4.4.2.1. Agricultural Intensification
Improved cultivars, irrigation, organic and inorganic fertilization, management
of soil acidity, green manure and cover crops in rotations, integrated pest
management, double-cropping, and crop rotation (including reduction of bare
fallow) are some of the ways to increase crop yields (see Fact
Sheet 4.1). Increasing crop yields results in more carbon accumulated in
crop biomass or in an alteration of the harvest index. The higher residue inputs
associated with those higher yields favor enhanced soil carbon storage (Paustian
et al., 1997a). Estimates and experimental data from around the world
indicate that the application of management practices to improve agricultural
productivity results in increased SOC content (Table 4-5).
For example, increases in biomass production resulting from advances in improved
crop germplasm and agronomy are estimated to sequester carbon at rates ranging
from 0.01-0.7 t C ha-1 yr-1, with a mean value of 0.27
t C ha-1 yr-1 (Lal and Bruce, 1999). This rate could provide
a carbon capture of 0.02-0.07 Gt C yr-1 in an area of 122-152 Mha.
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Table 4-5: Rates of potential carbon gain under
selected practices for cropland (including riceland) in various regions of the
world.
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|
| Practice |
Country/Region
|
Rate of Carbon Gain
(t C ha-1 yr-1)
|
Time1
(yr)
|
Other GHGs
and Impacts
|
Notes2
|
|
| Improved crop production and erosion control |
Global
|
0.05-0.76
|
25
|
+N2O
|
a
|
| - Partial elimination of bare fallow |
Canada
|
0.17-0.76
|
15-25
|
|
b
|
| |
USA
|
0.25-0.37
|
8
|
±N2O
|
q
|
| - Irrigation water management |
USA
|
0.1-0.3
|
|
|
c
|
| - Fertilization, crop rotation, organic amendments |
USA
|
0.1-0.3
|
|
+N2O
|
c
|
| - Yield enhancement, reduced bare fallow |
Tropical and subtropical China
|
0.02
|
10
|
|
e
|
| - Amendments (biosolids, manure, or straw) |
Europe
|
0.2-1.0
|
50-100
|
|
f
|
| - Forages in rotation |
Norway
|
0.3
|
37
|
|
d
|
| - Ley-arable farming |
Europe
|
0.54
|
100
|
|
m
|
|
| Conservation tillage |
Global
|
0.1-1.3
|
25
|
±N2O
|
a
|
| |
UK
|
0.15
|
5-10
|
|
g
|
| |
Australia
|
0.3
|
10-13
|
|
h
|
| |
USA
|
0.3
|
6-20
|
|
i
|
| |
|
0.24-0.4
|
|
|
c
|
| |
Canada
|
0.2
|
8-12
|
|
r
|
| |
USA and Canada
|
0.2-0.4
|
20
|
|
j
|
| |
Europe
|
0.34
|
50-100
|
|
k
|
| |
Southern USA
|
0.5
|
10
|
|
l
|
| |
|
0.2
|
10-15
|
|
s
|
|
| Riceland management |
|
|
|
|
|
| - Organic amendments (straw, manure) |
|
0.25-0.5
|
|
++CH4
|
n
|
| - Chemical amendments |
|
|
|
--CH4, +N2O
|
o
|
| - Irrigation-based strategies |
|
|
|
--CH4
|
|
|
1 Time interval to which estimated rate applies. This interval may or may
not be time required for ecosystem to reach new equilibrium.
2
a. Lal and Bruce (1999). Estimates of carbon gain shown represent range of values
presented by the authors for various regions throughout the world.
b. Dumanski et al. (1998). Estimates presented are for the 0-30 cm layer.
Estimated carbon gain for the 0-100 cm layer are twice those for the 0-30 cm
layer. Estimated rates of carbon gain are higher for conversion of fallow to
forages (0.48-0.76 Mg C ha-1 yr-1) than for conversion
to cereal crops (0.17-0.52 Mg C ha-1 yr-1).
c. Lal et al. (1999b).
d. Singh et al. (1994). Reported rate is from one long-term study.
e. Li and Zhao (1998). Rate of carbon gain based on total carbon gain (0.7 Tg
C yr-1 for 10 years) and total cropland area in the region (~40 Mha)
reported by the authors.
f. Smith et al. [1998, including data from Smith et al. (1997a,b)
and Powlson et al. (1998)]. Carbon gain from manure, sewage sludge, and
straw incorporation assumes that carbon in these materials would otherwise all
be lost as CO2. Rates reported here were calculated
from annual mitigation potential (Tg C yr-1) and area values in source
reference.
g. Mean carbon accumulation rate in four sites sampled after 5-10 years; from
literature data compiled and cited by Paustian et al. (1997a).
h. Mean carbon accumulation rate in two sites sampled after 10 or 13 years;
from literature data compiled and cited by Paustian et al. (1997a).
i. Mean carbon accumulation rate in 22 sites sampled after 6-20 years. Includes
one site from Canada.
j. Bruce et al. (1999). Rates of carbon accumulation assume "best management
practices," including no-till.
k. Smith et al. (1998). Based on data from 14 sites in UK and Germany,
ranging in duration from 2 to 23 years. Rates reported here were calculated
from annual mitigation potential (46.6 Tg C yr-1) and area of arable
land (135 x 106 ha).
l. Franzluebbers et al. (1998). Increase in soil carbon in soybean/wheat
double crop vs. soybean (averaged across tillage treatments).
m. Smith et al. (1997b). Rates reported here were calculated from annual
mitigation potential (73 Tg C yr-1) and area of arable land (135 x 106 ha).
n. Net local increase in carbon stored in organic matter; likely small net carbon
gain regionally, depending on fate of organic amendments if not applied as fertilizer.
o. Addition of sulfate, nitrate, or iron decreases activity of methanogens by
providing alternative electron acceptors and restricting availability of substrates
in submerged soils (e.g., Hori et al., 1990). Amendments tend to reduce
CH4 emissions by 0-77% in different experiments
(Schutz et al., 1989; Lindau et al., 1993; Wassmann et al.,
1993; Denier van der Gon and Neue, 1994). Amendments will likely result in net
loss of organic carbon, though estimates were not reported.
p. Drainage of field during cropping season. Oxygen availability stimulates
CH4 oxidation and reduces CH4
emission (Yagi et al., 1997). Reduced CH4
emission may be offset by increased CO2 emission
(soil carbon loss).
q. Peterson et al. (1998); values for increased carbon levels with continuous
crop rotations vs. wheat-summer fallow rotations for four experiments in Montana
and Colorado.
r. Janzen et al. (1998); mean for six experiments with no-till vs. tilled
treatments and continuous crop rotations. No apparent increases in soil carbon
with no-till were found for wheat-summer fallow rotations.
s. Potter et al. (1998); mean for no-till vs. conventional tillage treatments
at three sites (11 crop rotations) in Texas.
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Irrigated agriculture produces about one-third of the Earth's total crops,
including 40 percent of all crops harvested on only about one-sixth of the cultivated
cropland (Lal, 2000a). Because most irrigation is located in arid and semi-arid
regions, many irrigable soils are inherently low in soil organic carbon in their
native state. Converting these dryland soils to irrigated agriculture may increase
soil organic carbon content in the soil by 0.05-0.15 t C ha-1 yr-1,
with a modal rate of 0.10 t C ha-1 yr-1 (Lal et al.,
1998). Irrigation of arid and semi-arid soils can also affect the inorganic
soil carbon pool (carbonates) and its dynamics (Suarez, 1998). Although the
processes involving inorganic soil carbon dynamics are complex and poorly understood,
irrigation of arid and semi-arid soils may result in inorganic soil carbon sequestration
rates that are similar to the rates estimated for soil organic carbon (Lal et
al., 1998). Irrigated lands are susceptible to high levels of soil erosion
and salinization, however-both of which can reduce soil organic carbon levels
and increase emissions.
Improved management of drained croplands-through conversion to conservation
tillage and/or management of sub-surface drainage to keep soil moisture levels
high-can increase soil organic carbon. In some cases, practices that promote
higher productivity may entail greater energy use. For example, increased fertilization
and expansion of irrigation may lead to higher fossil fuel use (Schlesinger
1999). Soil carbon gains, therefore, may be partially offset by higher CO2
emissions from energy use.
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