Potential and sustainability for carbon sequestration with improved soil management in agricultural soils of China

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    Agriculture, Ecosystems and Environme1. Introduction

    Agricultural soils generally have lower organic matter

    content than natural lands because of reduced C input (due to

    annual harvest and removal of crop residue, etc.), high organic

    C decomposition (due to frequent tillage), increased soil

    erosion (Paustian et al., 1997; Bowman et al., 1999; Lal,

    2002a, 2004) and other factors. Many studies have demon-

    strated that improved management practices can increase the

    C content of arable soils towards the levels found in natural

    lands (Smith et al., 2000a; West and Post, 2002; Lal, 2004).

    Increasing agricultural soil stocks has been suggested as an

    important measure to sequester CO2 from the atmosphere to

    help stabilize atmospheric CO2 concentrations and has been

    estimated that 0.40.9 Pg C year1 can be sequestered withinglobal agricultural soils (Paustian et al., 1998). The Kyoto

    Protocol under Article 3.4 includes the component of C

    uptake by soil management in the framework of controlling

    greenhouse gases emissions and hence has generated a broad

    interest in studying C sequestration of agricultural soils

    through improved managements.

    The industrial C emissions of China are about

    1 Pg C year1, second only to the United States (Marlandet al., 2005). As a signatory country of the Kyoto Protocol,

    although it currently has no obligation to cut carbon dioxide

    emissions, China is looking for ways to curb C emission and

    to enhance C sequestration. The arable land in China covers* Corresponding author. Tel.: +86 10 6488 9808; fax: +86 10 6488 9399.

    E-mail address: yanhm@lreis.ac.cn (H. Yan).

    0167-8809/$ see front matter # 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.agee.2006.11.008Arable land soils generally have lower organic carbon (C) levels than soils under native vegetation; increasing the C stocks through

    improved management is suggested as an effective means to sequester CO2 from the atmosphere. Chinas arable lands, accounting for 13% of

    the worlds total, play an important role in soil C sequestration, but their potential to enhance C sequestration has not yet been quantitatively

    assessed. The C sequestration by agricultural soils is affected by many environmental factors (such as climate and soil conditions), biological

    processes (crop C fixation, decomposition and transformation), and crop and soil management (e.g. tillage and manure application).

    Estimation of the C sequestration potential requires the quantification of the combined effects of these factors and processes. In this study, we

    used a coupled remote sensing- and process-based ecosystem model to estimate the potential for C sequestration in agricultural soils of China

    and evaluated the sustainability of soil C uptake under different soil management options. The results show that practicing no-tillage on 50%

    of the arable lands and returning 50% of the crop residue to soils would lead to an annual soil C sequestration of 32.5 Tg C, which accounts for

    about 4% of Chinas current annual C emission. Soil C sequestration with improved soil management is highly time-dependent; the effect

    lasted for only 2080 years. Generally, practicing no-tillage causes higher rate and longer sustainability of soil C sequestration than only

    increasing crop residue into soils. The potential for soil C sequestration varied greatly among different regions due to the differences in

    climate, soil conditions and crop productivity.

    # 2006 Elsevier B.V. All rights reserved.

    Keywords: Carbon sequestration; Soil management; Process model; Remote sensingAbstractPotential and sustainability for c

    soil management in ag

    Huimin Yan *, Mingku

    Institute of Geographic Sciences and Natural Resource

    Anwai, Beiji

    Received 5 April 2006; received in revised f

    Available onlinon sequestration with improved

    ultural soils of China

    o, Jiyuan Liu, Bo Tao

    arch, Chinese Academy of Sciences, 11A Datun Road,

    101, China

    7 October 2006; accepted 7 November 2006

    ecember 2006

    www.elsevier.com/locate/agee

    nt 121 (2007) 325335

  • (Kern and Johnson, 1993; Freibauer et al., 2004; Lal, 2004;

    Paustian et al., 1995; Buyanovsky and Wagner, 1998).

    s andabout 124 Mha, accounting for about 13% of the worlds

    total. Chinas agricultural soils have relatively low C content

    level, because of intensive use, long cultivation history and

    the use of crop residue as fuels and feed for domestic

    animals, hence may have a great potential for C sequestra-

    tion through improved land management. It is estimated that

    about 90% of C uptake by agricultural systems would be

    emitted or returned to the atmosphere (Lin et al., 1997). C.S.

    Li et al. (2003) and K.R. Li et al. (2003) estimated that under

    conventional soil management Chinas cropland are losing

    1.6% of their soil organic carbon (SOC) while U.S.

    croplands are only losing 0.1%. However, although there

    are many studies describing agricultural SOC stocks (S.Q.

    Wang et al., 2005; X.B. Wang et al., 2005; Liu et al., 2006),

    SOC loss due to cultivation (Wu et al., 2003; Song et al.,

    2005), and the significance of improved soil management on

    increasing soil C sequestration (C.S. Li et al., 2003; K.R. Li

    et al., 2003; S.Q. Wang et al., 2005; X.B. Wang et al., 2005),

    few quantitative studies concerning agricultural soil C

    sequestration have been undertaken at the national level in

    China. Only Lin et al. (2002) and Lal (2002b) estimated soil

    C sequestration potential in China through proposed

    cropland management activities by IPCC, using the rates

    of C gain for various activities within the corresponding

    area. Therefore, there is a significant gap of spatially explicit

    quantification on C sequestration potential and its sustain-

    ability with improved soil management in the agricultural

    soils of China.

    Many studies have been conducted in other regions to

    assess the potential of agricultural soil sequestration at

    national or regional level, and have explored the options to

    enhance C sequestration (e.g. Smith et al., 2000a,b;

    Vleeshouwers and Verhagen, 2002; Marland et al., 2003;

    West and Marland, 2002, 2003; Dendoncker et al., 2004).

    Most of previous studies have used empirical approaches

    based on a comparison of measured organic C levels between

    arable and natural lands or on observations of the organic C

    change with improved management at limited sites (Lal and

    Bruce, 1999; Smith et al., 1997, 1998, 2000a,b; West and Post,

    2002). However, soil C sequestration is a complex process that

    is influenced by many factors, such as organic C inputs from

    crop residue or applied organic manure, climatic and soil

    conditions, and the original C levels, and thus has high spatio-

    temporal heterogeneity. A realistic estimate of the C

    sequestration potential at regional or national scales requires

    integrating the effects of various factors that affect C inputs to

    and loss from soils and accounting the inherent high spatial

    heterogeneity and temporal variability. Some studies have

    linked site level process-based model with GIS to extrapolate

    point measurements to regional scales (Falloon et al., 1998,

    2000; Zimmerman et al., 2005). A combination of process-

    based mechanistic modeling and satellite remote sensing is an

    effective approach to integrate the effects of various factors on

    soil C processes and to quantify the high spatial heterogeneity

    in the rates of the C sequestration. Remotely sensed surface

    H. Yan et al. / Agriculture, Ecosystem326properties combined with biogeochemical models have beenCurrently, no-tillage is practiced on only 5% of the worlds

    cropland (1379 Mha globally) (Lal, 2004). The amount of

    crop residue produced in the world is a large quantity, about

    3.5 Pg year1, but only 5060% of the residue producedmay be returned to the soil (Lal, 1999). In China, only about

    25% is returned to the fields (Ministry of Agriculture of

    China, 1998). Most crop residue was either used as fuel or

    feed for domestic animals in rural areas before 1980s, or

    burned on field after 1980s when farmers ceased taking crop

    straw as fuel due to improved living conditions. So, the

    objectives of this study were to: (1) use a coupled remote

    sensing- and process-based ecosystem model of CEVSA

    (Cao and Woodward, 1998; Cao et al., 2003) to make a

    spatially explicit quantification of the C sequestration

    potential of Chinas arable lands; (2) evaluate the effec-

    tiveness of different management options based on the

    modelled C uptake rate and its sustainability; (3) make a

    mechanistic analysis on the regional pattern of C

    sequestration potential. The soil management options

    considered in the present study are practicing no-tillage

    and increasing crop residue input into soils.

    2. Materials and methods

    Estimation of soil C sequestration requires quantification

    of the rates of C inputs and releases under improved soil

    management. The source of increment in SOC is the net C

    fixed by crops, usually measured as net primary productiv-

    ity (NPP), but only a part of the fixed C actually is

    incorporated into soils while the other parts are removed for

    harvest or for clearing the field. Soils lose C from

    heterotrophic respiration (HR), soil erosion by wind and

    water. In the present study, only HR is included in

    calculating soil C loss. The soil C sequestration was

    estimated using a coupled remote sensing- and process-

    based model (Fig. 1) that calculates the C inputs from crop

    growth and the decomposition of C under differentused to predict C fluxes into and out of ecosystems and to

    evaluate potential mechanisms of terrestrial CO2 sequestra-

    tion (Nemani et al., 2003; Cao et al., 2004).

    Soil C sequestration can be achieved by increasing the net

    flux of C from the atmosphere to the terrestrial biosphere by

    storing more of the C from net primary production in the

    longer-term C pools in the soil or by slowing decomposition.

    No-tillage can significantly reduce soil C release by

    reducing the turnover of soil aggregates and the exposing

    of young and labile organic matter to microbe decomposi-

    tion (Paustian et al., 2000). This method has been taken as an

    effective and environmentally friendly soil C sequestration

    strategy (Lal, 2004). No-tillage and preferential crop

    residues management (increasing C input) are two important

    and widely recognized measures to enhance C sequestration

    Environment 121 (2007) 325335management options.

  • s and2.1. Crop NPP and C inputs into soils

    The rate of C input into agricultural soils was calculated

    as the proportion of crop NPP that is returned into soils,

    including crop residues and manure application. The NPP in

    a given area at the present time was estimated using a remote

    sensing-based production efficiency model, GLO-PEM. The

    model consists of linked components that describe processes

    of canopy radiation absorption, utilization, autotrophic

    respiration, and the regulation of these processes by

    environmental factors such as temperature, water vapor

    pressure deficit, and soil moisture (Prince and Goward,

    1995; Cao et al., 2004). It calculates NPP as follows:

    H. Yan et al. / Agriculture, Ecosystem

    Fig. 1. A schematic representation of the model for estimating soil carbon

    sequestration. The meaning of acronyms are as follows: fraction of photo-

    synthetically active radiation (FPAR), light use efficiency (LUE), auto-

    trophic respiration (Ra), gross primary production (GPP), net primary

    production (NPP), soil organic matter (SOM).NPP X

    tStNteg Ra (1)

    where St is the incident PAR in time t, Nt the fraction of

    incident photosynthetically active radiation (PAR) absorbed

    by vegetation canopy (FPAR) calculated as a linear function

    of NDVI (Prince and Goward, 1995), eg the light useefficiency of the absorbed PAR by vegetation in terms of

    gross primary production, and Ra is the autotrophic respira-

    tion calculated as a function of standing aboveground bio-

    mass, air temperature, and photosynthetic rate.

    The NPP of crops consists of the organic C in the

    economic products (e.g. grain), roots and straw (stems,

    leaves, stalks, etc.), which were calculated using the C

    allocation parameters as given in Table 1.

    The C in the economic products of crops is usually

    removed from crop lands; a part of the C in straw and roots is

    removed away for other uses (e.g. fuel and livestock feed) or

    is burned for clearing the field, and only the remaining C

    enters into soil as the source of C. In addition, organic

    manure is also an important C source for agricultural soils,

    however, in China most of organic manure originated fromcrop products. At present, according to the national statistic

    data, altogether 25% residues are returned into soils with

    17% being directly incorporated into soils and 8% as organic

    manure input into soils in China (State Environmental

    Protection Administration of China, 2000). In the present

    study, the rate of C input into the soil was estimated as a part

    of NPP based on the C allocation among crop organs and the

    survey of actual data inputs, including from the applied

    organic manure. C input included all roots and returned

    straw, and the proportions of NPP to roots and straws were

    calculated by the mean value of different crops listed in

    Table 1 because the crop types are unavailable for the

    present.

    2.2. SOC decomposition

    The rate of C release from agricultural soils was

    calculated as the rate of heterotrophic respiration using a

    process-based ecosystem model, CEVSA (Cao and Wood-

    ward, 1998; Cao et al., 2003). In simulating organic carbon

    transformation, decomposition and accumulation, CEVSA

    divides soil organic matter into surface litter, root litter,

    microbes, and slow and passive C pools, each of which has a

    specific decay rate (Cao et al., 2003). All SOC transforma-

    tions and decomposition in these pools are treated as first-

    order rate reactions that are affected by temperature, soil

    moisture, nitrogen availability, soil texture, and the lignin/

    nitrogen ratio.

    The original CEVSA did not consider the effects of

    tillage on SOC dynamics. In natural lands, soil organic

    matter is physically protected in microaggregates to

    microbial decomposition. Tillage of arable lands enhances

    soil organic matter decomposition by disturbing the physical

    protection. It is estimated that the decay rate of SOM in

    cultivated soils is several times higher than that of natural

    Environment 121 (2007) 325335 327

    Table 1

    Carbon allocation to economic products, straw and roots (Li et al., 1994)

    Cropa Grain Straw Root

    Wheat 0.28 0.42 0.3

    Corn 0.3 0.44 0.26

    Cotton 0.19 0.48 0.33

    Soybeans 0.28 0.44 0.28

    a Scientific names: wheat (Triticum aestivum), corn (Zea mays), cotton

    (Gossypium hirsutum) and soybeans (Glycine max).lands (Balesdent et al., 2000; Six et al., 1998; Li et al., 1994).

    In the present study, The CEVSA model was modified to

    account the effect of tillage by using a parameter that

    measures the increases on the decay rates of different C

    pools relative to that in soils with no tillage (Leite et al.,

    2004) (Table 2).

    2.3. Data sources and model running

    We use the coupled models of GLO-PEM and CEVSA to

    calculate the average values of NPP, rates of SOC

  • study, we applied the methods of Lobell et al. (2002) at the

    county level in China to estimate the cropland NPP of all

    counties in 1996. The statistic-based NPP is computed by the

    following formula:

    NPP g C m2

    PN

    i1Yi1 MCi 0:45g C g1=HIi 0:9PNi1 Ai

    (2)

    where Y is the reported yield of crop i, MC is the typical

    harvest moisture content (mass water/harvested mass,

    g g1), HI is the harvest index (ratio of yield mass to

    s anddecomposition, and the rates of soil C sequestration for

    1990s. GLO-PEM is a top down model which estimates C

    assimilation based on light use efficiency (LUE) and

    ecophysiological processes, and its effect was highlighted

    by its avoidance of spatial extrapolation or landcover

    classification in NPP estimation over large scales (Goetz and

    Prince, 1999). The satellite data used in estimating NPP

    were obtained from the Pathfinder AVHRR Land (PAL) data

    at resolutions of 8 km and 10 days that were derived from

    channels 1, 2, 4 and 5 of AVHRR sensors aboard the NOAA-

    7, 9, 11, and 14 satellites (Cao et al., 2004). AVHRR/NDVI

    at 10 days temporal scale cannot only be used to calculate

    crop absorption of photosynthetically active radiation, but

    also provides regional applicable information about crop

    dynamics and cropping system by describing crop growth

    process. In fact, the multi-cropping system, including single-

    , double- and triple-cropping systems, have been subsumed

    in the calculation. However, with the coarse resolution of

    8 km, two or more crop types were comprised in one pixel,

    and so AVHRR is incapable to distinguish crop types.

    Spatial and temporal heterogeneity in vegetation LUE due to

    relative coarse spatial resolutions and complex cropping

    system would induce errors. Improvements in the approach

    to account for both spatial and temporal heterogeneity when

    implementing LUE model at coarse resolution and multiple

    cropping systems are important for future development of

    the model. Nonetheless, the AVHRR was taken as the most

    state-of-the-art, globally or regional applicable and free

    available satellite data in the past two decades, which

    provide a beneficial trade-off among the spatial resolution,

    repeat cycle and data volume for regional to global

    applications.

    The cropland distribution in China was derived from the

    National Land Cover Project (NLCD) database based on the

    Enhanced Thematic Mapper (ETM+) images acquired in

    1999 and 2000 for all of China (Liu et al., 2003, 2005). The

    information on soil properties was derived from the 1:

    4,000,000 soil map of China. The climate data (temperature,

    precipitation, relative humidity, sunshine duration, and wind

    speed) used were those interpolated for 0.18 grid based ondata from 730 meteorological stations for each 10 days from

    H. Yan et al. / Agriculture, Ecosystem328

    Table 2

    Parameters for the tillage effects on the decay rates (Leite et al., 2004)

    Carbon pool No tillage Conventional tillage

    Active 1 1.8

    Slow 1 4

    Passive 1 1.81991 to 2000.

    After obtaining crop NPP using the remote sensing data

    we ran the soil C model driven by the average values of NPP

    and climate data during the 1990s with seven soil

    management scenarios (including current status scenario,

    three realistic scenarios and three ideal scenarios) (Table 3).

    Then we calculated the differences in the rates of soil C

    change and in the soil C pool at the equilibrium levels underdifferent soil managements, so as to analyze the soil C

    potential and the effectiveness in enhancing soil C potential

    of the different options.

    2.4. NPP validation with statistic data

    Model validation over a large-scale is difficult because

    field measurements are not available to evaluate remotely

    sensed measures. Fortunately, county-level census data

    could provide agricultural ecosystems a comprehensive,

    independent measure of total plant production throughout

    the growing season, and thus C uptake for comparison to the

    satellite-derived estimates (Malmstrom et al., 1997). In this

    Environment 121 (2007) 325335

    Table 3

    Soil management scenarios for soil carbon sequestration estimates

    Measure Description

    Current soil management 25% crop residue return and

    conventional tillage

    50% CR Increasing crop residue return

    to 50% with conventional tillage

    50% NT Practicing no tillage in 50% of

    arable lands with current crop

    residue return

    50% CR + 50% NT Increasing crop residue return to

    50% and practicing no tillage in

    50% of arable lands

    100% CR Increasing crop residue return to

    100% with conventional tillage

    100% NT Practicing no tillage in 100% of

    arable lands with current crop

    residue return

    100% CR + 100% NT Increasing crop residue return to

    100% and practicing no tillage in

    100% of arable landsaboveground biomass) and A is the harvested area. In this

    study, 10 major crops are considered (N = 10) and it is

    assumed that 45% of crop biomass is C and 80% of NPP

    is allocated to aboveground parts. Potential variability

    within species for each of the factors used to convert yield

    to NPP was not considered in this study. Values for MC and

    HI were taken and adapted from Zhang and Zhu (1990),

    Fang et al. (1996) and Lobell et al. (2002) (Table 4).

  • counties for subsequent comparisons. According to the

    options separately. The upper limit of the C sequestration of

    Chinas agricultural soils is estimated to be 6 Pg C with all

    crop residue returns to soils and practicing no-tillage on all

    of the arable lands.

    3.2. Sustainability

    H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325335 329

    Table 4

    Harvest index (HI) and moisture content (MC) of major crops

    Cropa MC (%) HI

    Rice 14 0.43

    Wheat 12.5 0.37

    Corn 13 0.44

    Sorghum 14.5 0.39

    Millet 13 0.38

    Soybean 12 0.44

    Cotton 8.3 0.4

    Rapeseed 13 0.25correlation analysis, the estimates of crop NPP using this

    methodology are essentially consistent with NPP derived

    from county-level statistics (correlation coefficient

    R = 0.68) (Fig. 2). Although the state reported crop yield

    and the other agricultural statistics are likely biased due to

    underestimation of cropland area, the use of data at the

    county level provides a large number of samples for

    evaluating remote sensing estimates of NPP.

    3. Results

    3.1. The potential for C sequestration

    Under the improved land management options, the

    agricultural SOC pool of China could be increased by 2.3%To avoid errors in regions with sparse agriculture, we

    limited our analyses to those counties where cropland area

    accounted for more than 30% of the total county area

    according to statistical data. Furthermore, we limited our

    selection to those crops growing on more than one million ha

    according to 1996 data. This resulted in a total of 1171

    Potato 80 0.5

    Sugar beets 85 0.4

    a Scientific names: rice (Oriza sativa), sorghum (Sorghum bicolor),

    millet (Panicum miliaceum), rapeseed (Brassica napus), potato (Solanum

    tuberosum) and sugar beets (Beta vulgaris).up to 60.9%. The average rate of C sequestration (during the

    period from starting practicing improved management

    measures to reaching an equilibrium soil C status) ranges

    from 21.5 to 71.7 Tg C year1 (Table 5). The results showthat no-tillage is very effective in enhancing soil C stocks

    with high C sequestration potential. Increasing crop residue

    Table 5

    Carbon sequestration potential under no-tillage (NT) and crop residues (CR)

    Land management

    scenario

    Carbon sequestration

    potential (Tg C)

    Yearly average carbon sequestr

    potential (Tg C year1)

    50% CR 231.8 23.2

    100% CR 1146.2 57.1

    50% NT 1247.3 21.5

    100% NT 2497.2 43.0

    50% NT + 50% CR 2372.0 32.5

    100% NT + 100% CR 6024.0 71.7return can only produce a moderate C sequestration potential

    and rates. Increasing crop residue return to 50% could

    sequester 232 Tg C of soil C, only equivalent to 18% of the

    sequestrated C with practicing no-tillage on 50% of the

    arable lands. A combination of practicing no-tillage and

    increasing crop residue return is the most effective measure

    for sequestering C. 50% CR + 50% NT (increasing crop

    residue return to soils from 25% to 50% and practicing no-

    tillage in one-half of the arable lands, leaving another half

    under conventional tillage) can sequester 2372 Tg C, which

    almost double the sum of practicing the two management

    Fig. 2. Comparison of NPP derived from GLO-PEM model and from

    statistics. The production and sown area data were taken from national

    natural resource database (http://www.naturalresources.csdb.cn).The rates of agricultural soil C sequestration change

    significantly over time (Fig. 3A). The C sequestration rates

    are normally high in the early stage of practicing no-tillage

    and increasing crop residue return, and then decrease with

    consequent increases of soil C pool and finally became zero

    ation Duration

    (year)

    Saving of agricultural fossil

    fuel-carbon (Tg C year1)Total carbon mitigation

    potential (Tg C year1)

    10 23.2

    20 57.1

    58 1.9 23.4

    58 3.8 46.8

    73 1.9 34.4

    84 3.8 75.5

  • H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325335330

    Fig. 3. Annual soil C stock (A) and C sequestration rates (B) of converting

    traditional cropland management to no-tillage (NT), crop residue return

    (CR) and combination of NT and CR (NT + CR), and annual fractional

    increases of soil carbon sequestration caused by CR, NT and the interaction

    term of combining NT and CR (C).

    Fig. 4. Maps of carbon sequestration potential for (A) no-tillage (NT)

    practiced on all cropland scenario, (B) 100% crop residues returned into soil

    scenario (CR), and (C) combination of 100% NT and 100% CR (NT + CR)

    scenario (the other four scenarios have similar spatial patterns and different

    quantity under same measurements implemented). The number is the codes

    for nine agricultural regions of China.

    Table 6

    Temperature (T), precipitation (P), net primary production (NPP) and cropland area (A) of nine agricultural regions in China

    Names of agricultural regions Codea T (8C) P (mm) NPP (g C m2 year1) A (million ha)

    Northeast Region 1 2.2 573.3 425 31.1

    Innermongolia and the Great Wall Region 2 4.1 379.2 258 14.8

    Huang-Huai-Hai Region 3 13.5 676.4 565 30.4

    Loess Plateau Region 4 9.1 479.9 323 16.0

    Middle and lower reach of Yangtze River Region 5 16.7 1541.3 761 34.9

    Southwest Region 6 14.5 1097.1 581 30.0

    South China Region 7 20.1 1579.9 912 11.5

    Ganxin Region 8 5.7 150.6 110 8.8

    Tibet Region 9 0.9 430.0 127 1.5a Code number of the nine agricultural regions in China.

  • as soil C reaches a new equilibrium level. However, these

    changes differ among different management options. The

    rate of C sequestration by increasing crop residue return is

    higher than by practicing no-tillage in the first 5 years, but

    decreases more sharply and becomes lower (Fig. 3B), while

    the rate of C sequestration by combining no-tillage and

    increasing crop residue return is higher than the sum of the

    two managements practiced separately at any stage.

    Thereafter, the C sequestration capacity of practicing no-

    tillage lasted longer than increasing crop residue return, and

    a combination of practicing no-tillage and increasing crop

    residue return lasted longer than any other two measure-

    ments practiced separately (Fig. 3A and C). For example, it

    would take about 60 years for soil C to reach a new

    equilibrium level of practicing no-tillage, but would just take

    20 years with increasing crop residue return from 25% to

    100%. With a combination of practicing no-tillage and

    increasing crop residue return from 25% to 100%, soil C

    would take about 85 years to reach a new equilibrium level.

    3.3. Spatial pattern

    Fig. 4 shows the spatial pattern of C sequestration

    potential of arable lands in China under no-tillage and crop

    residues management scenarios. Although the value of soil C

    sequestration rates under different measures are remarkable

    different, its spatial variability is similar. The C sequestra-

    tion potential is higher in Southeast and lower in Northwest

    China. Such regional variation is generally related to the

    pattern of crop productivity and hence the amount of crop

    residue returns to the soils (Fig. 5). Among the nine

    agricultural regions in China (Table 6 and Fig. 3), NPP

    varied from 110 g C m2 year1 in Ganxin Region to912 g C m2 year1 in the South China Region. Due to

    H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325335 331

    Fig. 5. Map of carbon input with 100% crop residues return.Fig. 6. Carbon sequestration potential under 100% crop residue returned

    scenario (A) and no-tillage implemented on all cropland scenario (B) in nine

    agricultural regions of China that are lined up according the average C input

    in each region, (the former six regions (8, 9, 2, 4, 1, 3) are located in the

    north of China, and the latter three (6, 5, 7) are in the South of China). The

    dotted line are trendline of Carbon Sequestration Potential with increasing C

    input in the two parts of China, indicating there are regional differentiation

    on C sequestration capacity between North and South China.Fig. 7. Carbon sequestration efficiency (ratio of C input to C emission)

    under 100% crop residue returned scenario (A) and no-tillage implemented

    on all cropland scenario (B) in nine agricultural regions of China. The nine

    regions are lined up according the average C input in each region same asFig. 6.

  • the difference in crop productivity, soil C input would

    change from 55 to 456 g C m2 year1 when a half of cropresidue would be returned to soil. Because soil C

    sequestration rates are linearly correlated with C inputs

    (Fig. 6), high crop productivity and hence more crop residue

    return can cause more soil C sequestration.

    The regional pattern of soil C sequestration is

    controlled by climate conditions that affect SOC decom-

    position. The warm and moist climate in southern China

    can cause higher decomposition rates. Provided that the

    same amount of crop residue is input into the soil, there

    would be more C sequestered into the soil in northern

    China (upper dot-line in Fig. 6) than in southern China

    (lower dot-line in Fig. 6). Therefore, although having

    equivalent C input levels, the Huang-Huai-Hai Plain

    Region sequesters more SOC than the warmer and moister

    southwest Region (Fig. 7). For the regions where both

    temperature and moisture control organic matter decom-

    position (such as Tibet) had the highest C sequestration

    efficiency (ratio of C uptake to C input) (Figs. 7 and 8). In

    the arid and semi-arid regions (2, 4, 8) of Northwest

    China, the C sequestration efficiency is higher than the

    humid region (1, 3, 6, 5, 7) located in East and South of

    China (Fig. 7).

    H. Yan et al. / Agriculture, Ecosystems and332Fig. 8. Maps of C sequestration efficiency of soil (ratio of C input to Cemission) (A: CR; B: NT).4. Discussion and conclusions

    Chinas arable lands have a large potential for C

    sequestration. Theoretically, the upper-limit of the agricul-

    tural soil C sequestration potential with all crop residue

    return and practicing no-tillage in all cropland would be

    6.0 Pg C, close to the cumulative historic SOC loss of about

    7.1 Pg C from the cultivated soils in China (Wu et al., 2003).

    The realistic soil management option, 50% crop residue

    return and practicing no-tillage in 50% of cropland, having a

    C sink capacity of 2.4 Pg C (equivalent to 35% of the

    historic SOC loss), would lead to annual soil C uptake of

    32.5 Tg, 60% more than the current C uptake by all forests

    (Fang et al., 2001). Practicing no-tillage can also reduce C

    release with a decrease of energy use at the rate of

    23.8 kg C ha1 year1(Smith et al., 1998). The annual fossilfuel saving C in China would be 1.9 and 3.8 Tg C year1 for50% no-tillage and 100% no-tillage, respectively. Conse-

    quently, the total C mitigation potential is 75.5 Tg C year1

    in the case that all crop residues is returned into soils

    together with practicing no-tillage in all of the arable lands

    (Table 5). The enhanced C uptake and reduced C release

    from practicing 50% no-tillage and 50% crop residue return

    is estimated to be 34.4 Tg C year1, equivalent to 4.5% ofthe Chinas total C emission in 2000 (Marland et al., 2005).

    The no-tillage together with increasing crop residue

    return (NT + CR) strategy is the most effective measure for

    C sequestration through the combination of increasing SOC

    input and decreasing SOC decomposition. The relationship

    between effects of no-tillage and crop residue return is not

    linear and non-additive but interacts with each other and

    therefore intensifies soil C sequestration capacity. This

    results in not only more C sequestration than the sum of

    practicing the two management options separately, but also a

    longer duration of C sequestration (Table 3). Accordingly, C

    sequestration through the combination of practicing no-

    tillage and increasing crop residue return could be attributed

    to three terms: crop residue return (black area in Fig. 3C),

    no-tillage (dark grey area in Fig. 3C) and the interaction

    between crop residue return and no-tillage (light grey area in

    Fig. 3C). At the preliminary stage of practicing no-tillage

    together with increasing crop residue return, the contribu-

    tions of crop residue or no-tillage predominate over the

    interaction term; but with the attaining of new equilibrium

    state by practicing two managements separately, the

    interaction term begins to play a significant role, yielding

    almost one third of sequestered C could benefit when a new

    balance is attained (Fig. 3C). Relative to increasing crop

    residue return, no-tillage soils have higher potential for C

    sequestration by virtue of the resulted long sustainability of

    C uptake. Although no-tillage has a high C sequestration

    capacity, the average C sequestration rate is limited by low C

    input, especially in the northwest of China where crop NPP

    are generally low (Fig. 4A).

    Soil C accumulation is far from linear (Smith et al., 1997;

    Environment 121 (2007) 325335Freibauer et al., 2004), and this study shows that the rates of

  • s andC sequestration are highly time-dependent, they decrease

    with time and finally approach zero as soil organic C pool

    reaches a new equilibrium level. Our results also show that

    the rates of C uptake resulting from practicing no-tillage

    are smaller than the rates from increasing crop residue

    return in the early stage, but become higher after several

    years and last for longer. Some studies that used constant

    rates and duration of C uptake to estimate the C

    sequestration of agricultural soils can only give a reason-

    able result within restricted areas and periods, because the

    C sequestration rate and its sustainability are changed with

    the crop NPP, climate and soil conditions (Smith et al.,

    1998; Dendoncker et al., 2004). Our estimated C

    sequestration rates decrease with time, which are in

    agreement with the studies of Marland et al. (2003) and

    West and Post (2002), but the C sequestration rate does not

    coincide with experiments-based result at the initial years.

    The possible reasons here are that our estimates had

    assumed that NT was practiced at a presumed equilibrium

    state with lower SOC level after centuries of cultivation.

    Moreover, the assimilated C and climate conditions remain

    constant in this study. In reality, when the management is

    changed from CT (conventional tillage) to NT, there is a

    possibility that agricultural yields will also change, and the

    effect of decreased or increased yield is most apparent in

    the early years (Marland et al., 2003). In this study we

    examine soil C uptake capability under present conditions

    and do not consider any C loss or sequestration from crop

    yield by land use or climate changes. Thomson et al.

    (2006) showed that NT increased C sequestration rates by

    2080 g C m2 year1 on the Huang-Hai Plain of Chinausing a different model. This is comparable to our

    estimates in this region (with an average C sequestration

    rate of 25 g C m2 year1 under NT and46 g C m2 year1 under NT together with 100% cropresidues returned during the first 20 years). Our results are

    also consistent with the estimates of West and Post (2002),

    who used a global database of long-term agricultural

    experiments. They showed that change from CT to NT can

    sequester 57 14 g C m2 year1.The variations in crop productivity, residue return, and

    climate and soil conditions affect the regional pattern of soil

    C sequestration (Paustian et al., 1995, 1998; Freibauer et al.,

    2004). The crop NPP in China is generally higher in the

    southeast and lower in the northwest, in agreement with our

    estimated rates of agricultural C sequestration. Among the

    nine agricultural regions in China, the annual average

    temperature varies from 0.9 8C to the highest of 20.1 8C,and the annual precipitation from 150 mm to the maximum

    of 1580 mm (Table 6). The difference in climate conditions

    can explain the regional heterogeneity in our estimated C

    sequestration rates for China (Huang et al., 2002). Generally,

    warm and moist climate causes higher SOC decomposition

    rate, while in the regions where organic matter decomposi-

    tion is limited by low temperature, more C can be

    H. Yan et al. / Agriculture, Ecosystemsequestered into the soil.Regional quantification of soil C mitigation potential and

    sustainability is helpful for developing climate policies and

    land management strategies. Soil C sink is not infinite but

    will get to a saturation status after decades with the

    implementation of land management (Watson et al., 2000).

    The time taken for sink saturation to occur is highly variable

    dependent on soil type, temperature and soil moisture. In this

    study, we used a coupled remote sensing- and process-based

    ecosystem model to estimate the potential and sustainability

    for C sequestration of agricultural soils of China. This

    coupled model integrates the effects of various factors

    (climate, management, soil properties, etc.) on soil C

    processes, with which C sequestration potential in China is

    estimated with spatially explicit data on 0.18 0.18 grids,and the temporal profiles of C sequestration from measures

    practiced to a new C equilibrium status reached are

    described. The results obtained in this study are the greatest

    potential to accumulate C from the beginning of the

    implementation of measures until the time at which soil C

    reaches a new equilibrium through taking account of the C

    input, land management, climate, soil type, and many other

    factors. The model input data are mainly based on remote

    sensing measurements, which provide the most suitable data

    for large scale research, and are the best available data of

    cropland extent, climate and soil in China. The estimates of

    crop NPP using this methodology agree well with county-

    level crop statistics data. In reality, nevertheless, converting

    100% of arable agriculture to no-till farming or incorporat-

    ing 100% crop residues into soils is not achievable. Our aims

    of taking into account the three scenarios (including 100%

    NT, 100% CR, 100% NT + 100% CR) were to establish the

    potential for C sequestration that could be obtained under

    optimum conditions and to recognize the effects of C

    sequestration with time and space under different measures.

    Although the three realistic scenarios (including 50% NT,

    50% CR and 50% NT + 50% CR) may be possible in terms

    of soil and climate suitability, the application of no-tillage or

    crop residue return can be constrained by the economic

    conditions and the availability of technical equipment. Lal

    (2004) showed that potential soil C sink capacity of

    managed ecosystems equals approximately the cumulative

    historic C loss, whereas the attainable soil C sink capacity is

    only 5066% of the potential capacity. The physical,

    technical and economic feasibility of no-tillage agricultural

    and crop residues management are not taken into account in

    this study, so the C sequestration potentials may be

    overestimated. Incorporating the above-mentioned limita-

    tions into our modeling will be an important step forward to

    improve regional C sequestration potential estimates and

    regional variability assessment. Besides, incorporating the

    role of erosion into the model and using finer resolution

    satellite data with the ability for determination of the spatial

    distribution of crop types also are significant to narrow the

    uncertainties of model results. Nevertheless, the results

    presented here provided us with the basic information that

    Environment 121 (2007) 325335 333there is a large C uptake potential in agricultural soils of

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    Potential and sustainability for carbon sequestration with improved soil management in agricultural soils of ChinaIntroductionMaterials and methodsCrop NPP and C inputs into soilsSOC decompositionData sources and model runningNPP validation with statistic data

    ResultsThe potential for C sequestrationSustainabilitySpatial pattern

    Discussion and conclusionsAcknowledgementsReferences

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