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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.
| 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
|
|||
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| 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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