Advances in agronomy volume 76

Agronomy

DVANCES I N

VOLUME

76


Advisory Board
Martin Alexander

Ronald Phillips

Cornell University

University of Minnesota

Kenneth J. Frey

Kate M. Scow

Iowa State University

University of California, Davis

Larry P. Wilding
Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee
Lisa K. Al-Almoodi
David D. Baltensperger
Warren A. Dick
Jerry L. Hatfield
John L. Kovar

Diane E. Stott, Chairman
David M. Kral
Jennifer W. MacAdam
Matthew J. Morra
Gary A. Pederson
John E. Rechcigl

Diane H. Rickerl
Wayne F. Robarge
Richard Shibles
Jeffrey Volenec
Richard E. Zartman


Agronomy

DVANCES IN

VOLUME

76

Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware
Newark, Delaware

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Contents
CONTRIBUTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii
ix

THE POTENTIAL OF SOILS OF THE TROPICS TO SEQUESTER
CARBON AND MITIGATE THE GREENHOUSE EFFECT
R. Lal
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Soil-Related Constraints to Biomass Production. . . . . . . . . . . . . . . . . . . . . . . .
III. Soil Degradation and Emission of Greenhouse Gases
to the Atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Soil Carbon Pool and Dynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Historic Loss of SOC Pool from Soils of the Tropics . . . . . . . . . . . . . . . . . .
VI. Need for Soil Restoration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII. Strategies of Mitigating the Greenhouse Effect through Soil
Carbon Sequestration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VIII. Potential of SOC Sequestration in the Tropics. . . . . . . . . . . . . . . . . . . . . . . . . .
IX. Dynamics of Soil Inorganic Carbon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2
3
7
10
13
14
15
17
23
24
25

APPLICATIONS OF CROP/SOIL SIMULATION MODELS
IN TROPICAL AGRICULTURAL SYSTEMS
Robin Matthews, William Stephens, Tim Hess,
Tabitha Middleton, and Anil Graves
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Applications of Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. The Way Forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32
33
95
108

INTERORGANISMAL SIGNALING IN SUBOPTIMUM
ENVIRONMENTS: THE LEGUME–RHIZOBIA SYMBIOSIS
F. Zhang and D. L. Smith

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Symbiotic Nitrogen Fixation and Soil Fertility. . . . . . . . . . . . . . . . . . . . . . . . . .
v

126
127


vi
III.
IV.
V.
VI.

CONTENTS
Principles of Legume Nodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Legume Nodulation under Stressful Conditions. . . . . . . . . . . . . . . . . . . . . . . .
Legume Nodulation with Preactivated Rhizobium . . . . . . . . . . . . . . . . . . . . . .
Commercial Products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

128
138
146
150
153

SURFACE CHEMISTRY AND FUNCTION OF MICROBIAL BIOFILMS
M. A. Chappell and V. P. Evangelou
I. Introduction: Definition and Importance of Microbial Biofilms. . . . . . .

II. The Microbial Biofilm as an Interfacial Boundary Regulating
Solution Equilibrium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. Features and Properties of the Biofilm Surface. . . . . . . . . . . . . . . . . . . . . . . . . .
IV Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

164
169
177
193
194

CROP SCHEDULING AND PREDICTION—PRINCIPLES AND
OPPORTUNITIES WITH FIELD VEGETABLES
D. C. E. Wurr, J. R. Fellows, and K. Phelps
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Identification of Distinct Stages and Phases of Growth
and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. Prediction of Duration of Developmental Phases for Given
Temperature Regimes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Additional Effects of Other Abiotic Factors on the Duration
of Developmental Phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Experimental Approaches to the Construction of Scheduling
and Prediction Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. The Accuracy of Measurement of Abiotic Factors . . . . . . . . . . . . . . . . . . . . . .
VII. Methods of Planning Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VIII. Future Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IX. Concluding Comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

202
205
206
213
216
219
222
228
231
231
235


Contributors
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

M. A. CHAPPELL (163), Department of Agronomy, Iowa State University, Ames,
Iowa 50011
V. P. EVANGELOU (163), Department of Agronomy, Iowa State University,
Ames, Iowa 50011
J. R. FELLOWS (201), Horticulture Research International, Wellesbourne,
Warwick CV35 9EF, United Kingdom
ANIL GRAVES (31), Institute of Water and Environment, Cranfield University,
Silsoe, Bedfordshire MK45 4DT, United Kingdom
TIM HESS (31), Institute of Water and Environment, Cranfield University, Silsoe,
Bedfordshire MK45 4DT, United Kingdom
R. LAL (1), School of Natural Resources, The Ohio State University, Columbus,
Ohio 43210

ROBIN MATTHEWS (31), Institute of Water and Environment, Cranfield
University, Silsoe, Bedfordshire MK45 4DT, United Kingdom
TABITHA MIDDLETON (31), Institute of Water and Environment, Cranfield
University, Silsoe, Bedfordshire MK45 4DT, United Kingdom
K. PHELPS (201), Horticulture Research International, Wellesbourne, Warwick
CV35 9EF, United Kingdom
D. L. SMITH (125), Department of Plant Science, McGill University–Macdonald
Campus, Saint Anne de Bellevue, Quebec H9X 3V9, Canada
WILLIAM STEPHENS (31), Institute of Water and Environment, Cranfield
University, Silsoe, Bedfordshire MK45 4DT, United Kingdom
D. C. E. WURR (201), Horticulture Research International, Wellesbourne,
Warwick CV35 9EF, United Kingdom
F. ZHANG (125), Bios Agriculture, Inc., Saint Anne de Bellevue, Quebec H9X
3V9, Canada

vii


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Preface
Volume 76 contains five excellent reviews on topics of great interest to crop and
soil scientists as well as to others in various fields. Chapter 1 is concerned with the
potential of tropical soils to sequester carbon. Topics that are covered include: soil
inorganic and organic pools and dynamics, loss of soil organic pools from tropical
soils, and potential for C sequestration in tropical soils. Chapter 2 covers the applications of crop/soil simulation models in tropical agricultural systems. Chapter 3
deals with interorganismal signaling in suboptimum environments with emphasis on legume–rhizobia symbiosis. Chapter 4 discusses the surface chemistry and
function of microbial biofilms. The authors discuss biofilm formation and matrix

architecture and general features and properties. Chapter 5 deals with vegetable
crop scheduling and prediction. Topics that are covered include: identification of
stages of growth and development and experimental approaches for developing
scheduling and prediction models.
I appreciate the authors’ timely and thoughtful reviews.
DONALD L. SPARKS

ix


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THE POTENTIAL OF SOILS OF THE
TROPICS TO SEQUESTER CARBON AND
MITIGATE THE GREENHOUSE EFFECT
R. Lal
School of Natural Resources
The Ohio State University
Columbus, Ohio 43210

I.
II.
III.
IV.
V.
VI.
VII.

Introduction
Soil-Related Constraints to Biomass Production
Soil Degradation and Emission of Greenhouse Gases to the Atmosphere
Soil Carbon Pool and Dynamics
Historic Loss of SOC Pool from Soils of the Tropics
Need for Soil Restoration
Strategies of Mitigating the Greenhouse Effect through Soil Carbon
Sequestration
VIII. Potential of SOC Sequestration in the Tropics
A. Restoration of Degraded Soils and Ecosystems
B. Agricultural Intensification through Adoption of Recommended
Agricultural Practices
IX. Dynamics of Soil Inorganic Carbon
X. Conclusions
References

The tropics cover 8.2 billion hectares or approximately 40% of the world’s land
area. These regions are characterized by a large portion of the world’s rapidly increasing population, high risks of soil and environmental degradation because of
harsh climate and resource-poor farmers, and rapid decomposition of soil organic
matter because of continuously high temperatures. Predominant soils of the tropics include Oxisols, Aridisols, Ultisols, and Alfisols. Soil and ecosystem degradation
lead to emissions of greenhouse gases (e.g., carbon dioxide, methane, and nitrous
oxide) into the atmosphere. Anthropogenic activities that exacerbate gaseous emissions include deforestation and biomass burning, low- or no-input subsistence agriculture, plowing, drainage of wetlands, and elimination or shortening of restorative
fallows. Soils of the tropics contain about 496 Pg of soil organic carbon (SOC) or
32% of the global pool. The historic loss of the SOC pool, due to land-use change
and cultivation, may be 17–39 Pg compared with the global loss of 66–90 Pg. If 60–
80% of the SOC lost can be resequestered through land-use change and adoption
of recommended management practices, the potential of SOC sequestration in the
1
Copyright

C

Advances in Agronomy, Volume 76
2002 by Academic Press. All rights of reproduction in any form reserved.
0065-2113/02 $35.00


2

R. LAL
tropics is 12–27 Pg over a 25- to 50-year period. Important strategies of SOC sequestration include reduction in emission of greenhouse gases and sequestration
of carbon (C) in biomass and soils. The potential of C sequestration in soils and
biomass of the tropics is estimated at 10.0–25.0 Pg by effective erosion control, 5.7–
10.8 Pg through restoration of degraded soils and ecosystems, 58–115 Pg through
biofuel offset, 2.2–4.1 Pg through adoption of recommended practices on croplands, and 6.0–12.0 Pg through adoption of recommended practices on grazing
lands. Of this, the potential of SOC sequestration is only 13.9–26.9 Pg over the
50-year period. Realization of this vast potential is a challenge for researchers, land
C 2002 Academic Press.
managers, and policymakers.

I. INTRODUCTION
The tropics, regions between the Tropic of Cancer and the Tropic of Capricorn,
cover approximately 40% of the world’s land area. Subtropical regions are located
between 23◦ and 30◦ N and S of the Equator. The mean monthly temperature of all
months, corrected to sea level, is above 18◦ C in the tropics and below 18◦ C for one
or more months in the subtropics. On the basis of the length of the growing period
(LGP, period in days during the year when rainfed available soil moisture supply is
greater than half the potential evapotranspiration) and the daily mean temperature
during the growing period (DMT), tropics are divided into six different ecoregions
as follows (CGIAR, 1990):

1. Warm humid tropics These regions comprise LGP of 275–365 days and
DMT > 20◦ C. These regions stretch some five to ten degrees on either side
of the Equator. Constant heat throughout the year results in continuous evaporation, upward movement of air currents, and local rather than regional winds.
Air temperature ranges between 21 and 33◦ C with a mean of about 27◦ C.
These regions support the tropical rainforest (TRF) vegetation, and mean
annual rainfall is about 2000 mm.
2. Warm seasonally dry tropics These regions comprise (1) subhumid tropics
(LGP = 180–270 days), (2) semiarid tropics (LGP = 75–180 days), and (3)
arid tropics (LGP = 0–75 days) all with DMT of >20◦ C.
3. Cool tropics These regions comprise humid, subhumid, semiarid, and arid
regions with a DMT during the growing period in the range of 5–20◦ C. The
moderately cool tropics have a DMT during the growing period in the range of
15–20◦ C.
4. Warm/cool humid subtropics These regions are characterized by humid moisture zones in the subtropics and comprise (1) warm humid subtropics with
DMT > 20◦ C, and (2) cool humid subtropics with DMT < 20◦ C (15–20◦ C).


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

3

5. Cool subtropics with summer rainfall These regions comprise humid, subhumid, semiarid, and arid moisture zones. The DMT during the growing period
is in the range of 5–20◦ C. These regions include parts of China, Mongolia, and
Korea.
6. Cool subtropics with winter rainfall These regions comprise humid, subhumid, semiarid, and arid moisture zones. The DMT during the growing period is
in the range of 5–20◦ C. These regions include parts of Turkey, Argentina, and
Chile.
Total land area in different ecoregions includes 1925 million hectares (Mha) in
the humid regions, 2481 Mha in seasonally dry regions, 2875 Mha in arid regions,
and 946 Mha in the montanous regions. Combined tropics and subtropics cover

an area of 8.2 billion ha (Lal, 2000b).
Demographically, these are the ecoregions that support a large portion of the
world’s population, where the population is increasing rapidly, and where most of
the world’s poor, undernourished, and deprived inhabitants live. These are also the
regions where risks of soil and environmental degradation are high. Biophysical
processes of soil and environmental degradation are driven by socioeconomic,
political, and cultural factors leading to severe soil degradation, eutrophication, and
contamination of natural waters, and emission of radiatively active or greenhouse
gases (GHGs) from the soil into the atmosphere.
The objective of this chapter is to describe: (1) soil resources of the tropics,
(2) soil-related constraints to biomass production, (3) soil organic carbon (SOC)
and soil inorganic carbon (SIC) pools and dynamics, (4) factors and processes
affecting soil degradation with particular reference to flux of C between soil and
the atmosphere, and (5) strategies to sequester C in soil and terrestrial ecosystems.
This chapter addresses soil C pool and dynamics in warm humid tropics and
warm seasonally dry tropical ecoregions and excludes most of China, Mongolia,
Korea, Japan, Argentina, Chile, South Africa, Central and West Asia, and the
Mediterranean regions.

II. SOIL-RELATED CONSTRAINTS
TO BIOMASS PRODUCTION
Soils of the tropics, those that occur in the geographic tropics, are characterized
by “iso” soil temperature regime in which the difference between mean summer and
mean winter soil temperature is 5◦ C or less. Predominant soils of the tropics include
Oxisols (1151 million hectares, Mha), Aridisols (912 Mha), Ultisols (902 Mha),
and Alfisols (641 Mha) (Table I) (Lal, 1990). In the humid tropics, highly weathered
Oxisols, Ultisols, and Alfisols cover about 71% of the area. Moderately weathered


4

R. LAL
Table I
Land Area under Principal Soils of the Tropicsa
Order

Total area (Mha)

Alfisols
Andisols
Aridisols
Entisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Miscellaneous

641
168
912
326
29
457
23
1151
4

902
219
136

Total

4968

a Data from Buringh (1979); Van Wambeke (1990); Eswaran
et al. (1992, 1993b).

Inceptisols, Alfisols, and Mollisols cover about 9% of the tropics. In addition,
hydromorphic soils of alluvial regions cover about 10%, and other miscellaneous
soils occupy about 10% of the land surface (Moormann and Van Wambeke, 1978).
Highly weathered soils have low cation exchange capacity (CEC), low available
water capacity, and low plant nutrient reserves. Aridisols, Alfisols, Vertisols, and
Entisols comprise soils of the semiarid tropics. These soils have ustic moisture
regime (El-Swaify et al., 1984). Similarly, predominant soils of the arid regions
include Aridisols, Alfisols, Entisols, and Vertisols (Dregne, 1976 ). Inadequate soil
moisture in the root zone is the most important factor limiting biomass production
in soils of the arid tropics (Buringh, 1979; Van Wambeke, 1990; Eswaran et al.,
1993a). Similar to soil diversity, there is a wide range of rainfall regimes and
ecological characteristics.
Biomass productivity in soils of the tropics is limited by numerous soil-related
constraints. Soils of the humid tropics have severe chemical and other nutrient/
fertility-related constraints to biomass production. Sanchez and Logan (1992)
identified four principal soil chemical constraints: (1) Al toxicity on 1247 Mha,
(2) acidity without A1 toxicity on 1160 Mha, (3) high P fixation with Fe oxides and
allophanes on 1018 Mha, and (4) low nutrient reserves on 1854 Mha area (Table II).
Two ecoregional hot spots of soil degradation by numerous processes are

sub-Saharan Africa (SSA) and South Asia. In addition to widespread problems
of accelerated erosion and soil physical degradation, soils of SSA are severely
constrained by nutrient depletion (Smaling, 1993; Hartemink, 1997). The land
area affected by low CEC in soils of Africa is estimated at 1296 Mha (Table III). In
addition, there are 635 Mha affected by Al toxicity, 383 Mha by P fixation, 637 Mha


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

5

Table II
Principal Chemical Constraints in Soils of the Tropicsa

Soil constraint

Humid
tropics

Acid
savannas

Semiarid
tropics

Tropical
steeplands

Tropical
wetlands

929
808
257
537

287
261
264
166

166
132
298
94

279
23
177
221

193
23
164
0

165
6
29
8

13
5

19
0
0
0
2
0

63
80
0
20
5
15

2
60
—
—
26
—

2
6
40
38
0
33

1444

525

1012

1086

571

Low nutrient reserves
Aluminum toxicity
Acidity (without Al toxicity)
High P fixation with Fe oxides
and allophanes
Low CEC
Calcareous reaction
High soil organic matter
Salinity
High P fixation by allophanes
Alkalinity
Total areab
a In

Mha. Modified from Sanchez and Logan (1992).
area does not reflect the sum of the areas affected by different soil constraints,
because more than one constraint occurs on the same land.
b Total

by low K supply, and 107 Mha by high salinity and alkalinity. Accelerated soil erosion is a severe problem in regions with high population density, fragile soils, and
harsh environment (Ovuka, 2000). Van Lynden and Oldeman (1997) reported that
in South Asia the land area affected by different degradative processes includes
114 Mha by water erosion, 50 Mha by terrain deformation due to water erosion,
24 Mha by wind erosion, 72 Mha by terrain deformation due to wind erosion,
47 Mha by fertility decline, 17 Mha by salinization, and 8 Mha by waterlogging
(Table IV). Desertification affects vast areas in arid regions (Dregne, 1976, 1998).

Table III
Soil Chemical Constraints in Africaa
Chemical constraints

Area (106 ha)

Low CEC
A1 toxicity
P fixation
Low K supply
Salinity
Alkalinity

1296
635
383
637
76
31

Total

3058

a Data

from FAO (1986); Eswaran et al. (1993a).


6

R. LAL
Table IV
Land Area Affected by Moderate, Strong, and Extreme Forms of Soil
Degradation in South and Southeast Asiaa
Land area affected (106 ha)
Process

Strong and extreme

Moderate

Water erosion
Terrain deformation by water erosion
Wind erosion
Terrain deformation by wind erosion
Fertility decline
Salinization
Aridification
Compaction and crusting
Waterlogging

16.0
32.9
8.1
59.3
1.9
2.6
1.4
—
2.7

98.1
17.0
16.3
12.6
45.1
14.3
—
1.5
5.4

a Modified

from Van Lynden and Oldeman (1997).

Soil salinity is a serious issue in the arid tropics, covering 317 Mha of the land
area (Table V). In addition, it is the secondary salinization of irrigated land that
is a major cause of concern (Lal, 2000a). Secondary salinization is a particularly
serious issue in India, Pakistan, Australia, and other semiarid and arid regions with
large areas under irrigation. Middleton and Van Lynden (2000) estimated that land
area affected by secondary salinization in 11 countries of South Asia is 43.8 Mha.

Three countries (Bangladesh, Pakistan, and India) in South Asia all report > 6%
of the national land area affected by secondary salinization.
Goals of sustainable management of soils of the tropics are: (1) enhancing food
production to meet the demands of rapidly increasing population, (2) reversing
soil degradative trends and restoring degraded soils and ecosystems, (3) improving
quality of surface and ground water resources, and (4) sequestering C in soil and
Table V
Distribution of Salt-Affected Soils in the Arid Tropicsa
Region

Land area affected (Mha)

Mexico and Central America
South America
Africa
Southeast Asia
South and West Asia

2.0
129.2
80.5
21.5
83.6

Total

316.8

a Recalculated

from Balba (1995).


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

7

terrestrial ecosystems to minimize the risks of accelerated greenhouse effect. A
solution to the issues of achieving food security and improving the environment
lies in judicious management of soil resources.

III. SOIL DEGRADATION AND EMISSION OF
GREENHOUSE GASES TO THE ATMOSPHERE
Prior to human intervention, pedosphere or soil was in a dynamic equilibrium with its environment (e.g., hydrosphere, atmosphere, biosphere, and lithosphere) (Fig. 1). Drastic perturbations by anthropogenic activities, especially those
due to rapid increase in human population during the 20th century, have caused
widespread degradation of soils and environments (Fig. 2). Soil degradation, both
extent and severity, is more pronounced in the tropics than in the higher latitudes. Soil degradation has severe impacts on global food security (Oldeman,
1998) and on the environment. Relating agricultural production to climate change
is not a new issue (Russell, 1941). However, linking agricultural intensification
to enhancing production and mitigating the greenhouse effect is an innovative
strategy. Minimizing risks of soil degradation and restoring degraded soils and

Figure 1 The earth system is being altered by dominance of humans in the biosphere. The current
human population of about 6 billion in the year 2000 is about 12% of the estimated 50 billion people
that have lived on earth.


8

R. LAL

Figure 2 Environmental effects of anthropogenic activities include change in land use/land cover
with attendant loss of biodiversity, fossil fuel combustion and the greenhouse effect, soil degradation,
and decline in water quality.

ecosystems can have synergistic effects on productivity and environment quality.
Principal processes of soil degradation include: (1) loss of topsoil and reduction
in effective rooting depth due to soil erosion, (2) depletion of the SOC pool due
to cultivation and erosion, (3) reduction in plant available water capacity due to
decline in soil structure and reduction in the SOC pool, (4) loss of essential macro(N, P, K) and micronutrients (Zn, Cu, Mo) due to lack of or low rate of application of fertilizers and amendments, and, (5) increase in toxic concentration of salts
(saline or sodic soils) due to excessive irrigation with poor quality water or of some
elements (Al, Mn, Fe) due to leaching and acidification. Soil degradation, caused
by land misuse and soil mismanagement, leads to depletion of the SOC pool and
emission of GHGs (e.g., CO2, CH4, and N2O) into the atmosphere. For natural
ecosystems and in similar soils and moisture regimes, the quantity and quality
of SOC is similar in temperate and tropical ecoregions (Sanchez et al., 1982b;
Greenland et al., 1992). However, the magnitude and rate of SOC depletion is
more rapid in the tropics than in the temperate regions. The dynamic equilibrium,
however, is disturbed by anthropogenic activities. Principal agricultural activities
that cause depletion of the SOC pool and the attendant emission of GHGs include deforestation, biomass burning, subsistence agriculture, plowing, and lack
of restorative fallow (Table VI). Settled agriculture began 5–10 millennia ago in
several regions of the tropics. A gradual increase in population led to deforestation


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

9

Table VI
Anthropogenic Activities Leading to Soil Degradation and Depletion of SOC Pool

Activity

Processes leading to SOC depletion

1. Deforestation

Erosion, mineralization, leaching

2. Biomass burning

Mineralization, volatilization

3. Subsistence farming

Mineralization, leaching

4. Plowing

Erosion, mineralization

5. Lack of restorative fallow

Nutrient depletion

References
Nye and Greenland
(1960); Alegre and
Cassel (1986); Lal
(1996); Cassel and
Lal (1992); Hulugalle

et al. (1984)
Ghuman et al. (1991);
Hulugalle et al.
(1984); Andrae (1991)
Jenny and Raychaudhuri
(1960); King and
Campbell (1994);
Charreau (1972)
Lal (1976); Jenkinson
and Ayanaba (1977)
Smaling (1993); Pieri
(1989)

and conversion of natural to agricultural ecosystems. Deforestation and biomass
burning have been used as tools for conversion to agricultural ecosystems and are
important factors leading to emission of C from biomass and soil to the atmosphere. The data in Table VII show that 3410 Tg C/year is released by biomass
burning in the tropics. The global annual release of C by biomass burning is 3940
Tg/year (Crutzen and Andrae, 1990; Andrae, 1991). Deforestation leads to the
annual emission of 1.7–1.8 Pg C into the atmosphere (Table VIII; Houghton et al.,
1987; IPCC, 1996, 2000). Subsistence agriculture, based on low external input and
mining of soil fertility, is an important factor leading to depletion of the SOC pool.
Table VII
Biomass Burning in Tropical Regionsa
Region

C release by burning (Tg C/year)

Tropical America
Africa
Asia

Oceania

780
1450
980
200

Total

3410

a Data

from Andrae (1991); Crutzen and Andrae (1990).


10

R. LAL
Table VIII
Carbon Emission from Deforestation of TRFa
Region

Area under TRF (109 ha)

C emission (Tg/year)

Tropical America
Tropical Asia
Tropical Africa

1.2
0.4
1.3

665
621
373

Total

2.9

1659

a Data

from Houghton et al. (1987).

Smaling (1993) estimated that soils of sub-Saharan Africa have a negative nutrient
(N, P, K) balance of 30 Kg/ha/year. Mining soil fertility at this rate at the continental scale has severe adverse impacts on the SOC pool. Lal and Logan (1995)
estimated that shifting cultivation and other subsistence-type agricultural practices
cause emissions of CO2 and N2O into the atmosphere. These trends in emission of
GHGs from soils can be reversed through adoption of soil restorative measures on
degraded soils and of recommended agricultural practices (RAPs) on prime agricultural lands. In fact, restoration of degraded soils requires improvements in key
soil properties which affect SOC pool and dynamics. Some soil properties are relatively more easily restored than others. In general, restoration of topsoil depth and
increasing SOC content are more difficult than augmenting the nutrient pool. The
SOC pool can be increased only if the input (root and shoot biomass, crop residue,
manure, compost, etc.) exceed the output (oxidation, erosion, leaching, etc.)

IV. SOIL CARBON POOL AND DYNAMICS
The soil C pool, composed of soil organic carbon (SOC) and soil inorganic
carbon (SIC), plays an important role in the global C cycle. In addition, the soil C
pool is also an important factor that affects soil’s productivity and its environment
moderating capacity (e.g., the water quality and air quality including atmospheric
concentration of trace gases). There are several estimates of the C pool in soils of
the tropics ranging from 308–506 Pg for SOC and 149–218 Pg for SIC (Table IX).
Soils of the tropics contribute 32% of the total SOC pool of 1550 Pg in world soils.
Estimates of the regional distribution of the mean SOC pool of 496 Pg in the tropics
include 201 Pg in soils of Africa (40.5%), 198 Pg in tropical America (39.9%), and
97 Pg in tropical Asia (19.6%) (Table X). Soils of the tropical forest ecosystem
contain 206 Pg or 41.5% of the SOC pool in soils of the tropics (Table XI).
The SOC pool is in a dynamic equilibrium with its environment, with a balance
of input and output at a steady state level. The mean SOC content in soils of


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

11

Table IX
Estimates of the Global Soil C Pool to Depth of 1 ma
Soil organic carbon (SOC)

Soil inorganic carbon (SIC)

Region

A

B

A

B

Tropics
World

308–403
1462–1548

408–506
1526–1576

203–218
695–748

149–162
940–946

a In Pg C. Sources of A: Batjes (1996); Batjes and Sombroek (1997); Batjes
and Dijkshoorn (1999). Sources of B: Eswaran et al. (1993a, 1995, 2000); Kimble
et al. (1990).

the tropics may be 2.7 Kg m−2 for Aridisols, 18.4 Kg m−2 for Andisols, and
23.6 Kg m−2 for Spodosols (Table XI). For similar soils (e.g., Ultisols, Alfisols,
and Mollisols) and moisture regimes (e.g., humid, subhumid, or semiarid), the
mean SOC content is similar in soils of the tropics and the temperature regions
(Sanchez et al., 1982b; Greenland et al., 1992; Kimble et al., 1990).

In addition to deforestation and biomass burning, other agricultural activities
leading to C emission from soil to the atmosphere include plowing, residue removal, and drainage of wetlands. Numerous studies on soil management have
shown drastic reductions in SOC content due to plowing for row crop farming.
In India, Jenny and Raychaudhuri (1960) studied 522 soils and observed losses
of up to 80% of soil organic matter (SOM) due to cultivation. In West Africa,
Siband (1974) observed that the SOM (SOM = SOC × 1.731) content of the surface layer decreased from 2.8% in the natural ecosystem to 0.58% after 180 years
of cultivation. Pieri (1989) modeled this decline and established that even after
Table X
Regional Distribution of Organic Carbon Pool by Soil Ordera
Order

Africa

America

Asia

Total

Alfisols
Andisols
Aridisols
Entisols
Inceptisols
Mollisols
Oxisols
Ultisols
Vertisols

42.4

1.6
37.2
20.7
11.1
0.1
74.8
11.3
2.1

11.6
23.6
2.2
6.9
49.1
6.6
74.8
22.6
0.1

7.7
1.1
0.04
0.7
49.2
0.1
7.4
28.3
2.5

61.7

26.3
39.4
28.3
109.4
6.8
157.0
62.2
4.7

201.3

197.5

97.0

495.8

Total
a In

Pg C. Data from Kimble et al. (1990); Rosell and Galantini (1998).


12

R. LAL
Table XI
Soil Organic C Pool in Soils of the Tropicsa

Order

Mean SOC content
(Kg m−2)

All tropical soilsb

Tropical forest soilsb

Alfisols
Andisols
Aridisols
Entisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Miscellaneous

5.2
18.4
2.7
10.2
—
10.4
8.8
9.7
23.6

8.3
6.2
—

30
47
29
19
100
60
2
119
2
85
11
2

4
25
0
1
100
2
0
43
0
30
1
0

506

206

Total
a Adapted
b

from Kimble et al. (1990); Eswaran et al. (1993b).

In Pg.

90 years, a true equilibrium had not been established. In western Nigeria, Lal (1996)
observed that the SOC of the surface 10-cm layer declined by about 50% within
10 years after deforestation and cultivation. In the Ivory Coast, Traor´e and Harris
(1995) reported that the SOC content declined steadily with duration of cultivation.
Following 10 years of cultivation by 1980, SOC content had decreased to 76.7% of
its original value without and 80.3% with residue return. Following 20 years of cultivation by 1990, the SOC content had decreased to 61.6 and 62.1% of the original
value without and with residue return, respectively. In Australia, Dalal and Carter
(2000) reported that the SOC pool in the 10-cm layer decreased with cultivation
duration from 7.5–22.1 to 4.4–10.2 Mg C/ha. The rate of loss was lower in clayey
than in coarse-textured soils. In Argentina, Rosell and Galantini (1988) reported
a decrease in SOC content of a Haplustoll from 2.71–1.54% after 11 years of
cultivation.
The depletion of the SOC pool upon conversion from natural to agricultural
ecosystems is accentuated by soil degradation, especially that caused by soil erosion and nutrient depletion. Accelerated soil erosion causes preferential removal
of fine soil particles and the light fraction. Because SOC is concentrated in the
vicinity of the soil surface and has low density, it is easily removed and carried
away by runoff water or by blowing wind. Consequently, the SOC pool of eroded
soils is lower than those of uneroded soils (Lal, 1976; Roose, 1967, 1977; Pierri,

1989). The enrichment ratio of C in sediments is usually >1 (Lal, 1976) and often
as much as 20. On a global scale, accelerated erosion may have caused emission


POTENTIAL OF TROPICAL SOILS TO SEQUESTER CARBON

Figure 3

13

Soil erosion in the tropics and dynamics of soil organic carbon.

of 19–32 Pg of C into the atmosphere at an annual rate of 1.14 Pg C/year (Lal,
1995). Lal (1995) estimated C displacement by erosion from tropical regions at
1.59 Pg/year with a range of 0.80–2.4 Pg C/year. If 20% of the total amount displaced is emitted into the atmosphere, the annual rate of C emission due to soil
erosion in the tropics may be 0.2–0.5 Pg C (Fig. 3). Soil erosion control can lead
to a reduction in emission of some of this carbon. With a cumulative historic loss
of SOC of 66–90 Pg from world soils (Lal, 1999), the global loss of SOC by
oxidation due to plowing may be 47–58 Pg.
There are several mechanisms leading to emission of C from soil to the atmosphere by accelerated soil erosion. Important among these are: (1) breakdown of
aggregates and exposure of previously protected C to microbial activity, (2) redistribution of C over the landscape where it may be easily mineralized, (3) methanogenesis of C deposited in depressional sites under anaerobic conditions, and
(4) oxidation of dissolved organic carbon (DOC) from aquatic ecosystems.

V. HISTORIC LOSS OF SOC POOL FROM SOILS
OF THE TROPICS
It is difficult to obtain reliable estimates of the historic loss of the SOC pool in
the tropics. The magnitude of SOC loss depends on the land use and management.
Land use in the tropics includes 418 Mha of cropland, 51 Mha of permanent
corps, 1226 Mha of permanent pastures, 1205 Mha of forest and woodlands, and
1265 Mha of miscellaneous land uses. Of the total arable land, rice is cultivated

on about 100 Mha (FAO, 1996). Soil and crop management practices within a


14

R. LAL

land use, along with other ecological factors, have an important impact on the
magnitude of the historic loss of the SOC pool.
Lal and Logan (1995) estimated the SOC loss from the tropical ecosystem
caused by a wide range of agricultural activities. Estimates of the historic SOC
loss included 2–13 Pg C by deforestation. The current rate of SOC loss is 90–
219 Tg C/year because of tropical deforestation, 3.8–9.2 Tg C/year by shifting
cultivation, 112–276 Tg C/year by annual burning of grasslands, 38–92 Tg C/year
by plowing of cropland, 55–133 Tg C/year from pastures, and 2–3 Tg C/year from
cultivation of peat soils (Lal and Logan, 1995). Assuming that 20–30 Mg C/ha
has been lost from all croplands and land under permanent crops, the total loss of
SOC may be 9–14 Pg. If all pastoral lands have lost 5–10 Mg C/ha, the total loss
from pastureland may be 6–12 Pg. Therefore, total historic loss of SOC from soils
of the tropics may be 17–39 Pg (mean of 28 Pg) compared with global loss of
SOC estimated at 66–90 Pg (Lal, 1999). Assuming that 60–80% of the SOC lost
can be resequestered, the potential of SOC sequestration in soils of the tropics is
12–27 Pg over a 25- to 50-year period.

VI. NEED FOR SOIL RESTORATION
Restoration of degraded soils and ecosystems in the tropics is a high priority both
for reasons of food security and for environment quality. In terms of food security,
the per capita arable land area is rapidly decreasing, especially in densely populated
countries. By 2025, the per capita land area in China and India will be 0.05 and
0.07 ha, respectively (Lal, 2000a). In addition to soil degradation, decline in per

capita land area is exacerbated by conversion of agricultural land to industrial,
urban, and recreational uses. Meeting basic needs for food and other commodities
from per capita land area of 0.05 ha is a major challenge. In terms of environmental
quality, degraded soils and ecosystems contribute to: (1) emission of GHGs to the
atmosphere, and (2) eutrophication of surface water and contamination/pollution
of groundwater.
Estimates of land area affected by strong and extreme forms of different soil
degradative processes include 150 Mha by water erosion, 16 Mha by wind erosion,
115 Mha by loss of nutrients or fertility depletion, 43 Mha by salinization, and
33 Mha by physical degradation (Table XII). Restoring these lands would increase
the production base and improve environmental quality. Identification of effective
restorative strategies, however, would require knowledge of the resilience characteristics of soil (Lal, 1997). The resilience, the ability of soil to restore/enhance
its quality leading to improvement in productivity and environment moderating
capacity, depends on inherent characteristics (endogenous factors) and management as influenced by climate and ecoregional parameters (exogenous factors).