction (%), and F is the
content of the fraction in the soil (g fraction kg−1 soil).
www.agronomy.org • www.crops.org • www.soils.org
To better characterize the changes in SOC and total N stocks
after reclamation, we selected two parameters: (i) absolute change
(Mg ha−1) and (ii) relative change (absolute change divided
by SOC or total N stock in native grassland, as a percentage).
A two-tailed t-test was performed to determine whether each
change was significantly different from zero, and 95% confidence
intervals were used as threshold for the minimum significant
difference (Zinn et al., 2005; He et al., 2009).
All data are represented as mean ± 1 SD (n = 5). One-way
ANOVA (with Duncan’s post hoc test for multiple comparisons)
was used to evaluate the effect of cultivation on C and N stocks
in bulk soil, in soil fractions, and at different sites. A significance
level of P = 0.05 was used for all the tests. All analyses were
conducted using the program SPSS (ver. 11.0).
Results
Changes in Soil pH, Bulk Density,
and Soil Particle-Size Fractions
Soil pH decreased significantly at the 0- to 10-cm and 10to 30-cm layers in all sites under cultivation (P < 0.05) and in
the 30- to 50-cm layer only in Sites A and B. Soil BD values in
the 0- to 10-cm layer significantly differed among the land-use
types at all sites (P < 0.05 for all sites), and it increased to some
extent with the duration of cultivation (Table 1). The effect of
cultivation on BD declined with soil depth. Bulk density in the
10- to 30-cm layer significantly differed at Sites B and D (P <
0.05), whereas there were no apparent changes in the 30- to
50-cm and 50- to 100-cm layers.
Sand was the most abundant particle in the soil particle-size
fractions at all sites. In the 0- to 10-cm layer, the sand fraction
accounted for 58 to 75% of the total soil weight, silt accounted
for 22 to 39% of the total soil weight, and the clay content was
small (Table 2). Cultivation significantly decreased the sand
content and increased the silt content in the 0- to 10-cm soil
layer at Sites A, C, and D (P < 0.05) due to a possible effect of
wind erosion enhanced by cultivation. However, no significant
changes in the soil fractions were observed at Site B. In the 10- to
30-cm layer, cultivation significantly enhanced the silt content
(P < 0.05) at Sites A and C. The effect of cultivation on the soil
fractions was not evident in the 30- to 50-cm layer.
Changes in Soil Carbon and Nitrogen Stocks
The SOC and total N content decreased with soil depth,
and there were significant differences between the upper and
underlying horizons (P < 0.05). The SOC stock in the 0- to
100-cm layer was significantly higher at Sites C and D than
at Sites A and B for not only native grasslands but also for
croplands (P < 0.05) (Fig. 2). Similarly, the total N stock in
native grasslands was significantly higher at Site D than at Sites
A and B (P < 0.05) (Fig. 3), whereas in croplands, it was higher
at Sites C and D than at Sites A and B (P < 0.05).
The practice of cultivation in the Inner Mongolian grasslands
decreased significantly the SOC stocks in the 0- to 100-cm layer
at Sites B, C, and D, but the decrease was not significant at Site A
(Fig. 2). Soil organic C stock in the 0- to 10-cm and 10- to 30-cm
layers declined significantly with the duration of reclamation (P <
0.05) except in the 10- to 30-cm layer at Site C (Fig. 2; Table 3).
Soil organic C stock in the 0- to 10-cm and 10- to 30-cm soil layers
1509
1510
7.6a
(0.2)
7.3b
(0.2)
7.2a
(0.2)
7.5a
(0.3)
7.2b
(0.3)
6.5c
(0.1)
7.7a
(0.2)
7.4b
(0.1)
7.1c
(0.2)
7.5a
(0.2)
7.1b
(0.2)
7.2b
(0.1)
pH
0–10 cm
C
N
BD†
content content
g cm–3
— g kg−1 —
1.30a‡ 17.0a
1.7a
(0.01)
(0.4)
(0.2)
1.33ab 15.3b
1.6ab
(0.03)
(1.6)
(0.1)
1.34b
13.8b
1.5b
(0.01)
(1.0)
(0.2)
1.26a
20.9a
2.0a
(0.02)
(2.0)
(0.1)
1.32b
15.5b
1.8b
(0.01)
(1.0)
(0.1)
1.41c
9.0c
1.0c
(0.01)
(0.5)
(0.1)
1.29a
18.4a
1.8a
(0.03)
(2.5)
(0.1)
1.31ab 16.3ab
1.8a
(0.03)
(2.4)
(0.1)
1.34b
14.4b
1.5b
(0.01)
(0.7)
(0.1)
1.16a
26.1a
2.2a
(0.03)
(2.8)
(0.2)
1.36b
19.0b
1.5b
(0.02)
(2.7)
(0.1)
1.32c
15.7c
1.2c
(0.01)
(0.5)
(0.1)
10.1a
(1.3)
9.9a
(1.4)
9.6a
(1.9)
10.5a
(1.6)
8.4b
(0.6)
9.1a
(1.0)
10.3a
(1.8)
9.2a
(1.4)
9.4a
(0.4)
11.8a
(0.4)
10.7a
(1.6)
11.3a
(1.3)
C:N
ratio
7.8a
(0.2)
7.3b
(0.2)
7.1b
(0.3)
7.7a
(0.3)
7.3b
(0.3)
6.7c
(0.1)
7.9a
(0.2)
7.5b
(0.1)
7.3b
(0.3)
7.7a
(0.2)
7.3b
(0.1)
7.4b
(0.3)
pH
g cm–3
1.34a
(0.02)
1.35a
(0.02)
1.35a
(0.02)
1.27a
(0.01)
1.34b
(0.02)
1.36b
(0.01)
1.33a
(0.02)
1.33a
(0.04)
1.34a
(0.01)
1.31a
(0.02)
1.37b
(0.02)
1.34b
(0.03)
BD
10–30 cm
C
N
content content
— g kg−1 —
13.9a
1.4a
(1.4)
(0.2)
13.6a
1.3a
(1.4)
(0.1)
11.2b
1.1b
(1.3)
(0.1)
19.7a
2.1a
(0.8)
(0.1)
16.7b
2.0a
(2.1)
(0.1)
11.7c
1.3b
(1.0)
(0.1)
15.2a
1.6a
(1.6)
(0.1)
14.7a
1.6a
(2.9)
(0.1)
13.9a
1.5a
(1.0)
(0.1)
16.9a
1.8a
(1.8)
(0.2)
14.2b
1.4b
(2.2)
(0.2)
11.9b
1.2b
(1.4)
(0.2)
10.2a
(0.4)
10.9a
(1.3)
10.1a
(1.7)
9.4a
(0.1)
8.4a
(0.6)
9.0a
(0.4)
9.5a
(1.3)
8.9a
(1.8)
9.6a
(0.4)
9.4a
(0.6)
10.2a
(2.2)
9.9a
(1.6)
C:N
ratio
8.6a
(0.2)
8.2b
(0.2)
8.1b
(0.3)
8.5a
(0.3)
8.2a
(0.3)
8.3a
(0.5)
8.7a
(0.2)
8.4b
(0.1)
8.3b
(0.3)
8.5a
(0.2)
8.3a
(0.3)
8.2a
(0.1)
pH
30–50 cm
C
N
BD
content content
g cm–3
— g kg−1 —
1.40a
9.6a
0.9a
(0.01)
(1.0)
(0.1)
1.39a
10.0a
0.9a
(0.01)
(0.8)
(0.1)
1.39a
10.0a
0.9a
(0.02)
(1.6)
(0.1)
1.34a
15.0a
1.6a
(0.04)
(2.2)
(0.2)
1.38a
11.2b
1.2a
(0.01)
(0.6)
(0.1)
1.38a
10.8b
1.2a
(0.04)
(1.4)
(0.2)
1.35a
14.3a
1.6a
(0.01)
(1.1)
(0.1)
1.37b
10.7b
1.3b
(0.01)
(1.9)
(0.1)
1.36ab 12.0b
1.2b
(0.01)
(1.1)
(0.1)
1.38a
10.8a
1.1a
(0.05)
(1.8)
(0.4)
1.39a
10.0a
0.9a
(0.02)
(1.1)
(0.1)
1.40a
9.7a
1.0a
(0.03)
(1.9)
(0.2)
‡ Data are represented as mean (±1 SD) (n = 5). Different letters in the same column indicate significant difference at the P = 0.05 level.
† Bulk density.
19-yr cultivation
(DC19)
39-yr cultivation
(DC39)
39-yr cultivation
(CC39)
51-yr cultivation
(CC51)
Site D grassland (DC0)
19-yr cultivation
(BC19)
39-yr cultivation
(BC39)
Site C grassland (CC0)
19-yr cultivation
(AC19)
49-yr cultivation
(AC49)
Site B grassland (BC0)
Site A grassland (AC0)
Experimental
plot
Table 1. Changes in pH, bulk density, and carbon and nitrogen contents of soil along the cultivation chronosequences in Inner Mongolian grasslands.
pH
8.4a
(0.2)
8.1a
(0.4)
8.1a
(0.4)
8.3a
(0.3)
8.1a
(0.4)
8.1a
(0.1)
8.4a
(0.2)
8.3a
(0.1)
8.3a
(0.3)
8.3a
(0.2)
8.1a
(0.6)
8.2a
(0.4)
C:N
ratio
10.2a
(0.9)
11.3a
(0.6)
11.2a
(1.6)
9.5a
(0.2)
9.6a
(0.6)
8.7a
(1.4)
8.7a
(0.7)
8.4a
(1.5)
9.7a
(0.7)
11.3a
(1.1)
10.7ab
(0.6)
9.7b
(0.4)
g cm–3
1.44a
(0.02)
1.45a
(0.03)
1.45a
(0.02)
1.40a
(0.02)
1.40a
(0.03)
1.43a
(0.02)
1.40a
(0.02)
1.41a
(0.02)
1.41a
(0.01)
1.42a
(0.03)
1.40a
(0.03)
1.42a
(0.02)
BD
50–100 cm
C
N
content content
— g kg−1 —
7.4a
0.6a
(1.2)
(0.1)
7.0a
0.6a
(1.8)
(0.1)
6.9a
0.6a
(1.2)
(0.1)
9.3a
0.9a
(1.7)
(0.2)
8.9a
0.8a
(1.9)
(0.1)
8.3a
0.9a
(1.5)
(0.1)
9.8a
1.0a
(1.6)
(0.1)
9.7a
1.0a
(0.4)
(0.1)
9.1a
0.9a
(1.1)
(0.1)
8.1a
0.7a
(2.2)
(0.2)
8.3a
0.7a
(1.3)
(0.1)
8.3a
0.7a
(0.8)
(0.1)
12.5a
(1.3)
12.5a
(2.2)
12.4a
(1.3)
11.1a
(2.1)
10.8a
(0.7)
9.8a
(1.2)
10.1a
(1.6)
10.1a
(0.9)
9.9a
(1.9)
11.0a
(0.7)
11.7a
(1.0)
11.5a
(0.6)
C:N
ratio
significantly decreased by 2.27 to 12.69 Mg
C ha−1 (10.44–52.60%) and 0.89 to 12.15
Mg C ha−1 (2.32–28.92%), respectively; the
decreases were significantly higher in the
plots with longer reclamation in terms of
absolute and relative changes in SOC stock.
Cultivation significantly decreased
the total N stock in the 0- to 100-cm
layer at Sites A, B, and D (P < 0.05),
but the decrease was not significant at
Site C (Fig. 3; Table 3). The practice of
cultivation had a significant effect on the
total N stock in the 0- to 10-cm and 10to 30-cm layers at Sites A, B, and C (P <
0.05), whereas the effect was small in the
30- to 100-cm layers (Fig. 3). Moreover,
the absolute and relative changes in the
total N stock in the 0- to 30-cm layer
were significant at Sites B and D.
Changes in Carbon and Nitrogen
Stocks in Soil Fractions
The C and N concentrations in the
soil fractions were in the order clay > silt
> sand in the 0- to 10-, 10- to 30-, and
30- to 50-cm layers at all sites. Cultivation
significantly decreased the C content and
stock in the sand fraction in the 0- to
10-cm and 10- to 30-cm layers except in
the 10- to 30-cm layer at Site A (Fig. 4;
Tables 2 and 4). The decrease in the C
stock in the sand fraction ranged from 1.60
to 10.51 Mg C ha−1 (11.91–79.73%) in
the 0- to 10-cm soil layer and from 0.65 to
10.82 Mg C ha−1 (−2.01 to 45.26%) in the
10- to 30-cm layer (Table 4). Cultivation
declined the C stock in the silt and clay
fractions only slightly (Table 4).
Cultivation significantly decreased
the N stock in the sand fraction in the
0- to 10-cm layer at all sites (Fig. 5), and
the decrease varied from 0.16 to 0.95 Mg
N ha−1 (16.92–80.38%) (Table 5). The
N stock in the sand fraction in the 10- to
30-cm soil layer was significantly lower
in croplands than in native grasslands
at Sites B and D. As in the case of the C
stock in soil fractions, cultivation had a
small effect on the N stock in the silt and
clay fractions (Fig. 5; Table 5).
Changes in the Carbon to Nitrogen
Ratio in Soil and Soil Fractions
Cultivation had no apparent effect on
the soil C:N ratio in the 0- to 10-, 10- to
30-, 30- to 50-, and 50- to 100-cm layers
except in the 0- to 10-cm layer at site B (P
< 0.05) and the 30–50 cm layer at site D
Journal of Environmental Quality
Table 2. Changes in soil particle-size fractions and their carbon and nitrogen content along different experimental plots.
Depth
cm
0–10
10–30
30–50
Land use
types
Site 1 grassland
19-yr
cultivation
49-yr
cultivation
Site 2 grassland
19-yr
cultivation
39-yr
cultivation
Site 3 grassland
39-yr
cultivation
51-yr
cultivation
Site 4 grassland
19-yr
cultivation
39-yr
cultivation
Site 1 grassland
19-yr
cultivation
49-yr
cultivation
Site 2 grassland
19-yr
cultivation
39-yr
cultivation
Site 3 grassland
39-yr
cultivation
51-yr
cultivation
Site 4 grassland
19-yr
cultivation
39-yr
cultivation
Site 1 grassland
19-yr
cultivation
49-yr
cultivation
Site 2 grassland
19-yr
cultivation
39-yr
cultivation
Site 3 grassland
39-yr
cultivation
51-yr
cultivation
Site 4 Grassland
19-yr
cultivation
39-yr
cultivation
Sand†
Content
C content
g kg−1 soil
g C kg−1
753.6a‡ (26.6) 10.7a (1.2)
707.3ab (8.2)
9.7a (2.2)
N content
g N kg−1
0.9a (0.1)
0.8b (0.1)
Silt
Content
C content
g kg−1 soil
g C kg−1
223.6a (24.9) 34.7a (1.4)
271.3ab (6.8) 27.6b (3.4)
N content
g N kg−1
3.9a (0.4)
3.3a (0.3)
Content
g kg−1 soil
22.8a (2.9)
21.4a (2.1)
Clay
C content
g C kg−1
50.6a (3.9)
42.1b (5.0)
719.2a (39.5)
7.6b (0.4)
0.7b (0.1)
256.3b (37.2) 28.6b (4.9)
3.4a (0.6)
24.5a (8.1)
45.8ab (4.3) 5.6a (1.2)
709.0a (35.1)
693.1a (33.2)
14.7a (1.7)
11.0b (0.8)
1.3a (0.2)
0.9b (0.2)
286.6a (39.8) 24.9a (3.0)
274.7a (27.1) 23.8a (1.4)
2.5a (0.1)
2.4a (0.4)
24.4a (7.9)
27.9a (3.7)
63.5a (15.6) 7.4a (1.9)
50.6ab (3.6) 5.8ab (0.6)
685.5a (24.3)
3.1c (0.3)
0.3c (0.0)
292.8a (27.9) 20.4b (2.4)
2.2a (0.3)
25.8a (7.8)
39.1b (4.5)
726.3a (28.2)
636.2b (21.5)
11.1a (1.5)
10.4a (1.1)
0.9a (0.2)
0.9a (0.0)
237.3 a (18.2) 33.9a (5.4)
324.1b (27.1) 23.2b (4.4)
3.6a (0.7)
2.9b (0.2)
32.4a (8.8)
39.7a (9.7)
69.5a (13.7) 8.5a (0.9)
53.2b (10.5) 7.2b (0.4)
663.1b (40.3)
7.9b (1.3)
0.7b (0.1)
296.4b (30.1) 24.4b (2.7)
2.7b (0.2)
40.5a (10.6) 49.9b (5.6)
612.2a (16.8)
576.4b (21.3)
21.2a (2.4)
14.8b (2.9)
1.8a (0.3)
0.9b (0.1)
343.7a (10.8) 30.8a (3.2)
392.3b (19.6) 22.0b (4.7)
3.4a (0.4)
2.6b (0.2)
44.1a (4.5)
31.4a (5.3)
56.0a (6.5) 7.9a (1.2)
58.0a (14.6) 7.5a (2.3)
599.3b (18.8)
10.4c (1.6)
0.7b (0.1)
372.5b (18.1) 21.7b (1.7)
2.2c (0.2)
30.9a (7.5)
44.6a (4.1)
5.6a (1.1)
736.6a (8.9)
694.0b (15.6)
7.5a (1.2)
7.6a (0.2)
0.6a (0.1)
0.5a (0.1)
241.6a (9.8)
30.8a (3.2)
3.5a (0.4)
287.9b (15.9) 25.9ab (4.0) 2.8b (0.4)
21.8a (3.4)
18.1a (3.2)
44.8a (3.2)
46.9a (5.9)
5.0a (0.7)
5.4a (0.3)
727.3ab (33.9)
6.2b (0.9)
0.5a (0.2)
251.9ab (34.1) 22.8b (5.3)
2.7b (0.4)
20.8a (2.1)
40.9a (5.4)
4.8a (1.1)
646.0a (34.6)
639.7a (19.6)
12.5a (0.8)
10.0b (1.7)
1.0a (0.0)
0.8b (0.0)
326.1a (30.5) 21.2a (1.8)
338.0a (21.9) 18.2b (1.5)
2.0a (0.1)
1.8ab (0.3)
27.8a (8. 7) 56.7a (22.6) 6.1a (2.7)
24.3a (3.4) 54.4a (14.1) 5.7a (0.6)
617.4a (17.1)
7.3c (1.7)
0.6c (0.1)
355.6a (15.9) 16.6b (1.1)
1.7b (0.1)
31.4a (5.5)
43.1a (9.0)
673.3a (14.2)
648.8a (11.7)
7.7a (0.9)
7.1a (1.1)
0.6a (0.1)
0.6a (0.1)
296.5a (13.9) 28.4a (3.2)
346.9b (11.7) 23.6b (2.8)
2.9a (0.2)
2.3b (0.1)
30.1a (4.1)
30.9a (8.1)
50.8a (7.6) 5.7a (1.3)
53.7a (18.2) 5.7a (0.7)
647.5a (25.3)
6.0a (1.7)
0.5a (0.1)
336.7a (23.5) 24.2b (1.2)
2.4b (0.3)
32.4a (5.4)
48.7a (7.6)
5.5a (1.4)
650.6a (11.6)
654.8a (33.6)
13.8a (1.5)
9.1b (1.7)
1.1a (0.1)
0.7b (0.2)
309.6a (14.8) 19.7a (3.2)
305.6a (36.2) 21.3a (3.3)
2.2a (0.3)
2.3b (0.3)
39.7a (4.4)
39.6a (9.4)
45.1a (9.1)
41.6a (5.2)
5.6a (0.9)
5.8a (0.5)
649.1a (32.4)
7.4b (1.3)
0.5c (0.1)
313.4a (33.5) 21.6a (1.1)
2.2b (0.2)
37.5a (4.3)
44.6a (10.1) 5.4a (0.6)
706.8a (19.3)
686.7a (8.9)
3.5a (0.3)
4.1a (0.5)
0.3a (0.1)
0.2a (0.1)
269.3a (21.7) 23.4a (2.9)
2 90.5a (8.0)
21.5a (1.7)
2.4a (0.2)
2.1b (0.2)
23.9a (4.8)
22.8a (4.7)
35.2a (4.5) 4.2a (0.4)
40.5a (13.2) 4.2a (1.2)
690.9a (43.1)
4.2a (1.6)
0.3a (0.1)
289.1a (46.0) 21.5a (2.5)
2.1b (0.2)
20.0a (3.0)
39.8a (4.1)
4.4a (0.4)
665.7a (32.8)
677.8a (23.4)
6.5a (0.7)
6.3a (0.5)
0.5a (0.1)
0.5a (0.1)
303.5a (32.9) 20.5a (2.4)
297.8a (20.4) 19.9a (0.9)
2.0a (0.2)
2.0a (0.2)
30.8a (0.9)
24.4b (4.2)
37.5a (7.9)
44.1a (4.1)
4.5a (0.8)
5.6a (0.6)
666.9a (14.5)
5.4a (1.4)
0.4a (0.0)
304.6a (8.7)
19.7a (1.4)
2.0a (0.2)
28.4a (4.6)
40.0a (4.1)
4.9a (1.2)
679.8a (25.2)
651.1a (24.7)
7.0a (0.8)
7.2a (0.5)
0.5a (0.1)
0.4a (0.1)
279.6a (22.7) 22.1a (4.4)
303.0a (29.1) 19.3a (2.4)
2.2a (0.3)
2.0a (0.1)
40.6a (3.7)
45.9a (3.3)
40.0a (6.2)
35.2a (8.1)
4.3a (0.4)
4.5a (0.4)
673.2a (28.1)
5.6a (1.9)
0.4a (0.1)
284.7a (28.1) 23.3a (4.8)
2.2a (0.3)
44.1a (5.7)
36.8a (4.8)
4.2a (1.0)
669.4a (11.2)
681.5a (20.4)
7.2a (1.0)
6.9a (1.4)
0.5a (0.1)
0.5a (0.1)
284.8a (9.4)
19.9a (2.8)
274.2a (27.7) 20.4a (2.5)
1.9a (0.3)
2.0a (0.2)
45.8a (5.7)
44.3a (9.1)
32.0a (10.6) 3.9a (1.3)
35.0a (3.3) 4.2a (0.2)
668.5a (24.3)
7.2a (1.7)
0.5a (0.1)
287.1a (26.4) 18.7a (1.9)
1.8a (0.1)
44.4a (5. 8) 34.1a (6.0)
N content
g N kg−1
5.9a (0.4)
5.1a (0.6)
5.1a (0.7)
6.9b (0.7)
4.5a (0.6)
4.1a (0.3)
† Sand, 50–2000 μm; silt, 2–50 μm; clay, <2 μm.
‡ Data are represented as mean (±1 SD) (n = 5). Different letters in the same column indicate significant difference at the P = 0.05 level.
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1511
Fig. 2. Changes in soil C stock due to cultivation in Inner Mongolian
grasslands. Data are represented as mean ± 1 SD (n = 5). Different
lowercase letters in a group indicate significant differences among
different plots (P < 0.05). See Table 1 for plot abbreviations.
(P < 0.05). The C:N ratio in the soil fractions was in the order
sand > silt > clay, and no significant changes were observed in
the C:N ratio in these fractions among different land use types.
Discussion
Cultivation had apparent effects on soil pH, BD, and C and
N contents in Inner Mongolian grasslands. Along the cultivation
chronosequences, soil pH decreased significantly and soil BD
increased to some extent in the upper layers. This is consistent
with previous results, which attribute high soil BD in croplands
to the duration and intensity of cultivation (Mikhailova et al.,
2000). Wang et al. (2009) reported that soil BD increased with
cultivation chronosequence in Inner Mongolian grasslands,
attributing it to a significant decrease (70%) in root biomass in
the upper layers in croplands, increased compaction of topsoil,
and destruction of initial roots. However, some studies have
reported that cultivation leads to a decrease in soil BD (Ellert
and Bettany, 1995; Lee et al., 2009). The varying effects of
reclamation on BD in different regions require further research.
1512
Fig. 3. Changes in soil N stock due to cultivation in Inner Mongolian
grasslands. Data are represented as mean ± 1 SD (n = 5). Different
lowercase letters in a group indicate significant differences among
different plots (P < 0.05). See Table 1 for plot abbreviations.
Conversion of native grassland into croplands in Inner
Mongolia in China, on an average, decreased soil C (9.7%) and
N (9.9%) stocks in the 0- to 100-cm layer after reclamation.
Wu et al. (2003) and Wang et al. (2008) estimated 10 to 40%
and 4 to 22% soil C loss in the 0- to 100-cm layer after decades
of cultivation in Inner Mongolian grasslands, respectively.
In our study, the soil C and N stocks in the 0- to 30-cm layer
decreased by 5. 5 to 37.8% and 4.6 to 34.3%, respectively; this
was consistent with the data of Wang et al. (2009). Li et al.
(2007) reported that soil C stock under annul oats decreased by
26 to 42%. Using meta-analysis, Guo and Gifford (2002) found
that soil C stock declined globally by 59% after conversion of
pasture into cropland. Mikhailova et al. (2000) reported that
the soil C and N stocks in a 50-yr continuous-fallow cropland
with Russian chernozem soil decreased by 38 to 43% and 45 to
53%, respectively. Luo et al. (2010) reported that cultivation in
Australian agro-ecosystems caused 51% soil C loss in the 0- to
10-cm layer. Soil properties and crop management practices such
as crop rotation, fallow cultivation, residue management, and
fertilizer addition and their interactions determine the loss rates
Journal of Environmental Quality
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‡ Not significant.
† Data represent absolute changes in soil C and N stocks (n = 5); data in parentheses represent changes (%).
** Significant at P < 0.01 for the two-tailed t test (n = 5).
39-yr cultivation (DC39)
19-yr cultivation (DC19)
Site D
51-yr cultivation (CC51)
39-yr cultivation (CC39)
Site C
39-yr cultivation (BC39)
19-yr cultivation (BC19)
Site B
49-yr cultivation (AC49)
19-yr cultivation (AC19)
* Significant at P < 0.05 for the two-tailed t test (n = 5).
Changes in soil N stock
0–10 cm
10–30 cm
0–30 cm
0–100 cm
————————————— Mg N ha–1 —————————————
−0.20 ± 0.32‡
−0.31 ± 0.42‡
−0.51 ± 0.70‡
−0.85 ± 0.31**
(−7.80 ± 13.34)
(−7.67 ± 98.2)
(−7.84 ± 10.39)
(−6.70 ± 2.26)
−0.30 ± 0.39‡
−0.66 ± 0.28**
−0.96 ± 0.62*
−1.32 ± 0.79*
(−12.83 ± 16.61)
(−17.70 ± 7.02)
(−15.99 ± 9.86)
(−10.31 ± 6.16)
−0.46 ± 0.19**
−0.38 ± 0.24*
−0.84 ± 0.28**
−1.42 ± 1.30‡
(−19.68 ± 7.67)
(−10.41 ± 6.36)
(−14.02 ± 4.04)
(−10.02 ± 9.00)
−1.11 ± 0.25**
−0.86 ± 0.18**
−1.97 ± 0.42**
−2.29 ± 1.57*
(−47.91 ± 7.18)
(−23.51 ± 3.67)
(−32.94 ± 5.11)
(−16.28 ± 10.65)
−0.06 ± 0.21‡
−0.25 ± 0.42‡
−0.31 ± 0.52‡
−0.19 ± 1.15‡
(1.91 ± 9.08)
(−5.99 ± 9.44)
(−4.62 ± 7.35)
(−1.13 ± 8.15)
−0.34 ± 0.30‡
−0.38 ± 0.56‡
−0.71 ± 0.80‡
−0.83 ± 1.07‡
(−13.83 ± 12.14)
(−8.92 ± 12.57)
(−10.89 ± 11.66)
(−5.71 ± 7.26)
−1.08 ± 0.34**
−0.60 ± 0.24**
−1.68 ± 0.53**
−1.99 ± 1.08*
(−34.54 ± 8.05)
(−14.44 ± 6.31)
(−22.91 ± 6.94)
(−12.67 ± 5.67)
−1.53 ± 0.279**
−1.00 ± 0.32**
−2.53 ± 0.40**
−2.54 ± 1.65*
(−49.33 ± 6.31)
(−23.23 ± 6.39)
(−34.33 ± 4.23)
(−15.97 ± 9.07)
Changes in soil C stock
0–10 cm
10–30 cm
0–30 cm
0–100 cm
————————————— Mg C ha–1 —————————————
−2.27 ± 1.87†‡
−0.89 ± 1.22‡
−3.16 ± 2.35*
−4.34 ± 20.33‡
(−10.44 ± 8.44)
(−2.32 ± 3.22)
(−5.45 ± 3.97)
(−2.85 ± 14.60)
−4.18 ± 1.31**
−7.21 ± 6.49‡
−11.38 ± 7.52*
−13.21 ± 13.96‡
(−18.82 ± 5.80)
(−18.26 ± 15.26)
(−18.68 ± 11.39)
(−9.69 ± 10.33)
−4.37 ± 2.95*
−6.52 ± 4.21*
−10.89 ± 4.98*
−13.93 ± 8.75*
(−17.45 ± 11.91)
(−15.61 ± 10.17)
(−16.62 ± 7.60)
(−9.53 ± 6.05)
−12.69 ± 2.55**
−12.15 ± 3.51**
−24.84 ± 1.38**
−31.65 ± 19.29*
(−52.60 ± 6.32)
(−28.92 ± 7.35)
(−37.81 ± 2.24)
(−20.57 ± 11.95)
−2.73 ± 3.67‡
−2.13 ± 4.13‡
−4.86 ± 5.57*
−6.21 ± 12.26‡
(−10.62 ± 15.54) (−4.80 ± 10.08)
(−7.17 ± 8.26)
(−3.87 ± 7.97)
5.08 ± 2.61*
−4.06 ± 2.72*
−9.13 ± 5.29*
−14.50 ± 8.01*
(−20.62 ± 8.62)
(−9.85 ± 6.13)
(−13.86 ± 7.09)
(−9.40 ± 4.99)
−8.79 ± 4.87*
−7.24 ± 7.43‡
−16.02 ± 8.38*
−15.51 ± 17.30‡
(−28.17 ± 18.32) (−15.51 ± 17.13)
(−21.10 ± 10.20)
(−9.11 ± 9.37)
−12.39 ± 3.87**
−9.50 ± 7.06*
−21.89 ± 6.32**
−21.33 ± 16.76*
(−40.26 ± 8.36)
(−20.38 ± 14.39)
(−29.11 ± 7.25)
(−12.68 ± 8.69)
Site A
Conversion of native grasslands into croplands resulted
in significant decreases in the soil C and N stocks in inner
Mongolian grasslands. Soil C and N stocks declined significantly
in the 0- to 10-cm and 10- to 30-cm layers during reclamation,
and the effect of cultivation declined with soil depth. The effect
Experimental plot
Conclusions
Table 3. Absolute and relative changes in soil carbon and nitrogen stocks as affected by cultivation.
and amounts of C and N stocks (Elberling et al., 2003). We
assumed that the practice of fallow cultivation and the biomass
of weeds, which would increase the C input into the soil and
recycle the N absorbed by the weeds, can partially explain the
lower soil C loss in Inner Mongolian grasslands.
Soil C loss in Inner Mongolian grasslands subjected to
decades of cultivation can partly be explained by the following
reasons: (i) Decreasing input of C and N through root growth
and turnover after reclamation results in decreases in soil C
and N stocks (Gill and Jackson, 2000; Wang et al., 2009). It
was estimated that root biomass in the 0- to 30-cm layer in
croplands (630 g m−2) was 23% of that in native steppe (2750
g m−2) (Wang et al., 2009). (ii) Removal of native vegetation
and cultivation reduce the physical protection of SOM from
decomposition because of destruction of soil structure, which
enhances microbial decomposition of SOM (Guo and Gifford,
2002; Urioste et al., 2006; Dawson and Smith, 2007). (iii)
Disruption of soil’s initial structure by the practice of cultivation
renders the surface soil susceptible to wind erosion in the region
(Liu and Tong, 2003; Yan et al., 2005; He et al., 2008).
Some studies have pointed out that most management
practices or land use changes exert greater effects on the SOM
in the sand fraction than on that in the silt and clay fractions
(Balesdent et al., 1998; Christensen, 2001; Jolivet et al., 2003; Li
et al., 2007; He et al., 2009). Our results showed that the effect
of cultivation on the C and N stocks in the soil fractions was in
the order sand > silt > clay, which could be partially attributed
to higher wind erosion by cultivation (Liu et al., 2004). Li et
al. (2007) found that soil C stock in the 0- to 30-cm soil layer
in annual oat croplands decreased by 51, 45, and 33% in the
sand, silt, and clay fractions, respectively. Jolivet et al. (2003)
reported that 30-yr cultivation induced a 70% decrease in the
soil C stock in the sand fraction. Conversely, the recovery rates
of soil C and N stocks were higher in the sand fraction than in
the silt and clay fractions when croplands were converted into
perennial grasslands (Preger et al., 2010) or when degraded
grasslands were restored by grazing exclusion (He et al., 2009).
These studies suggest that sand fraction is a more important
contributor to the loss or gain of C and N stocks as a whole
than silt or clay fraction under land use changes.
Most of the SOM associated with sand fraction is labile with
rapid turnover, whereas that in silt and clay fractions is relatively
stable (Christensen, 2001). Using the natural 13C abundance,
some studies have found that the C turnover in all soil fractions is
affected by new C inputs, and the formerly protected C becomes
less stable with cultivation (Balesdent et al., 1998; Desjardins et
al., 2006). Balesdent et al. (1998) reported that C stock in particle
organic matter (POM) decreased by 51 and 82% in 7- and 35-yrold croplands derived from forest, respectively. We did not quantify
the amount of C and N transferred among the soil fractions in
this study, although transfers from POM to finer SOM should be
essential for the C turnover model (Balesdent, 1996).
1513
Fig. 4. Changes in C stock in soil fractions due to cultivation. Data are represented as mean ± 1 SD (n = 5). Different lowercase letters in a group
indicate significant differences among different plots (P < 0.05). See Table 1 for plot abbreviations.
Table 4. Absolute and relative changes in C stocks in soil fractions as affected by cultivation.
Experimental plot
Site A
19-yr cultivation (AC19)
49-yr cultivation (AC49)
Site B
19-yr cultivation (BC19)
39-yr cultivation (BC39)
Site C
39-yr cultivation (CC39)
51-yr cultivation (CC51)
Site D
19-yr cultivation (DC19)
39-yr cultivation (DC39)
Depth
cm
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
Change in C stock
Sand (50–2000 μm)
Silt (2–50 μm)
Clay (<2 μm)
—————————————— Mg C ha−1 ——————————————
−1.60‡ (−11.91)†
−0.35‡ (−0.67)
−0.32* (−21.09)
−0.65‡ (−2.01)
0.13‡ (15.57)
−0.37‡ (−12.57)
1.06* (15.96)
−0.11‡ (2.12)
0.11‡ (12.59)
−3.43* (−31.23)
−0.70‡ (−3.52)
0.05‡ (−1.97)
−4.61** (−30.84)
−2.24‡ (−10.10)
−0.36‡ (−11.33)
1.26‡ (23.74)
−0.02‡ (−0.13)
−0.13‡ (−2.47)
−3.58* (−26.04)
−0.68* (−7.06)
−0.11‡ (−1.84)
−4.21* (−20.63)
−2.00‡ (−9.20)
−0.32‡ (−8.74)
0.17‡ (−0.91)
−0.72‡ (−3.82)
−0.21‡ (−3.87)
−10.51** (−79.73)
−1.53‡ (−16.10)
−0.65‡ (−31.42)
−9.10** (−44.15)
−2.73* (−14.38)
−0.32‡ (−7.39)
−1.87‡ (−14.86)
−0.48‡ (−1.87)
0.05‡ (4.42)
−1.86* (−16.96)
−0.69‡ (−5.84)
−0.18‡ (−24.05)
−1.46‡ (−9.66)
−0.74‡ (−2.03)
0.06‡ (10.61)
0.17‡ (−0.41)
−0.53‡ (−2.02)
0.04‡ (5.04)
−3.59* (−33.39)
−1.11‡ (−9.43)
−0.38‡ (−22.32)
−2.35‡ (−23.49)
−0.74‡ (−3.78)
0.03‡ (3.78)
−2.61‡ (−19.16)
0.78* (3.54)
0.01‡ (3.44)
−5.44** (−35.69)
−2.60‡ (−19.39)
−0.76‡ (−24.95)
−7.94* (−32.45)
1.21‡ (7.52)
−0.51‡ (−7.54)
−0.36‡ (−0.46)
−0.30‡ (−0.22)
0.33‡ (14.27)
−7.97** (−51.92)
−3.11** (−14.27)
−1.31‡ (−45.61)
−10.82 ** (−45.26)
1.69‡ (4.65)
−0.38‡ (−4.65)
0.09‡ (−0.46)
−0.79‡ (−14.82)
0.20‡ (14.82)
* Significant for two-tailed t test at P < 0.05 (n = 5).
** Significant for two-tailed t test at P < 0.01 (n = 5).
† Data represent absolute changes in C stock in soil fractions; data in parentheses represent relative changes (%).
‡ Not significant.
1514
Journal of Environmental Quality
Fig. 5. Changes in N stock in soil fractions due to cultivation. Data are represented as mean ± 1 SD (n = 5). Different lowercases in a group indicate
significant difference among different plots (P = 0.05). See Table 1 for plot abbreviations.
Table 5. Absolute and relative changes in the nitrogen stock in soil fractions as affected by cultivation.
Experimental plot
Site A
19-yr cultivation (AC19)
49-yr cultivation (AC49)
Site B
19-yr cultivation (BC19)
39-yr cultivation (BC39)
Site C
39-yr cultivation (CC39)
51-yr cultivation (CC51)
Site D
19-yr cultivation (DC19)
39-yr cultivation (DC39)
Depth
cm
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
0–10
10–30
30–50
Change in N stocks
Sand (50–2000 μm)
Silt (2–50 μm)
Clay (<2 μm)
————————————— Mg N ha−1 —————————————
−0.19* (−20.41)†
0.02‡ (4.43)
0.03‡ (−15.69)
−0.20‡ (−15.72)
−0.07‡ (−3.08)
−0.03‡ (−9.07)
−0.02‡ (2.23)
−0.10‡ (−5.01)
−0.03‡ (−2.00)
−0.28* (−16.81)
0.02‡ (1.22)
0.01‡ (−0.46)
−0.16‡ (−15.05)
−0.47* (−19.81)
−0.02‡ (−4.18)
0.06‡ (23.51)
−0.15‡ (−7.95)
−0.04‡ (−8.86)
−0.38* (−31.59)
−0.06‡ (−5.67)
−0.04‡ (8.08)
−0.23** (−14.61)
−0.11‡ (−5.81)
−0.02‡ (−2.68)
−0.06‡ (−6.58)
0.03‡ (0.96)
0.01‡ (1.35)
−0.95** (−80.38.43)
0.10‡ (10.01)
−0.06‡ (−25.19)
0.67** (−42.45)
−0.15‡ (−8.45)
−0.04‡ (−9.24)
−0.13** (−14.65)
0.09‡ (10.24)
0.01‡ (1.35)
−0.16‡ (−16.92)
0.10‡ (10.12)
0.01‡ (21.61)
−0.10‡ (−5.49)
−0.16‡ (−6.94)
0.01‡ (6.46)
−0.10‡ (−10.24)
−0.06‡ (−3.41)
0.04‡ (8.02)
−0.28* (−30.70)
−0.05‡ (3.38)
0.01‡ (2.56)
−0.23‡ (−17.31)
−0.15‡ (−6.50)
0.01‡ (6.46)
−0.14‡ (−9.48)
0.03‡ (2.94)
0.04‡ (8.02)
−0.72** (−54.62)
0.21‡ (14.22)
−0.15‡ (−32.87)
−0.66* (−35.27)
0.05‡ (3.06)
0.01‡ (9.71)
−0.14‡ (−11.18)
0.05‡ (4.06)
0.02‡ (16.58)
−0.85** (−64.61.92)
−0.47** (−33.14)
−0.21** (−51.95)
−0.95** (−50.02)
0.01‡ (1.00)
−0.05‡ (−4.46)
−0.10‡ (−9.38)
−0.03‡ (0.58)
0.01‡ (13.88)
* Significant for the two-tailed t test at P < 0.05 (n = 5).
** Significant for the two-tailed t test at P < 0.01.
† Data represent absolute changes in N stocks in soil fractions; data in parentheses represent relative changes (%).
‡ Not significant.
www.agronomy.org • www.crops.org • www.soils.org
1515
of cultivation on the C and N stocks in the soil fractions was in the
order sand > silt > clay. On an average, the C and N stocks in the
sand fraction in croplands decreased by 4.74 Mg C ha−1 (35.86%)
and 0.48 Mg N ha−1 (41.3%) in the 0- to 30-cm layer, respectively,
whereas the declines in the silt and clay fractions were low. The
significant decreases observed in the C and N stocks in the soil and
sand fraction provide the basic data to evaluate the response of soil
C and N to cultivation in semiarid grasslands.
Acknowledgments
This research was funded by the Natural Science Foundation of China (no.
31070431 and 40803024) and the State Key Basic Research Development
Program in China (no. 2010CB833504). The authors thank Li Li and
Qiang Li of the Inner Mongolia Grassland Ecosystem Research Station,
CAS, for assistance with field samplings and chemical analysis.
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Journal of Environmental Quality