Potential and sustainability for carbon sequestration with improved

soil management in agricultural soils of China

Huimin Yan *, Mingkui Cao, Jiyuan Liu, Bo Tao

Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road,

Anwai, Beijing 100101, China

Received 5 April 2006; received in revised form 27 October 2006; accepted 7 November 2006

Available online 12 December 2006

www.elsevier.com/locate/agee

Agriculture, Ecosystems and Environment 121 (2007) 325–335

Abstract

Arable 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. China’s arable lands, accounting for 13% of

the world’s 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 China’s current annual C emission. Soil C sequestration with improved soil management is highly time-dependent; the effect

lasted for only 20–80 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 sensing

1. 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

* 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.2006.11.008

important measure to sequester CO2 from the atmosphere to

help stabilize atmospheric CO2 concentrations and has been

estimated that 0.4–0.9 Pg C year�1 can be sequestered within

global 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 year�1, second only to the United States (Marland

et 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

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335326

about 124 Mha, accounting for about 13% of the world’s

total. China’s 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 China’s 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

properties combined with biogeochemical models have been

used 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

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

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

Currently, no-tillage is practiced on only 5% of the world’s

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

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

3.5 Pg year�1, but only 50–60% of the residue produced

may 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 China’s 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 different

management options.

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335 327

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).

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).

2.1. 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:

NPP ¼X

t½ðStNtÞeg � 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 use

efficiency 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 from

crop 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

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

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335328

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 1.8

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 lands

decomposition, 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 on

data from 730 meteorological stations for each 10 days from

1991 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 under

different 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

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 ðg C m�2Þ

¼PN

i¼1ðYið1�MCiÞ � 0:45ðg C g�1ÞÞ=ðHIi � 0:9ÞPN

i¼1 Ai

(2)

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

harvest moisture content (mass water/harvested mass,

g g�1), HI is the harvest index (ratio of yield mass to

aboveground 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).

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335 329

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 0.25

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).

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).

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

counties for subsequent comparisons. According to the

correlation 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%

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 year�1 (Table 5). The results show

that 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 year�1)

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 71.7

return 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

options separately. The upper limit of the C sequestration of

China’s 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

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 year�1)

Total carbon mitigation

potential (Tg C year�1)

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) 325–335330

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 m�2 year�1) 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 1.5§

a Code number of the nine agricultural regions in China.

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335 331

Fig. 5. Map of carbon input with 100% crop residues return.

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

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.

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 m�2 year�1 in Ganxin Region to

912 g C m�2 year�1 in the South China Region. Due to

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 as

Fig. 6.

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335332

the difference in crop productivity, soil C input would

change from 55 to 456 g C m�2 year�1 when a half of crop

residue 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).

Fig. 8. Maps of C sequestration efficiency of soil (ratio of C input to C

emission) (A: CR; B: NT).

4. Discussion and conclusions

China’s 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 ha�1 year�1(Smith et al., 1998). The annual fossil

fuel saving C in China would be 1.9 and 3.8 Tg C year�1 for

50% no-tillage and 100% no-tillage, respectively. Conse-

quently, the total C mitigation potential is 75.5 Tg C year�1

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 year�1, equivalent to 4.5% of

the China’s 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;

Freibauer et al., 2004), and this study shows that the rates of

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335 333

C 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

20–80 g C m�2 year�1 on the Huang-Hai Plain of China

using a different model. This is comparable to our

estimates in this region (with an average C sequestration

rate of 25 g C m�2 year�1 under NT and

46 g C m�2 year�1 under NT together with 100% crop

residues 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 m�2 year�1.

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

sequestered 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 50–66% 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

there is a large C uptake potential in agricultural soils of

H. Yan et al. / Agriculture, Ecosystems and Environment 121 (2007) 325–335334

China, and the mitigation potential is noteworthy in C

mitigation options. These results are also useful to make

climate policy and land management strategies by virtue of

the spatial variability and temporal sustainability of C

uptake potential under different soil management measures.

However, we should note that soils have a finite capacity to

store C, and gains in soil C can be reversed if proper

management is not maintained; on the other hand, many C

sequestration measures have not only positive effects but

also potential negative effects to the environment and farm

profitability. Typically, practicing NT would increase

pesticide usage, promote N2O emission, increase the need

for high initial equipment costs and raise the risk of fungal

attack in wetter areas (Freibauer et al., 2004). Therefore,

managements and policies to sequester C in soils need to

have a discreet and comprehensive consideration.

Acknowledgements

This study was supported by Chinese Academy of

Sciences International Partnership Project ‘‘Human Activ-

ities and Ecosystem Changes’’(CXTD-Z2005-1), National

Key Basic Research Project (G2002CB412507), Chinese

Academy of Sciences ‘‘Hundred Talents’’ Project (Mingkui

Cao) and the Project of National Science Foundation of

China (30590384). We thank Dr. Changhui Peng, Dr. Heqing

Huang and two anonymous reviewers for their very helpful

comments during the finalization of this manuscript.

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