IS CONSERVATION TILLAGE THE ANSWER
TO GLOBAL WARMING?
Daniel Walters
Institute of Agriculture and Natural Resources
University of Nebraska – Lincoln U.S.A.
SOILS, AGRICULTURE AND GLOBAL WARMING
During the past 150 years, the conversion of native ecosystems to agricultural
lands in both temperate and tropical regions represents the most extensive
ecological disturbance experienced in recorded history. A consequence of this
disturbance has been the loss of enormous quantities of carbon and nitrogen to
the atmosphere from soils and native biomass. The net loss C from changes in
land use in the past 150 years is estimated at 120 Pg (1.2x10 17 g), one-third of
which came from the oxidation of soil carbon (8). Atmospheric gases such as
carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are called
greenhouse gases because of the effect they have on preventing the emission of
longwave radiation from the earth. Agricultural activity represents an estimated
26, 65 and 92 percent of anthropogenic emissions of these three gases,
respectively, and contributes to an estimated 23% of global warming (5) (Fig. 1).
Based upon mean lifetime and radiative forcing potential, per molecule, CH 4 has
21 times and N2O 290 times more global warming potential than CO2 (17). As
soils represent a sizable sink for carbon and nitrogen, there has been
considerable interest in sequestration of soil C (and N) through conservation
tillage. This paper will present a brief overview of agriculture’s impact on carbon
losses and the potential for conservation tillage to reverse global warming trends.
C
F
C
'
s
,
1
7
%
O
t
h
e
r
I
n
d
u
s
t
r
i
a
l
,
3
%
A
g
r
i
c
u
l
t
u
r
a
l
P
r
a
c
t
i
c
e
s
,
1
5
%
E
n
e
r
g
y
U
s
e
5
7
%
L
a
n
d
U
s
e
M
o
d
i
f
i
c
a
t
i
o
n
,
8
%
Figure 1. Contribution of anthropogenic activities to global warming (5, 20).
CURRENT INVENTORIES
World Population and Grain Demand: The major driving force behind agricultural
expansion has been unprecedented population growth in the past several
decades. According to the United Nations Department of Economic and Social
Affairs, world population currently stands at 6 billion persons and is growing at a
rate of 1.33 percent per year (19 ). The best estimate of long-range population
increase projects a stabilization of growth rate at replacement level around 2050
with a maximum population of 10 billion by 2150-2200. (Fig 2).
U
.
N
.
W
O
R
L
D
P
O
P
U
L
A
T
I
O
N
P
R
O
J
E
C
T
I
O
N
S
6
0
4
0
2
0
H
I
G
H
M
E
D
I
U
M
L
O
W
1
0
8
6
WORLDPOULATION(bilons)
4
2
1
1
8
5
0
1
9
0
0
1
9
5
0
2
0
0
0
2
0
5
0
2
1
0
0
2
1
5
0
Y
E
A
R
Figure 2. United Nations long-term population projections. Stabilization @ 8.9
million in 2050 with: Medium = fertility @ replacement level; High = fertility
½ a child higher than medium; Low = fertility ½ a child lower than medium
( UN ).
If the medium population growth scenario is realized, the demand for major
cereals will increase at a rate of about 1.2 percent per annum for wheat and rice
and 1.5 percent for maize (6, 14). The projected demand for cereal production in
the next 30 years stands at 43% for rice, 44% for wheat and 56% for maize
above current production levels (2 ) (Table 1).
Table 1. World wide harvested area of the major cereals in 1967 and 1997,
average growth rate in production since 1967, projected annual growth
rate in cereal demand, and projected production increase for UN medium
population growth scenario. (2 )
Harvest area,
ha x 106
Annual growth rate, %
1967-97
Production
Grain
demand
projected
to 2030
Increase in production
to meet 2030 demand,
% of present
Crop
1967
1997
Wheat
219
229
2.5
1.22
44
Rice
128
150
2.5
1.19
43
Maize
112
142
2.6
1.49
56
Arable Land: Increases in agricultural production have kept pace with population
growth over the past 100 years primarily from the expansion of agriculture into
virgin territory. Worldwide area in agriculture (cropland) exceeds 1.5 x 109 ha (6 )
yet the rate of land conversion to agricultural production has declined since 1950
owing to a decline in land area that is suitable for agriculture (7, 13) (Fig.3). The
majority of land clearing today occurs from tropical deforestation. Between 1850
and1980, 60% of the 900 million acres cleared for agricultural production has
been in tropical regions (8). Although these statistics are formidable, it is
surprising that the FAO attributes only a fraction of total deforestation to the
establishment of permanent agricultural land. Estimates put the expansion of
agricultural lands at only 27% of total deforestation between 1980 and 1985 (8),
with a large percentage classed as abandoned “other land” (6).
Figure 3. Worldwide area in agriculture (- - -) and rate of land clearing ( ), (7).
Global Carbon Pools and Fluxes: The earth contains an estimated 1023 g (108
Pg) of carbon. Only 0.04 percent of this carbon is involved in relatively “active”
biogeochemical exchange (18). The largest pool of “active” carbon resides as
dissolved carbonates in the ocean and fluxes to the atmosphere from the ocean
are approximately 175% greater than the flux of CO2 from soil to the atmosphere
(16). Soil comprises the largest biologically active terrestrial C pool and stores of
soil C are estimated to be approximately 1500 Pg (1.5 x 10 8 g) with a flux to the
atmosphere of approximately 55 to 60 Pg yr-1 (16 ) (Fig 4). Prior to 1960, the
principal source of CO2 enrichment came from the oxidation of soil and plant
biomass due to land use change (agriculture and deforestation) (7), (Fig. 5).
Current estimates of anthropogenic CO2 flux from terrestrial land management
indicate that it represents approximately 25% of the loss generated from fossil
fuel use (15) (Fig 6). Of the current 1.6 Pg yr-1 of C loss from soil and
deforestation, 90 percent is generated from land use changes in the tropics (7)
(Fig 7). Most C losses from land use changes are associated with the oxidation
of plant material with approximately one third of the 120 Pg lost in the past 150
years coming from soil (8). At the current rate of fossil fuel consumption, it takes
only eight years to equal the loss of C from soil over the past 150 years.
G
L
O
B
A
L
C
A
R
B
O
N
P
O
O
L
S
O
c
e
a
n
s
,
3
8
,
0
0
0
P
g
A
t
m
o
s
p
h
e
r
e
,
7
5
0
P
g
M
i
s
c
e
l
l
a
n
e
o
u
s
,
1
5
0
P
g
T
e
r
r
e
t
r
i
a
l
,
2
1
0
0
P
g
(
S
o
i
l
s
,
1
5
5
0
P
g
&
B
i
o
t
a
,
5
5
5
P
g
)
Figure 4. “Active” global carbon pools. (Pg = petagram = 1 x 1015 grams). (10)
Figure 5. Net flux of carbon to the atmosphere as a result of land-use change
and deforestation () and as a result of the combustion of fossil fuel (----).
(7).
S
I
N
K
S
S
O
U
R
C
E
S
D
e
f
o
r
e
s
t
a
t
i
o
n
&
L
a
n
d
u
s
e
1
.
6
P
g
/
Y
r
U
n
a
c
c
o
u
n
t
e
d
f
o
r
,
1
.
8
P
g
Y
r
A
t
m
o
s
p
h
e
r
e
3
.
2
P
g
/
Y
r
F
o
s
s
i
l
F
u
e
l
,
5
.
4
P
g
/
Y
r
O
c
e
a
n
s
,
2
.
0
P
g
/
Y
r
Figure 6. Principal anthropogenic CO2 sources and global CO2 sinks. (10)
Figure 7. Net flux of carbon to the atmosphere as a result of land-use change
and deforestation in Latin America, Tropical Africa, North America and
Europe. (7).
POTENTIAL FOR CONSERVATION TILLAGE TO ENHANCE
SOIL CARBON SEQUESTRATION
Tillage affects soil C in a number of ways. The combination of reduced residue
decomposition rate and less soil mixing under conservation tillage tends to
reduce overall soil-C oxidation rate and lower soil temperatures under CT tend to
retard residue decomposition rate. Evidence of enhanced soil C sequestration
through reduced tillage has been mixed. In cases where reduced tillage has
limited crop productivity and therefore residue-C input, there has been little
change in soil C over time (11). Intensification of cropping in traditionally wheatfallow regions has been shown to significantly favor reduced tillage systems in
both grain yield and increased soil C (3). Generally, high surface residue
systems result in higher C sequestration when compared to intensive tillage
systems when water is limiting and temperature or drainage does not inhibit crop
productivity (12).
Much of the work reporting tillage impacts on global C budgets relies on the use
of process models that simulate the effects of edaphic, climatic and biological
factors as well as C inputs on soil C dynamics. Soil C enhancement is a relatively
long-term process. Given the fact that CT has been practiced for a relatively
short time, there are few tillage studies, per-se, that have been in place long
enough to study organic matter build-up from reduced tillage. On the other hand,
there is a larger world laboratory of long-term studies and sites that have
recorded the decline of soil C from agricultural practices. These have been
largely used to test and validate process models.
One such study attempted an assessment of alternative management practices
affecting soil C in agroecosystems of the central United States (4). Simulations
were run using the Century model over a database of soils that represented 60%
of the agricultural land area of the continental U.S. The Century model simulates
biomass production and conversion to soil organic matter under defined climates,
soils and management practices. Soil organic matter pools are divided into
active, passive and stabilized fractions with turnover times representing short (15 yr), slow (20–40 yr) and recalcitrant (200 to 1500 yrs). As Century is driven by
C inputs, prediction of future trends in soil C input in this study were simulated
with biomass increasing at a rate of 0.5, 1 or 1.5% yr-1.
The Century model performed quite well and predicted the documented decline
in soil organic C to approximately 53% of original 1907 level (Fig. 8). With the
onset of improved crops and fertilizer use in the1960’s, the rate of SOC
increased to 61% of original levels by 1990 consistent with most published
observations. With increased conservation tillage after 1990, the projections
were for an almost linear increase in SOC at rates of 0.6, 0.9 and 1.2% yr-1.
These projected scenarios would result in a sequestration of 1.0, 1.36 and 1.8 Pg
C for the study area by 2030. These values were compared to a more empirical
simulation made by Kern and Johnson (9). The Century estimates were found to
be within the limits or slightly lower than Kern and Johnson’s empirical estimates.
In the Century simulation, the maximum C sequestration potential had the
capacity to fix only 0.1% of the total global CO2 emissions from fossil fuel. Kerr
and Johnson’s best estimate (@76% no-till nationwide) would only sequester 0.5
% of global fossil fuel emissions. These projections become somewhat less
favorable if one considers the added fuel cost associated with increased nitrogen
fertilization necessary for high levels of crop productivity and the potential for
greater N2O emissions under conservation tillage (1).
Many other factors have not been adequately considered in evaluating the
impact of increased C sequestration from the practice of CT. The reduction in
total farm fuel consumption and the potential for reduced fertilizer N consumption
as soils become more biologically stable under CT may significantly alter
conclusions from these previous scenarios. The impact of “C fertilization” from
elevated atmospheric CO2 on crop biomass production, nutrient use efficiency
and soil C sequestration potential is unclear. It seems reasonable, however, to
promote integrated soil conservation systems designed to remove marginal land
from crop production to forest or biomass fuel production. This would have
considerably greater impact on CO2 emissions than wholesale conversion of all
cultivated lands to CT. A program for global reforestation of degraded lands
offers the fastest means to sequestration of atmospheric CO2 but it will require
controls on population growth, reduction in the current rate of deforestation for
agricultural purposes and a concerted energy conservation policy especially by
the industrialized nations.
Figure 8. Simulated total soil carbon from 1907 to 2030 for historic (<1990) and
three alternative levels of future crop yield increases. Future projections
assume 27% reduced till and 3% no-till for the region.( 4)
REFERENCES
1. Aulakh, M.S., D.A. Rennie, and E.A. Paul. 1984. Gaseous nitrogen losses
form soils under zero-till as compared with conventional tillage-management
systems. J. Envir. Qual. 13:130-136.
2. Cassman, K.G. 1999. Ecological intensification of cereal production systems:
Yield potential, soil quality, and precision agriculture. Proc. Nat. Acad. Sci.
USA. Vol. 96:5952-5959
3. Cole, C.V., I.C. Burke, W.J. Parton, D.S. Schimel, D.S. Ojima, and J.W. B.
Stewart. 1988. Analysis of historical changes in soil fertility and organic matter
levels of the North American Great Plains. p. 436-438 In: P.W. Unger, T.V.
Sneed, W. R. Jordan, and R.W. Jensen (eds.) Challenges in Dryland
Agriculture: A Global Perspective. Proc. Intl. Conf. On Dryland Farming. Aug.
15-19. Amarilllo/Bushland, TX. Texas Agr. Expt. Sta.
4. Donigan, A.S., T.O. Barnwell, R.B. Jackson, A.S. Patwardhan, K.B. Weinrich,
A.L. Rowell, R.V. Chinnaswamy nad C.V. Cole. 1994. Assessment of
alternative management practices and policies affecting soil carbon in
agroecosystems of the central United States. EPA/600/R-94-067. USEPA,
Wash. D.C. 194 pp.
5. Duxbury, J,M., L.A. Harper, and A.R. Mosier. 1993. Contributions of
agroecosystems to global climate change. p. 1-18 In: D.E. Rolston, L.A.
Harper, A.R. Mosier and J.M. Duxbury (eds.) Agricultural Ecosystem Effects
on Trace Gases and Global Climate Change. ASA Soec. Publ. 25. ASA,
CSSA, SSSA, Madison, WI.
6. FAO. 1992. 1991 Production Yearbook. Rome, Italy.
7. Houghton, R.A., J.E. Hobbie, J.M. Melillo, B. Moore, B.J. Peterson, G.R.
Shaver, and G.M. Woodwell. 1983. Changes in the carbon content of
terrestrial biota and soils between 1860 and 1980: A net release of CO 2 to the
atmosphere. Ecological Monographs Vol. 53(3):235-262.
8. Houghton, R.A. 1995. Changes in the storage of terrestrial carbon since
1850. In: R. Lal, J. Kimbal, E. Levine and B.A. Stewart (eds.) p.45-65. Soils
and Global Change. CRC Press, Boca Raton, FL.
9. Kern, J.S., and M.G. Johnson. 1993. Conservation tillage impacts on native
soil and atmospheric carbon level. Soil Sci. Soc. Am. J. 57:200-210.
10. Lal, R. J. Kimbal, E. Levine, and C. Whitman. 1995. World soils and
greenhouse effect: An overview. P.1-7 In: R. Lal, J. Kimbal, E. Levine and
B.A. Stewart (eds.) p.45-65. Soils and Global Change. CRC Press, Boca
Raton, FL.
11. Paul, E.A., E. Paustian, E.T. Elliott and C.V. Cole (eds.). 1997. Soil Organic
Matter in Temperate Ecosystems: Long Term Experiments in North America.
CRC Press. Boca Raton, FL.
12. Prasad, R. and J.F. Power. 1991. Crop Residue Management. P 215 In
Advances in Soil Science. Vol. 15. Springer-Verlag, NY.
13. Richards, J.F. 1990. Land transformation. In: B.L. turner, W.C. Clark, R.W.
Kates, J.F. Richards, J.T. Mathews, and W.B. Meyer (eds) p. 163-178. The
Earth As Transformed by Human Action.
Cambridge Univ. Press,
Cambridge, U.K.
14. Rosegrant, M.W., Leach, N., and Gerpacio, R.V. 1998. Alternative Futures for
World Cereals and Meat Consumption. (International Food Policy Institute,
Washington, DC. http://www.apps.fao.org
15. Rotty, R.M. and C.D. Masters. 1985. Carbon dioxide from fossil fuel
combustion: Trends, resources, and technological implications. p. 63-80 In:
J.R. Trabalka (ed.) Atmospheric Carbon Dioxide and the Global Carbon
Cycle. United States Dept. of Energy. DOE/ER-0239.
16. Schlesinger, W.H. 1995. An overview of the carbon cycle. p. 9-25 In: R. Lal, J.
Kimbal, E. Levine and B.A. Stewart (eds.) p.45-65. Soils and Global Change.
CRC Press, Boca Raton, FL.
17. Shine, K.P., R.G. Derwent, D.J. Wuebbles, and J.J. Morcrette. 1990.
Radiative forcing of climate. P.41-68. In J.T. Houghton, G.J. Jenkins, and J.J.
Ephraums (eds.) Climate Change: The IPCC Scientific Assessment.
Cambridge University Press, Cambridge, U.K.
18. Solomon, A.M., J.R. Trabalka, D.E. Reichle, and L.D. Voorhees. 1985. The
global cycle of carbon. p.1-14 In: J.R. Trabalka (ed.) Atmospheric Carbon
Dioxide and the Global Carbon Cycle. United States Dept. of Energy.
DOE/ER-0239.
19. United Nations–Department of Economic and Social Affairs, Population
Division. http://www.popin.org/pop1998/4.htm
20. U.S. Environmental Protection Agency. 1990. Greenhouse gas emissions
form agricultural systems. Vol.1, Summary Report. p. 19-24 In Proc. Of IPCCRSWG Workshop, Washington, DC. 12-14 Dec. 1989. USEPS Climate
Cchange Div., Wash. DC.