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Soil & Tillage Research 43 (1997) 81-107
ELSEVIER
Residue management, conservation tillage and soil
restoration for mitigating greenhouse effect by
CO,-enrichment
R. La1 *
School
of Natural
Resources,
The Ohio State University,
Columbus,
OH, USA
Abstract
This manuscript reviews the potential impact of residue management, conservation tillage and
soil restoration on carbon sequestration in world soils. The greenhouse effect is among four
principal ecological issues of global concern that include: (i) adequacy of land resources to meet
needs of present and future generations; (ii) role of world soils and agricultural practices in the
‘greenhouse’ effect; (iii) potential of crop residue management, restoration of degraded soils, and
conservation tillage in carbon sequestration in soil; and (iv) minimizing risks of soil degradation
by enhancing soil resilience and soil quality. Annual increase in CO, concentration in the
atmosphere is 3.2 X 1015 g, and there exists a potential to mitigate this effect through C
sequestration in soils. Just as world soils are an important active pool of organic carbon and play a
major role in the global carbon cycle, crop residue is a major renewable resource which also has
an important impact on the global carbon cycle. I have estimated the annual production of crop
residue to be about 3.4 billion Mg in the world. If 15% of C contained in the residue can be
converted to passive soil organic carbon (SOC) fraction, it may lead to C sequestration at the rate
of 0.2 X lOI5 g/yr. Similarly restoring presently degraded soils, estimated at about 2.0 billion ha,
and increasing SOC content by O.Ol%/yr may lead C sequestration at the rate of 3.0 Pg C/yr.
Conservation tillage is an important tool for crop residue management, restoration of degraded
soil, and for enhancing C sequestration in soil. Conservation tillage, any tillage system that
maintains at least 30% of the soil surface covered by residue, was practised in 1995 on about
40 X lo6 ha or 35.5% of planted area in USA. It is projected that by the year 2020, conservation
tillage may be adopted on 75% of cropland in USA (140 X IO6 ha), 50% in other developed
countries (225 X lo6 ha), and 25% in developing countries (172 X lo6 ha). The projected
conversion of conventional to conservation tillage may lead to a global C sequestration by 2020 at
a low estimate of 1.5 X 1015 g, and at a high estimate of 4.9 X lOI g of C. These potentials of C
’ Corresponding author.
0167-1987/97/$17.00
PII
SO167-1987(97)00036-6
0 1997 Elsevier Science B.V. All
rights
reserved.
82
R. Lal/Soil
& Tillage
Research
43 (1997)
81-107
sequestration can be realized through adoption of regional, national and global soil policy that
stipulate appropriate use of world soil resources. 0 1997 Elsevier Science B.V.
Keywords:
ment
Greenhouse effect; Aggregation; Soil carbon
dynamics;
Land
restoration;
Crop
residue
manage-
1. Introduction
There are four principal issues of global concern with regards to agricultural
production. The first is related to the finite extent of land resources, second to the impact
of agricultural activities on environmental quality in general, but the ‘greenhouse’ effect
in particular, third to the role of residue management and conservation tillage (CT) in
carbon sequestration, and fourth to restoration of degraded soils by enhancing soil
resilience and quality. An important strategy is to restore degraded lands, and intensify
agricultural production while mitigating the greenhouse effect (Fig. 1). More than 97%
of the world’s food is produced on land. While the world population is increasing, its
land resources are finite and unequally distributed. As recently as 197Os, the increase in
food production in most developing countries was achieved by bringing new land under
agricultural production. Presently, however, reserves of potentially arable prime agricultural land are rapidly dwindling. Whatever potentially arable land exists is located within
fragile or ecologically-sensitive ecoregions, e.g., tropical rainforest, acid savannas,
Crop residue management
and
tillage methods
4
Conventional tillage
and
residue removed
Conservation tillage
and
residue returned
1
1
- Low risks of soil degradation
- Increased soil resilience
- Enhanced soil quality
* High risks of soil degradation
* Decreased soil resilience
l Reduced soil quality
Degradative
effects
Fig. I. Interaction
residue
management
* Agricultural sustainability
l
GHG emissions
+ Water quality
between soil degradation,
(GHG = greenhouse).
soil resilience
I
1
I
I
Restorative
effects
and soil quality
as influenced
by tillage method
and
R. Lul/
Soil & Tillage
Research
43 (1997)
81-l
07
83
steeplands, and the West African Sahel. The number of developing countries with per
capita arable land area of less than or equal to 0.10 ha was five in 1990, and will
increase to 29 by the year 2025 (Table 1). Therefore, agricultural intensification with
improved and science-based technology is inevitable, especially for countries that
presently practice predominantly resource-based or subsistence agriculture.
Agricultural intensification is also an important factor influencing the greenhouse gas
(GHG) emissions (Table 2). A report of the Intergovernment Panel on Climate Change
(Intergovernmental Panel on Climate Change, 1995) estimated that 20% of the greenhouse effect is related to agricultural activities. Therefore, producers, scientists and
planners are faced with a challenge to increase agricultural production without accentuating risks of GHG emissions. In this regard, the management of soil resources, in
general, and that of the soil organic carbon (SOC), in particular, is extremely important.
Soil resources of the world may be the key factor in the creation of an effective carbon
sink and mitigation of the greenhouse effect.
Table 1
Developing
Engelman
Country
Emt
China
Bangledesh
Vietnsm
Kenya
Sri Lanka
Somalia
Indonesia
Guinea
Philippines
Tanzania
Nepal
Haiti
El Salvador
Yemen
Jordan
Rwanda
Peru
Pakistan
Malawi
Ghana
Guatemala
Zaire
Laos
Mozambique
Mali
Burundi
Madagascar
Cote d’Ivoire
countries
with per capita
and Le Rov. 1995)
arable land area of less than 0.1 ha by the year 2025 (recalculated
Per capita arable land area (ha)
1990
2025
0.05
0.08
0.09
0.10
0.10
0.11
0.12
0.12
0.13
0.13
0.13
0.14
0.14
0.14
0.14
0.14
0.17
0.17
0.17
0.18
0.18
0.20
0.21
0.22
0.22
0.23
0.24
0.25
0.31
0.03
0.06
0.05
0.05
0.04
0.08
0.05
0.08
0.05
0.08
0.05
0.07
0.07
0.08
0.05
0.05
0.07
0.10
0.07
0.07
0.07
0.09
0.08
0.09
0.09
0.09
0.10
0.09
0.10
from
R. La1 / Soil & Tillage
84
Table 2
Relevant references
showing
the impact
Research
of soil processes
43 (1997)
on greenhouse
81 -I 07
gas emissions
Subject
Reference
1, Soils and the greenhouse effect
2. Soil organic carbon pool
Bouwman
(1989), La1 et al. (19951, Schlesinger
Eswaran et al. (1993, 19951, Houghton
(19951,
Sanchez et al. (19821, Tans et al. (1990)
Stevenson (1982). Tate (1987)
3. Properties
of SOC and humus
(1990, 1995)
Post et al. (19901,
Soil quality refers to its capacity to produce economic goods and services and
regulate environment. Soil resilience is its ability to restore its life support processes and
environmental regulatory functions after a major anthropogenic perturbation (Lal, 1993).
Soil quality and resilience have a profound impact on productivity and environmental
quality, and soil quality is affected by crop residue management and tillage methods
(Doran et al., 1994). Agricultural intensification may have adverse impact on environmental quality (Fig. 2) through emissions of greenhouse gases, eutrophication of surface
water, and contamination of groundwater. Therefore, the objective of sustainable
management systems is to minimize the adverse effects of agricultural intensification. In
other words, sustainable use of soil resources involves decreasing the adverse impact of
degradative processes, enhancing soil resilience, and improving its quality (Fig. 2).
Decreasing
Per Capita
Arable Land Area
Finite
+
So&-Economic
land i-e~oui-ces
Agricultural
Intensification
Emissions of
* co,
. CH;
. NO,
. N,O
I
Fig. 2. Land
resilience.
shortage
and its effect
on agricultural
sustainability,
I
environmental
quality,
soil quality
and soil
R. Lal/Soil
& Tillage
Researclt
43 (1997)
85
81-101
The objective of this manuscript is to collate, review and evaluate available information on: (i) per capita land resources of the world and the need for agronomic
intensification, (ii) soil resources of the world and the global SOC pool, (iii) renewable
crop residue production and its role in carbon sequestration, and (iv) carbon sequestration through adoption of conservation tillage, and restoration of degraded soils. The
manuscript is based on literature review, and projections of carbon sequestration on
assumptions of the land area under CT, and possible increase in SOC resulting from
conversion from conventional to CT. Also included are reviews of subject matter at
different scales. While processes of C sequestration are discussed at the scale of soil
separate or microaggregates, SOC dynamics in soil under tillage methods are discussed
at field plot scale, and projections of carbon sequestration and adoption of CT are made
at global scale.
2. Soil resources and the soil organic carbon pool
Principal soils of the world are Aridisols, Inceptisols, Alfisols, Entisols, Oxisols and
Ultisols, which together account for 81% of the earth’s land area (Table 3, Eswaran et
al., 1993; Eswaran et al., 1995). Agriculturally productive and highly fertile soils are
Inceptisols, Entisols, Mollisols, Histosols and Andisols that constitute about 33% of the
world soils. Predominant soils of the tropics are Oxisols, Ultisols, Aridisols, Alfisols,
Inceptisols and Entisols accounting for 88% of tropical land area (Table 4). Agriculturally productive and relatively fertile soils of the tropics cover about 20% of the tropics
(Eswaran et al., 1993).
The SOC content is an important factor affecting soil quality, and is an important
source of plant nutrients, especially in subsistence agriculture. The important effect of
SOC on productivity and environmental quality is through its role in supplying nutrients,
Table 3
Soil organic
Order
carbon
reserves
of world
Area
lo6 ha
Alfisols
Andisols
Aridisols
Entisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Miscellaneous
Total
soils (recalculated
1828.3
255.2
3174.3
1492.1
174.5
2158.0
548.0
1177.2
487.8
1133.0
328.7
744.4
13521.5
from
Eswaran
Soil organic
%
13.5
1.9
23.5
11.0
1.3
16.0
4.1
8.7
3.6
8.4
2.4
5.6
100
O-25
cm
et al., 1993, 1995)
carbon
(10”
O-50
73
38
57
37
26
162
41
88
39
74
11
-
100
61
95
52
54
215
52
128
53
96
21
-
652
927
g)
cm
O-100
136
69
110
98
390
267
12
150
98
101
38
18
1555
cm
86
Table 4
Soil organic
Order
Alfisols
Andisols
Aridisols
Entisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Miscellaneous
Total/Average
R. Id/Soil
carbon
reserves
& Tillage
in soils of the tropics
Research
(recalculated
Land Area
43 (1997)
from
81-107
Eswaran
Soil organic
lo6 ha
% of the tropics
Total
641.1
168.3
911.7
325.6
28.6
456.5
23.4
1151.2
4.0
901.8
218.9
135.8
4966.9
12.9
3.4
18.3
6.6
0.6
9.2
0.5
23.1
0.1
18.2
4.4
2.7
100
30
47
29
19
100
60
2
119
2
85
11
2
506
(Pg)
et al., 1993)
carbon
in the tropics
% of the world
soils
24
60
26
13
28
17
3
100
3
81
58
11
32
nutrient recycling, improving soil/plant available water reserves, increasing soil buffer
capacity, and stabilizing soil structure (Doran et al., 1994). World soils, an important
and active pool of organic carbon, play a major role in the global carbon cycle. The
SOC pool, estimated at about 1550 Pg (Pg = petagram = 1015 g, Table 3), is about twice
that of the atmospheric pool of 750 Pg and about three times that of the biotic pool of
550 Pg (La1 et al., 1995). The SOC pool of soils of the tropics is about 506 Pg or 32% of
the global soil pool (Table 4; La1 and Logan, 1995). The atmospheric pool of carbon has
increased steadily, partly at the expense of the SOC pool. Small changes in global SOC
pool can have a large effect on atmospheric concentrations of CO,. Since agricultural
expansion in the 19th and 20th centuries, decomposition of SOC has contributed to
increase in atmospheric CO, (Houghton, 1995). Agricultural activities that enhance CO,
emissions include deforestation, biomass burning and tillage (Intergovernmental Panel
on Climate Change, 1995). Subsistence agricultural systems based on nutrient-mining
and fertility-depleting practices accentuate depletion of SOC and contribute to CO,
emissions to the atmosphere.
3. Crop residue and its role in soil organic carbon management
Crop residue and other biomass constitute an important resource. Residue management, quantity and quality of biomass applied to the soil, has a significant impact on soil
quality and resilience, agronomic productivity, and GHG emissions from soil to the
atmosphere. The quantity of crop residue produced depends on the arable land area,
crops and cropping systems, and soil and crop management. The global arable land area
is about 1.4 billion ha (Table 5) with about 31% in Asia, 20 per cent in North and
Central America, 17% in the former USSR, and 12% in Africa (FAO, 1993). Based on
the mean residue:grain ratio for different crops, annual production of crop residue is
R. IA/Soil
Table 5
Arable land use in the world
& Tillage
in 1992 (FAO,
Research
43 (I9971
1993)
Reeion
Arable
Africa
North central America
South America
Asia (excluding
USSR)
Europe (excluding
USSR)
Oceania
Former USSR (excluding
Asia and Europe)
World
163.6
264.4
96.8
424.8
122.6
51.6
221.5
1351.3
Table 6
Land area and production
production
of different
Crop
Area (lo6
Wheat
Rice
Barley
Maize
221.1
147.5
74.5
127.4
13.3
19.6
35.6
41.8
18.1
9.1
16.0
2.9
1.0
24.0
2.9
8.9
9.9
3.4
57.8
20.5
17.4
8.1
1.4
18.2
20.7
6.2
3.1
1.1
30.8
962.9
Rye
Oats
Millet
Sorghum
Potatoes
Sweet Potatoes
Cassava
Yam
Taro
Beans
Broad Beans
Peas
Chick Peas
Lentils
Soybeans
Ground Nuts
Sugar Cane
Beet
Castor
Sunflower
Rapeseed
Sesame
Linseed
Safflower
Cotton
Total
Residue
estimates
are made from
crops in the world
ha)
Production
in 1993 (FAO,
(lo6
residue:grain
Mg)
land (lo6
1993),
ha)
and estimates
Estimated
residue
847
791
256
471
39
35
40
58
72
31
154
28
6
16
4
16
6
3
111
25
260
70
564.5
527.4
170.4
470.6
26.2
35.4
26.4
57.7
288.2
123.8
153.6
28.1
5.6
16.2
4.0
16.0
6.6
2.7
111.0
25.1
1040.6
281.7
1.2
20.5
26.2
2.5
2.2
0.8
49.1
4084.3
standard
87
81-107
21
26
3
2
1
49
3442
ratios
for field crops (Lal,
1995).
of crop residue
(lo6
Mg)
88
R. La1 / Soil & Tillage
Research
43 (1997)
81-107
estimated at 3.4 Pg in the world (Table 6) and 0.4 Pg in the USA. These estimates are
similar to those produced earlier for the USA (Larson et al., 1978; USDA, 1978) and the
world (Lal, 1995). In terms of soil and water conservation and soil surface management,
the most useful crop residue is that of small-grain cereals. The amount of residue
produced by cereals is usually high because of a high straw:grain ratio, low decomposition rate, and high C:N ratio. Residue production by all grain cereals is estimated at 2.5
Pg for the world (Table 6) and 0.31 Pg for USA (data not shown). These are
undoubtedly large quantities of renewable resources that can be used to enhance soil
quality and regulate the environment.
There is a potential for C sequestration through management of crop residues.
Assuming the mean carbon content of 45%, total carbon assimilated annually in the crop
residue is about 1.5 Pg in the world and 0.2 Pg in USA. If 15% of the carbon assimilated
in the residue can be converted to humus fraction, it may lead to carbon sequestration at
the rate of 0.2 Pg/yr or 5.0 Pg of cumulative C sequestration in the world by the year
2020. If 15% carbon in residue were converted to humus, carbon sequestration in USA
may be about 0.75 Pg by the year 2020, an estimate similar to that made by Kern and
Johnson (1993). Assuming soil bulk density of 1.5 Mg/m3, an increase of 5.0 Pg of C
in world arable land area of 1500 X lo6 ha to l-m depth would increase mean SOC
content of O.OOl%/yr. These are realistically attainable goals.
4. Conservation
tillage: definitions
Conservation tillage (CT) is a practical tool to use crop residues for soil and water
conservation and of soil quality enhancement. Understanding the role of CT is important
to develop strategies and identify policies for sustainable use of soil and water resources,
for mitigating the greenhouse effect, and improving environmental quality. The Conservation Tillage Information Center (CTIC, 1990, 1995) defines CT as “any tillage and
planting system that maintains at least 30% of the soil surface covered by residue after
planting to reduce water erosion; or where wind erosion is a primary concern, maintain
at least 1000 kg/ha of flat, small grain residue equivalent on the surface during the
critical wind erosion period.” There has been a change in the definitions and concepts
involved in the CT system through its evolution since the 1930s. CTIC has revised its
definitions since 1989. In fact, CT is a generic term that refers to “any tillage system
that reduces loss of soil or water relative to conventional tillage” (Mannering and
Fenster, 1983). Under this generic term, there are several types of CT systems that are
based on the principle of crop residue management. The latter includes a year-round
system beginning with the selection of crops that produce sufficient quantities of residue
including the use of cover crops. Basic concepts and applications of CT systems have
been described by Unger and McCalla (1980) Cannel1 (1985) La1 (1989) Blevins and
Frye (1993) and others. CTIC (1995) defined a range of CT systems included under
CRM as follows:
4.1. No-tillage
The soil is left undisturbed from harvest to planting except for plant nutrient
application. Any tillage system that causes less than 25% of row width disturbance by
R. Lal/Soil & TillageResearch43 (1997)81-107
89
planting equipment (e.g., coulters, disk openers, in-row chisels, roto-tillers) is considered
a no-tillage system. Weed control is primarily through herbicides, but cultivation may be
used for emergency weed control.
4.2. Ridge-tillage
The soil in ridge-tillage is also left undisturbed from harvest to planting, except that
planting is completed in a seedbed prepared on ridges with sweeps, disk openers,
coulters or row cleaners. Residue is left on the surface between ridges. Weed control
may be accomplished with herbicides and/or cultivation.
4.3. Mulch-tillage
The soil is disturbed prior to planting by tillage tools such as chisels, field cultivators,
disks, sweeps or blades. Weed control is accomplished with herbicides and/or cultivation.
4.4. Reduced-tillage
Any seedbed preparation system that leaves 15 to 30% residue cover after planting or
500 to 1000 kg/ha of small grain residue equivalent throughout the critical wind erosion
period is considered a reduced-tillage system.
4.5. Conventional-tillage
Tillage methods that leave less than 15% residue cover after planting, or less than
500 kg/ha of small grain residue equivalent throughout the critical wind erosion period
come under the category of conventional-tillage systems.
+
&mservation-till
> 30% crop residue
on soil surface
&J&l
~25% of
row-width
disturbance
w
Planting is
on ridges
Fig. 3. Crop residue
15.30% crop residue
on soil surface
,&VW,lltiOtl&ilI
< 15% crop residue
on soil surface
@her tillare tm
< 30% of crop residue
on soil sufwx
Mulch-till
soil is disturbed
by chisel disc
etc.
management
systems
and types of tillage
methods.
R. La1 / Soil & Tilluge
90
Research
43 (1997)
N-107
Generic classification of these tillage systems based on crop residue management is
outlined in Fig. 3. The term conventional tillage must be differentiated from the
traditional tillage. The former refers to motorized tillage operations based on moldboard
plow or other soil-inversion tillage tools, and the latter to nonmotorized (manual or
animal-drawn) methods of seedbed preparation based on native tools and implements.
5. Conservation
tillage
Conservation tillage is widely being adopted in North America (Carter, 19941, Europe
(Riley et al., 1994; Christian and Ball, 1994; Ehlers and Claupein, 1994; Masse et al.,
19941, and the Pacific (Choudhary and Baker, 1994; Steed et al., 1994). However,
estimates of area under CT in world and even in USA are difficult to make because of
lack of a standard definition, and changes in definition over time. Estimates of area
under CT in USA are based on two separate definitions, before and after 1989 (CTIC,
1995). Keeping in view all limitations, there has been a steady increase in the area under
Table 1
Arable land area in conservation
tillage in USA (the data of 1968-1986
those from 1987-1995
are those compiled by CTIC, 1995)
Year
Land area (lo6
ha)
are those
% of planted
1968
2.43
2.0
1969
1970
3.24
4.05
4.45
2.0
3.4
3.6
4.86
4.1
4.1
1971
1972
1973
1974
6.01
6.88
7.29
1975
1976
1911
1978
1979
1980
1981
1982
1983
1984
1985
8.10
9.12
5.9
1.0
12.55
9.2
13.36
15.79
9.5
10.9
17.41
11.8
26.72
28.34
35.22
18.2
22.6
25.3
27.8
38.46
1986
1987
1988
32.9
31.6
32.3
25.7
39.68
34.84
35.64
29.03
29.65
1989
1990
1991
26.1
28.1
31.4
34.9
32.04
35.91
t992
1993
1994
39.33
40.21
40.03
1995
There
5.2
5.6
was a change
in definition
35.0
35.5
of conservation
tillage
from
1989 onward.
compiledby S&r& 1988;
area
R. La1 / Soil & Tillage
Research
43 (1997)
H-107
91
CT in USA from 2.4 X lo6 ha (2.0% of the planted area in 1968) to 40.0 X lo6 ha
(35.5% of the planted area in 1995)(Table 7).
There have been several attempts to project the future growth of CT in USA (Schertz,
1988). USDA (1975) estimated that ‘minimum tillage’ would be used on 95% of the
cropland in USA by 2010. Crosson (1981) estimated that economic factors could easily
induce farmers to adopt CT on 50-60% of the cropland by 2010. The Office of
Technology Assessment (1982) estimated that 72% of US cropland would be farmed by
a CT system by 2010. Schertz (1988) projected that CT may be adopted on 63-82% of
the total planted cropland by the year 2010. Considering all economic and ecologic
factors, it is probable that a CT system may be used on about 75% of planted cropland
in USA by the year 2020.
Spread of CT in Europe, Australia, New Zealand and Canada may be at a slower rate
than that of USA, and may be adopted on about 50% of planted cropland in these
regions of developed economies by 2020. Gatson et al. (1993) estimated that 86% of
agricultural land in the former Soviet Union (181 X lo6 ha) was suitable for no-tillage
farming. Spread of CT in developing countries of the tropics and subtropics may be slow
because crop residues are used for other purposes. It is expected, therefore, that only
about 25% of the planted area in the tropics may be under a CT system by the year
2020. The actual adoption rate may depend on several factors including practices and
policies on burning crop residues, availability of herbicides and appropriate machinery,
market forces and other socioeconomic factors.
4. Mechanisms of C sequestration in soil
The SOC content is a function of soil management, and change in management can
alter SOC content. The rate of change (sequestration or release per unit time), however,
depends on the net SOC content under the new management system. The net SOC
content in soil depends on several interacting mechanisms (Fig. 4), most of which are set
in motion by addition of biomass to the soil. The use of a CT system affects C
sequestration in soil through its effect on C dynamics, aggregation and soil structure,
and interaction with cropping system (Table 8). Carbon sequestration in soil depends on
two factors: (i) turnover time, and (ii) physical or chemical protection against microorganisms and soil erosion (Carter, 1995). The turnover time depends on the SOC
fraction, its quality and physical location within the soil (Table 9). Labile or rapidly
decomposable fractions have low turnover time of less than 5 years and comprise
microbial and fresh biomass (Oades and Waters, 1991; Woomer et al., 1993). Fractions
with moderate turnover time of 5 to 20 years comprise particulate or light fractions. The
SOC fractions with slow turnover time of 20 to 50 years comprise humus and light
fractions located between microaggregates. Passive or nonlabile SOC fractions are: (i)
protected from microbes, (ii) generally located within the stable microaggregates (< 250
,um), (iii) physically protected or blocked by the clay domains (adsorbed on the inner
clay domains), and (iv) complex polymers have a turnover time of 50 to 100 years. The
passive fraction may also be chemically protected by formation of resistant/stable
organo-mineral complexes of the form [(clay-P-OM),],,
where P is polyvalent cation,
OM is organic molecule, and x and y refer to number of molecules joined together
92
R. L.ul/Soil
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43 (1997)
81-107
* More biomass
returned to the soil
humas fraction
Fig. 4. Mechanisms
of carbon
sequestration
in soil (SOC
= soil organic
carbon).
(Tisdall, 1995). Other possible modes of formation of organo-mineral complexes may be
clay-P-clay or OM-P-OM (Tisdall, 1995; Elliott and Coleman, 1988; Oades and Waters,
1991; Chaney and Swift, 1986a,b; Collis-George and Lal, 1970; Tisdall and Oades,
1982). In Andosols containing amorphous material, the mechanism of aggregation may
be: amorphous Al silicates-Fe (Al) oxides-OM-allophone (Wada and Higashi, 1976).
Soil erosion, another degradative process with adverse effect on SOC, can also be
curtailed through several soil-protective mechanisms. Formation of stable aggregates is
an important mechanism for increasing aggregate strength and reducing soil erodibility.
Physical protection by crop residue mulch and CT is the other viable and important
Table 8
Conservation
tillage,
cropping
R. Ld/Soil
& Tillage
systems,
and cultivation
practices
43 (I9971
93
81-107
for carbon
sequestration
Reference
Subject
1, Soil organic
carbon
2. Soil organic
carbon,
3. Cropping
Research
systems
content
and dynamics
aggregation
and soil structure
and tillage interaction
Angers et al. (1993), Arshad et al. (1990)
Carter (1995) Doran (1980), Follett et al. (1987),
Jenkinson (1991) Chaney et al. (1985),
Rasmussen
and Rohde (1988),
Rasmussen
and Collins (1991), Lal et al. (1994)
Dalal and Bridge (1995) Feller et al. (1995)
Tanchandsponge
and Davidson
(1970)
Albrecht et al. (1986)
Goldberg et al. (1988)
Lucas et al. (1977), Bouwman et al. (1990)
Feller (1988) Chihacek and Ulmer (1995),
Glendining
and Powlson (1995), Havlin et al. (1990),
Huggins et al. (1995) La1 et al. (1990a,b),
Unger (1982) Bauer and Black (1981), Mann (1986).
Lepsch et al. (1982) Resck and da Silva (1990),
Skidmore et al. (1986)
option to reduce the impact of erosion (Lal, 1989). Deep placement of SOC beneath the
plow zone and useful strategy of physical protection of humus, may be achieved by: (i)
growing deep-rooted plants, (ii> translocation of SOC by activity of soil fauna (e.g.,
earthworms and termites), and (iii) transport of SOC through macropore flow.
7. Soil particle size distribution
and C sequestration
The humic fraction of SOC is usually associated with fine silt (2-50 urn) and clay
fraction (< 2 pm). Clay fraction generally contains more than 50% of the SOC (Bonde
et al., 1992). Christensen (1995) observed that between 48 and 69% of the SOC was in
clay, 21 to 43% in silt, and 2 to 10% in sand fraction. For soils containing less than 20%
clay, the proportion of SOC in clay increases with increasing clay content (Balesdent et
al., 1991). Gregorich et al. (19891, Gregorich et al. (1994) observed that after complete
Table 9
Turnover time of soil organic carbon depending on quality and physical location within the soil (Carter, 1995;
Gregorich
et al., 1994; Theng et al., 1989; Stevenson and Elliott, 1989; Oades and Waters, 1991; Woomer
et
al., 1993; Balesdent et al., 1988)
Type of organic
Microbial
biomass
Litter
Light fraction
Particulate
Humus
Humus
Humus
matter
Location
Turnover
Year
Category
Pores, particle/aggregate
surface
Soil surface, pores
Voids, aggregate surface
Voids, biopores
Inter-microaggregate
Adsorbed
on intra-microaggragate
Adsorbed on intra-microaggragate
0. I-0.5
l-5
5-15
5-20
20-50
50-1000
1000-3000
Labile
Rapid
Moderate
Moderate
Slow
Passive
Passive
time
94
R. L.al/Soil
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dispersion in laboratory, SOC content was distributed as 65% with clay (< 2 pm), 30%
with silt (Z-50 pm) and 5% with sand (50-2000 pm). Similar trends have been
reported by Balesdent et al. (1991) and Morra et al. (1991). Experiments conducted in
West Africa by Feller et al. (1991a,b,c,d) showed a strong correlation of SOC with clay,
but SOC in cultivated soils being about 60% of that in uncultivated soil. Regression
equation relating SOC with clay content for a cultivated soil is shown in Eq. (1):
SOC(g/kg) = 0.294(clay%) + 0.31 Y = 0.95 * *
(1)
Lepsch et al. (1982) observed a similar equation for cultivated soils of Brazil (Eq. (2)):
SOC(g/kg) = 0.325(clay%) + 0.77 Y = 0.81* *
(2)
Tillage reduces SOC content in all size fractions (Tiessen and Stewart, 1983; Cerri et
al., 1985). Relative decrease, however, varies with particle size and is generally more in
fine clay and sand fractions than in fine silt and coarse clay. In Australia Dalal and
Mayer (19&6a,b,c,d) observed that cultivation of a Vertisol caused a rapid decline in
SOC content in sand fraction, increase in SOC content of clay, and no change in silt
fraction. Similar observations were made on African savanna by Martin et al. (1990) and
Balesdent et al. (1988). Possible reasons for tillage-induced decline in SOC are decrease
in aggregation, increased rate of decomposition by microorganisms, and accelerated soil
erosion.
8. Aggregation and C sequestration
A principal mechanism of C sequestration in soil is through the formation of stable
microaggregates. The higher the SOC the more and stable are the aggregates. Microaggregates are developed around decomposing particulate organic matter because of the
formation of humic polymers and organo-mineral complexes (Elliott and Coleman,
1988; Beare et al., 1994a; Beare et al., 1994b). These micro-aggregates consist of clay
particles, clay domains, hydrous oxides of Al and Fe, and organo-mineral complexes.
Therefore, a strong correlation exists between aggregation and SOC (Table 10, Hamblin
and Davies, 1977; Douglas and Goss, 1982; Chaney and Swift, 1984; Haynes et al.,
1991). However, the degree of correlation depends on climate, soil type, texture, clay
mineralogy, and cropping history. In pasture soils with high SOC, a substantial portion
Table 10
Relationship
Regression
between
aggregation
and soil organic
Correlation
equation
% WSA (16% clay) = 2.0.x - 11.5
% WSA (39% clay) = 1.33x - 14.0
%WSA (49% clay) = 1.54.x -58.0
MWD (mm) = 0.24.x +0.31
%WSA > 2 mm =21.5x
-20.3
%WSA > 0.25 mm = 158.9x -9.5
%WSA > 20 pm = 74.3 + 6.3 pnX
Dispersible
clay (%) = 2.39 - 0.42 x
Y = organic
carbon
carbon
(%), WSA
= water
Y=
I =
Y=
R2
R2
R2
R”
R2
stable
(x)
content
coefficient
0.86
0.73
0.95
= 0.86
= 0.93
= 0.87
= 0.58
= 0.53
aggregation,
Reference
Douglas and Goss (1982)
Douglas and Goss (1982)
Douglas and Goss (1982)
Haynes et al. (1991)
Tisdall and Oades (1980)
Tyagi et al. (1982)
DalaJ and Bridge (1995)
Dalal and Bridge (1995)
MWD
= mean weight
diameter.
R. Lal/Soil
& Tillage
Researcla
43 (1997)
82-107
95
of §OC is not involved in aggregation, and the correlation is often low. In soils with low
SOC, mechanisms of aggregation are different, and the correlation coefficient OF SOC
with aggregation is also low. Soils with higher clay content usually require more SOC
content for maintaining a given level of aggregation and aggregate stability than those
with low clay content (Douglas and Goss, 1982). Differences in clay content also cause
differences in soil moisture regime. Aggregate stability often increases with decreasing
soil moisture content (Perfect et al., 1990).
Similar to the degree of aggregation, aggregate stability is also related to SOC
content. Water-stable aggregates usually contain more SOC than those that are unstable
(Elliott, 1986). Source of crop residue is also a factor in aggregate stability. Skidmore et
al. (1986) observed that application of sorghum (Sorghum bicolor) residue produced
more stable aggregates than that of wheat (Triticum aestivum) residue.
9. Conservation
tillage and carbon sequestration
There are several merits of a CT system (Blevins and Frye, 1993) the principal being
a possible increase in SOC content leading to enhancement in soil quality and resilience.
Conventional tillage practices involving soil turnover are usually detrimental to SOC.
Plowing decreases particulate SOC and the light fraction (Beare et al., 1994a; Beare et
al., 1994b; Camberdella and Elliott, 1992; Robertson et al., 1991; Angers et al., 1993).
The additional SOC in CT is usually held as inter-macroaggregate material. Conversion
to CT may increase macroaggregation and aggregate stability (Haynes and Swift, 1990;
Elliott, 1986; Haynes et al., 1991). In general, CT leads to relatively high SOC content
near the soil surface compared with the plow-based or conventional tillage (Lal, 1989;
Carter, 1992; Dick et al., 1986a,b). However, when comparisons are made on the basis
of either the whole soil profile or on mass basis (considering soil bulk density)
‘Yesences in SOC between tillage methods are either less, or in some cases in favor of
conventional tillage (Dalal and Mayer, 1986a,b,c,d). In some cases, mixing and soil
turnover by plowing may enhance formation of organo-mineral complexes and aggregation, as was observed in soils of the semiarid regions of the West African Sahel
(Charreau and Nicou, 1971).
For some soils, especially those with coarse texture and in arid climates, conversion
to CT when soil has been under cultivation for a long time may, however, have little
effect on SOC content (Powlson and Jenkinson, 1981; Haynes and Knight, 1989).
Similarly, when soils under native vegetation or pastures are converted to arable land
use, the decline in SOC over time is more pronounced with conventional than with a CT
system (Blevins et al., 1983a; Blevins et al., 1983b; Dick, 1983; Beare et al., 1992).
Higher SOC content in CT may lead to higher and stable aggregation (Home et al.,
1992; La1 et al., 1994), because of several mechanisms including the following: (i)
fungal dominated microflora (Beare et al., 1993; Beare et al., 199.5) (ii) higher
earthworm activity (Edwards et al., 1993) and (iii) formation of platy structure with
greater bulk density. Plowing leads to breakdown of aggregates, and conversion to CT
can lead to increase in aggregation (Hamblin, 1980; Ike, 1986; Prove et al., 1990; Dalal,
1989). Structural-enhancing effects are, however, generally more pronounced in humid
than in arid environments (Lal, 1989). Long-term experiments in Ohio showed higher
96
R. Lal/Soil
& Tillage
Research
43 (2997)
al-107
Table 11
Tillage methods effects on water stable aggregation
(WSA) and mean weight diameter (MWD)
of aggregates
in the row zone of two soils under corn-corn
rotation in Ohio (modified
from Mahboubi
et al., 1993)
Tillage
method
Wooster
WSA
No-till
Chisel plow
Moldboard
plow
LSD (0.05)
Tillage (T)
Soil (S)
TxS
silt loam
Crosby
(%)
MWD
44.4
28.4
20.0
1.1
0.7
0.5
6.0
5.5
9.5
0.2
0.2
0.4
WSA
(mm)
silt loam
(%)
MWD
61.3
29.8
28.6
(mm)
1.9
0.7
0.5
aggregation and bigger mean weight diameter (MWD) in aggregates from CT than those
from conventional tillage (Table 11, Mahboubi et al., 1993).
An example of the favorable effects of CT on SOC is shown in Fig. 5 from Ohio. The
Soil Organic Carbon (Mg ha-l )
0
40
2
4
- 6
6
10
Continuous
Corn
12
14
16
Corn-Soybean
Fig. 5. Tillage method effects on soil organic carbon profile of Wooster
Typic Fragiudalf)
in Ohio (recalculated
from Dick et al., 1986a.b).
silt loam (fine-loamy,
mixed,
mesic
R. L&/Soil
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43 (1997)
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97
data in the figure show higher total SOC content with CT than with conventional tillage
practices (Dick et al., 1986a,b). Kern and Johnson (1991, 1993) developed the following
regression equations relating SOC content in no-till (SOC,,) with that in conventional-till
(SOC,,) for O-8 cm depth (Eq. (3)) and 8-15 cm depth (Eq. (4)).
SOC,,=
(1.283 XSOC,,)
SOC,,=
(1.16 x SOC,,) -0.180,R2
R2=0.75n=
+0.510,
=0.89,n
15
(3)
= 17
(4)
These equations are based on surveys of several long-term experiments conducted
throughout the USA (Dick, 1983; Bauer and Black, 1981; Clay et al., 1985; and others).
It seems that SOC reserves in no-tillage soil may be about 10 to 20% more in the top
l-m depth in soils of the temperate region. The data in Fig. 5 show that SOC content in
the top 45 cm was greater in no-tillage than conventional tillage for all rotations, e.g.,
59.6 vs. 52.6 Mg/ha for continuous corn, 55.8 vs. 47.8 Mg/ha for corn-soybean, and
61.6 vs. 60.4 Mg/ha for corn-oats-meadow rotation. These levels of increase in SOC
content under the no-tillage system may be facilitated by frequent use of cover crops in
the rotation cycle (Frye et al., 1988; Utomo, 1986; Utomo et al., 1990). The time
required for attainment of the steady state level of SOC when changed over from
conventional to no-tillage system may be 10 years in the temperate climate (Kern and
Johnson, 199 1, 1993).
The data in Fig. 6 from an Alfisol in western Nigeria show that total SOC in the top
15-cm layer was more in no-tillage than conventional tillage treatment. However, total
SOC content in the 15-30 cm layer was more in conventional tillage than no-tillage
treatment. Higher SOC content beneath the surface layer in conventional tillage treatment may be due to turnover of crop residue in the subsoil and its protection from
erosion and microbial decomposition. Is it possible, therefore, that periodic (once every
5 to 7 years) plowing under of no-till plots may sequester SOC accumulated in the
Soil Organic
0
Fig. 6. Tillage
1982).
method
effects
0
2
~"""',',""""""""""'I
on soil organic
4
6
carbon
Carbon
6
profile
(Mg
10
ha’
12
for an Alfisol
)
14
in Nigeria
16
(recalculated
from
Lal,
98
R. L.&/Soil
& Tillage
Research
43 (1997)
81-107
surface layer into the subsoil? Long-term experiments are needed on different soils and
diverse ecoregions to test this hypothesis.
10. Global impact of conservation tillage on C sequestration
There are several mechanisms of C sequestration in soil by CT including saving in
fuel, application of crop residues, reduced mineralization due to differences in soil
temperature and moisture regimes, enhanced aggregation and aggregate stability, and
reduction in soil erosion. Projections of C sequestration in world soils is made by
estimating the area under CT and assuming a possible increase in SOC content.
Estimates reported herein are based on projected land area under CT by the year 2020
shown in Table 12. The increase in land area under CT is estimated about 100 m ha in
USA; 180 m ha in Canada, Europe, Australia and New Zealand combined, 80 m ha in
Asia, 32 m ha in Africa, and 2.5 m ha in Latin America. Based on the projections made
by Gatson et al. (1993) for the former Soviet Union, my estimates may be rather
conservative for Europe.
Low estimates of C sequestration by conversion from conventional till to CT are
made on the basis of a relatively low increase in SOC in the top l-m depth and
assuming a bulk density of 1.4-1.45 Mg/m3 (Table 13). These calculations show a
potential of C sequestration of about 350 Tg in USA and 1480 Tg in the world.
Kern and Johnson (1993) estimated that if CT were adopted on 57% of the arable
land by 2020, carbon sequestration in soils of USA would range from 80 to 129 Tg C. In
the event of CT being adopted on 76% of the arable land, C sequestration in USA would
be 286 to 468 Tg C. A similar approach was used by Lee et al. (1993) who used the
EPIC model to evaluate the benefits of erosion control in C sequestration by CT
systems. They concluded that adoption of CT would increase SOC content of the top 15
cm of soil by 0.2 kg/m2 during the next 100 years. Adoption of no-tillage or no-tillage
plus a cover crop may increase SOC content by 0.4 to 0.8 kg/m2. Gatson et al. (1993)
estimated that about 10% increase in SOC by complete conversion to CT in the former
Soviet Union would sequester 3.3 Pg of C.
High estimate of C sequestration by conversion from conventional tillage to CT are
made on the basis of assuming relatively high increase in SOC ranging from 0.05% in
the top 1 m depth for tropical and subtropical regions to 0.1% for developed agricultural
Table 12
Author’s
estimate
of projected
arable
land area under
Region/country
tillage
by 2020
lo6 ha
Arable
1. USA
2. Europe, Canada,
3. Asia
4. Africa
5. Latin America
Total
conservation
Australia
New Zealand
186
441
425
164
130
1352
land area
Area under
CT
1995
2020
Change
40
45
20
8
I
120
140
225
100
40
32
531
100
180
80
32
25
417
R. Lal/Soil
Table 13
Estimates of potential
Region
carbon
sequestration
& Tillage
Research
by conservation
43 (1997)
99
81-107
tillage
Increase in area
under conservation
tillage (lo6 ha)
Change in soil organic
oganic carbon
in l-m depth (%)
A. Low estimates
USA
Developed
countries
Asia
Africa
Latin America
Total
100
180
80
32
25
417
2.5 x
3.0x
2.0x
1.5x
2.0x
B. High estimates
USA
Developed
countries
Asia
Africa
Latin America
Total
100
180
80
32
25
417
+0.1
+0.1
+ 0.05
+ 0.05
+ 0.05
10-2
10-2
10-s
lo-’
10-z
Soil bulk
density
Carbon
sequestration
(Mg/m3)
(Tg)
1.40
1.40
1.45
1.45
1.45
3.50
156
232
IO
13
1481
1.40
1.40
1.45
1.45
1.45
1400
2520
580
232
181
4913
economies in the temperate climates. These calculations show a potential of C sequestration at a global scale ranging from 1.5 Pg in USA to 4.9 Pg in the world (Table 13).
11. Carbon sequestration
by restoration
of degraded soils
Oldeman (1994) estimated that 1965 X IO6 ha of soil are degraded worldwide. This
includes 1094 m ha degraded by water erosion, 549 m ha by wind erosion, 239 m ha by
chemical degradation, and 83 million ha by physical degradation. Most degraded soils
are low in SOC content. Soil restoration, by planting trees or sowing vigorously growing
cover crops, would enhance SOC content and lead to improvements in soil quality, If
SOC of these soils may be increased by O.Ol%/yr in the top l-m depth, it would lead to
carbon sequestration at the rate of 3.0 Pg/yr assuming a mean bulk density of 1.5
Mg/m3 (Eq. (5)).
SOC Sequestration = 1.965 X lo9 ha X lo4 m2/ha X 1 m X 1.5 Mg/m3
X 10 A4 g/g = 3 Pg/yr
(5)
This rate of C sequestration is about equal to the present rate of annual increase in
carbon concentration in the atmosphere (La1 et al., 1995). This rate of increase in humus
content may be difficult to achieve in arid and semi-arid tropics, and if so only for a
limited period of time.
12. Conclusions
There are several strategies for carbon sequestration in soil (Fig. 7), and the most
effective strategies are based on proper land use and soil management. Inappropriate
100
R. Lal/Soil
& Tillage
Research
I
I
Strategies for
c Sequestration
1
SIrategies
Restoration of
degraded soils
43 (1997181-107
1
+
Improved cultivars
and new species
1
4
1
1
Crop residue
and
biomass management
Conservation
tillaee
1
1
* High amount of biomass returned to the soil
- Deep and extensive root system developmenr
* High soil biodiversity and biomass C in soil
Effects
Fig. 7. Strategies
for carbon
sequestration
in soil through
better land use.
agricultural activities have been responsible for emissions of GHG into the atmosphere
(Houghton, 1995), eutrophication of surface water (La1 and Stewart, 1994), and pollution of the environment. Nonetheless, agricultural intensification is an inevitable consequence of increasing population pressure and decreasing per capita land area. Adoption
of improved and science-based agricultural practices can be an important strategy to
bring about a quantum jump in per capita productivity and yet enhance environmental
quality. Restoration of degraded soils by enhancing soil quality is important to increasing productivity, improving water quality, and mitigating the greenhouse effect. Restoration of vast tracts of these degraded soils can reverse the trends by sequestering carbon
into the soil.
Crop residue is an important and a renewable resource. Developing techniques for
effective utilization of this vast resource is a major challenge. Improper use of crop
residues (e.g. removal, burning or plowing under) can accelerate erosion, deplete soil
fertility, and pollute environment through burning and eutrophication of surface and
contamination of groundwater. Residue management may save energy, recycle nutrients,
enhance soil fertility, improve soil structure, sequester carbon, and mitigate the greenhouse effect.
The CT system is an ecological approach to soil surface management and seedbed
preparation. It minimizes soil erosion risks, conserves soil water, decreases fluctuations
in soil temperature of the surface layer, improves SOC content, and enhances soil
structure. With the development of appropriate soil-specific package of cultural practices
R. La1 /Soil
Table 14
Feasibility
of carbon
sequestration
& Tillage
by different
Research
technological
43 (1997)
81-l
07
101
options
Technique
Global carbon
sequestration
rate (Pg/yr)
Global increase in SOC
content to l-m depth (%/yr)
Crop residue management
Conservation
tillage
Soil restoration
0.20
0.125
3.0
0.001
0.002
0.01
(e.g., crop rotations, cover crops etc.), CT can lead to C sequestration in soil. The
principal environmental and ecological benefit of a CT system lies in carbon sequestration in soil and mitigating the greenhouse effect. Implemented as a science-based
technique, conversion from conventional to CT system may increase SOC, improve soil
structure, and enhance soil quality and its environmental regulatory capacity.
Restoration of degraded soils is an important option for carbon sequestration and
mitigating the greenhouse effect. It is a win-win situation. While improving productivity
through enhancing soil quality, restoration of degraded soils can also sequester carbon
and minimize risks of the greenhouse effect. The data in Table 14 show that goals of
increasing SOC content by 0.001 to O.Ol%/yr by residue management, conservation
tillage, and restoration of degraded soils can effectively mitigate the current rate of
increase of atmospheric CO, concentration estimated at 3.2 Pg/yr.
Solution to global ecological issues require: (i) a global agenda, and (ii) implementation of appropriate policies to facilitate adoption of CT technologies. Policy issues that
require attention at regional, national and global scale include: (i) techniques of crop
residue management, (ii) restoration and rehabilitation of degraded soils, and (iii)
adoption of a CT system.
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