Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of China

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  • 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 CO2to 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. TheSOC inventories (1.901.95 kg m-2) in the 015 cm surface

    soils were similar among upper, middle, and lower slope

    positions on the converted land, while the SOC inventories

    (1.411.65 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, convertedsoils (especially in 05 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. XiongKey Laboratory of Mountain Surface Processes and Ecological

    Regulation, CAS, Chengdu 610041, Peoples Republic of China

    J. H. Zhang (&) F. C. Li Y. Wang D. H. XiongInstitute of Mountain Hazards and Environment, Chinese

    Academy of Sciences and Ministry of Water Conservancy,

    P O Box 417, Chengdu 610041, Peoples Republic of China

    e-mail: zjh@imde.ac.cn

    123

    Environmental Management (2014) 53:274283

    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 andgrassland 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 (300402800303900000N and10411034001045303600E; Fig. 1). The climate at the studysite 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 peryear. 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 theannual 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

    1025 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) 53:274283 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 2533 cm

    for converted soils and 3246 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 for24 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 thelocation of the study area in the

    Sichuan Basin of southwestern

    China

    276 Environmental Management (2014) 53:274283

    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 detectthe 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 analyseswere 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 05 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 010 cm soil layer

    to those in the 1020 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 (010/1020 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

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    Environmental Management (2014) 53:274283 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 015 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 (015 cm) significantly

    increased (P \ 0.001) after the conversion from cultivatedland 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

    Soil d

    epth

    (cm)

    TN concentration (g kg-1)

    Cultivated land

    Converted land

    Uncultivated land

    (c)

    0

    5

    10

    15

    20

    25

    30

    35

    40

    45

    Soil d

    epth

    (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

    Soil d

    epth

    (cm)

    TN concentration (g kg-1)

    Cultivated land

    Converted land

    Uncultivated land

    (a)

    (b)

    Fig. 3 Distribution of soil total nitrogen in converted land, cultivatedland, and original uncultivated land at different landscape positions,

    a upper slope, b middle slope, and c lower slope. The originaluncultivated 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

    Soil d

    epth

    (cm)

    SOC concentration (g kg-1)

    Cultivated land

    Converted land

    Uncultivated land

    (c)

    0

    5

    10

    15

    20

    25

    30

    35

    40

    45

    Soil d

    epth

    (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

    Soil d

    epth

    (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. Theoriginal 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) 53:274283

    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 1530 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

    015 cm layer notably increased compared to the culti-

    vated land, those of the 1530 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 1530 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 (05 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

    530 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 (05 cm) (P = 0.012 and 0.050,

    respectively, for the 05 and 530 cm soil layers).

    0

    0.5

    1

    1.5

    2

    2.5

    3

    3.5

    Upper Middle Lower

    SOC

    ratio

    of 0

    -10

    cm to

    10-

    20 c

    m

    Slope landscape position

    Cultivated landConverted landUncultivated land

    Fig. 4 Soil organic carbon concentration ratios of the 010 cm to1020 cm soil layers

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    3.0

    3.5

    SOC

    inve

    ntor

    y (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

    SOC

    inve

    ntor

    y (kg

    m-2 )

    Slope landscape position

    Cultivated landConverted landUncultivated land

    (a)

    (b)

    Fig. 5 Soil organic carbon inventories in the 015 cm surface soillayer (a) and the 1530 cm subsoil layer (b)

    0

    2

    4

    6

    8

    10

    12

    14

    16

    Upper Middle Lower

    C/N

    ratio

    Slope landcape position

    Cultivated landConverted land

    0

    2

    4

    6

    8

    10

    12

    14

    16

    Upper Middle Lower

    C/N

    ratio

    Slope landcape position

    Cultivated landConverted land

    (a)

    (b)

    aa

    aa

    a

    b

    aa

    a

    aa

    b

    Fig. 6 C/N ratios in the 05 cm surface soil layer (a) and the530 cm subsoil layer (b). The same letter adjacent to the error barsindicates no significant difference between the two types of land-uses

    based on Fishers LSD (0.05). Error bars represent the standard errors

    of the means

    Environmental Management (2014) 53:274283 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 05 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

    (05 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 010 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 3050 cm deep),

    beneath which rock stratum immediately emerges, irre-

    spective of land-use types. The SOC inventories in the

    surface layer (015 cm) of converted soils increased sub-

    stantially, but those in the subsoil layer (1530 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; OBrien 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; OBrien

    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 andslope landscape elements

    SOC Soil

    depth

    Converted land

    (n = 10)

    Cultivated land

    (n = 20)

    Slope

    length

    Slope

    gradient

    Slope

    length

    Slope

    gradient

    Concentration 05 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) 53:274283

    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 (030 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 030 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 05 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; OBrien 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 030 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) 53:274283 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 015 cm surface soils

    increased significantly. For the converted land, C/N ratios

    in the near surface soil (05 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

    530 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 (05 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 theNational Natural Science Foundation of China (41271242) and the

    135 Strategic Program of the Institute of Mountain Hazards and

    Environment, CAS (SDS-135-1206).

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    Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of ChinaAbstractIntroductionMaterials and MethodsStudy SiteSoil Sampling and AnalysisStatistical Analysis

    ResultsCharacteristics of Soil Depth and SOC Depth Distribution at Different Landscape PositionsStocks and Distribution of SOC along the Hillslope TransectC/N RatioRelationship Between SOC Concentrations and Landscape Elements

    DiscussionInherent SOC ConcentrationsLand-Use ConversionC/N Ratio EffectsLandscape Position Impacts

    ConclusionsAcknowledgmentsReferences

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