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4.4. Activities Categorized and Described
In this section, we define activities broadly, primarily to keep the complexity
and length of the presentation manageable. Some of the activities listed here,
such as agroforestry and restoration of degraded lands, cut across several of
the "land cover" boxes of the matrix in Figure 4-6.
Key practices are discussed in some detail in several of the Fact Sheets.
4.4.1. Overview of Rates and Duration of Potential Carbon Sequestration
Potential rates of carbon sequestration in response to improved management
vary widely as a function of land use, climate, soil, and many other factors
(refer to Tables 4-5, 4-6,
4-7, 4.8,
4-9, 4-10,
4-11 and 4-12).
Although research to date does not allow
definitive evaluation of potential rates of carbon gain for all regions and
management options, Table 4-4 presents rough estimates
for broad activity and eco-zone groupings. For each grouping, Table
4-4 shows the carbon-conserving practices that are most likely to achieve
substantial rates of carbon gain. The rates of carbon gain alongside each practice
reflect the best combination of these practices for that activity and eco-zone.
For example, estimated rates of potential carbon gain for cropland management
might reflect the effects of reduced tillage, better fertilization, and improved
crop rotation applied together on the same unit of land.
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Table 4-4: Summary of potential rates of carbon
gain and associated impacts for various activities.
|
|
| Activity |
Ecozonea
|
Keyb
Practices
|
Ratec
(t C ha-1 yr-1)
|
Confidenced
|
Duration
(yr)e
|
Other
GHGsf
|
Associated
Impacts
|
|
| Cropland management |
Boreal |
Ley/perennial forage crops, organic amendments |
0.3-0.6 (0.4)
|
M
|
40
|
+N2O
|
Increased food production, improved soil quality |
| Temperate - dry |
Reduced tillage, reduced bare fallow, irrigation |
0.1-0.3 (0.2)
|
H
|
30
|
+N2O
|
Increased food production, improved soil quality, reduced erosion, possibly
higher pesticide use |
| Temperate - wet |
Reduced tillage, fertilization, cover crops |
0.2-0.6 (0.4)
|
H
|
25
|
+N2O
|
Increased food production, improved soil quality, reduced erosion, possibly
higher pesticide use |
| Tropical - dry |
Reduced tillage, residue retention |
0.1-0.3 (0.2)
|
L
|
20
|
+N2O
|
Increased food production, improved soil quality, reduced erosion, possibly
higher pesticide use |
| Tropical - wet |
Reduced tillage, improved fallow management, fertilization |
0.2-0.8 (0.5)
|
M
|
15
|
+N2O
|
Increased food production, improved soil quality, reduced erosion, fertilizers
often unavailable, possibly higher pesticide use |
| Tropical - wet (rice) |
Residue management, fertilization, drainage management |
0.2-0.8 (0.5)
|
L
|
25
|
++CH4, +N2O
|
Increased food production |
|
| Agroforest management |
Tropical |
Improved management |
0.5-1.8 (1.0)
|
M
|
25
|
+N2O
|
|
|
| Grassland management |
Temperate - dry |
Grazing management, fertilization, irrigation |
0-0.3 (0.1)
|
M
|
50
|
±CH4, +N2O
|
Increased energy use, salinity, higher productivity |
| Temperate - wet |
Grazing management, species introduction, fertilization |
0.4-2.0 (1.0)
|
M
|
50
|
±CH4, ++N2O
|
Higher productivity, acidification, erosion, reduced biodiversity |
| Tropical - dry |
Grazing management, species introduction, fire management |
0.1-1.5 (0.9)
|
L
|
40
|
-CH4, ++N2O
|
Reduced soil degradation, higher productivity, woody encroachment (reduced
productivity) |
| Tropical - wet |
Species introduction, fertilization, grazing management |
0.2-3.9 (1.2)
|
L
|
40
|
-CH4, ++N2O
|
Increased productivity, reduced biodiversity, acidification |
|
| Forestland management |
Boreal and
Temperate - dry |
Forest regeneration, fertilization, plant density, improved species, increased
rotation length |
0.1-0.8 (0.4)
|
L
|
80
|
+N2O, +NOX
|
Leakage (rotation length), high cost efficiency |
| Temperate - wet |
Forest regeneration, fertilization, species change |
0.1-3.0 (1.0)
|
L
|
50
|
+N2O, +NOX
|
Leakage (rotation length), reduced biodiversity |
| Tropical - dry |
Forest conservation, reduced degradation |
(1.75)
|
L
|
40
|
|
Ecological improvement, high cost efficiency |
| Tropical - wet |
Reduced degradation |
3.1-4.6 (3.4)
|
L
|
40
|
|
Environmental improvement |
|
| Wetland management |
All |
Restoration |
0.1-1 (0.5)
|
L
|
100
|
++CH4, ±N2O
|
Increase in water quality, decrease in flooding, increased biodiversity
|
|
| Restoration of degraded land |
All |
Restoration of eroded lands, saline soil reclamation |
0.1-7 (0.25)
|
M
|
30
|
+N2O
|
Increased productivity, may be expensive |
|
| Urban land management |
All |
Tree planting |
(0.3)
|
M
|
50
|
|
Increased biodiversity |
|
| Conversion to agroforestry |
Tropics |
Conversion from cropland or grassland at forest margins |
1-5 (3)
|
L
|
25
|
|
Improved biodiversity, CH4 sinks, poverty alleviation, food security |
|
| Conversion (cropland to grassland) |
Temperate - dry |
Marginal cropland re-seeded to grassland |
0.3-0.8 (0.5)
|
H
|
50
|
-N2O; -CH4
|
Enhanced biodiversity, reduced erosion |
| Temperate - wet |
Surplus cropland seeded to grassland |
0.5-1.0 (0.8)
|
M
|
50
|
--N2O; - CH4
|
Enhanced biodiversity, reduced erosion |
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a "Wet" vs. "dry" based on potential evapotranspiration:precipitation ratio.
"Wet" <1 and "Dry" >1; "Tropical" = latitude <30º.
b List of practices that may yield largest gains in carbon stocks, roughly
in order of importance.
c Range of carbon increase rates that might reasonably be expected to occur
in response to adoption of best-possible complement of key practices. Actual
rate will depend on previous management practices (e.g., rates of gain may be
higher in a carbon-depleted system), climate, ecosystem properties (e.g., soil
carbon gain may be favored by higher clay content), and many other factors.
Value in parentheses is default estimate. Rates for tropical forest management
to recover carbon stocks on degraded forests apply only to the present area
of degraded forest (as of 1990) as reported by FAO (1996)-that is, closed-canopy
forest having full biomass stocks are excluded.
d Relative reliability of rate estimates. Generally, confidence increases
with the number of studies conducted in the activity-ecozone grouping. L = low,
M = medium, and H = high.
e An estimate of the time required for the system to approach a new steady
state after the adoption of the new practices.
f Relative magnitude of potential effects on emission of N2O, CH4, and other
GHGs. "+" denotes increased emission; "-" denotes reduced emission; number of
"+" or "-" denotes relative magnitude of possible effects.
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The rates of carbon gain in Table 4-4 refer to the
average accumulation rate from the time a practice is started until carbon storage
again reaches a new equilibrium (see Box 4-1). Almost
invariably, the rates in Table 4-4 are lower than those
from published studies, which are usually measured for time intervals shorter
than that needed to reach saturation. As shown, some of the rate values have
greater uncertainty than others, largely because of a paucity of studies in
many regions. The net effect of the practices on climate forcing is also affected
by impacts on other GHGs (Table 4-4). For example, where
the proposed practice increases N2O or CH4 emission, the rate of carbon sequestration
alone overestimates the net benefit of the practice to the atmosphere.
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Box 4-1. The Rate of Carbon Gain
|
|
The rate of carbon gain following application of a given practice that
stores carbon will decrease over time (see Chapter
1 and Section 4.2). Figure
4-8a shows an approximation of this relationship, which normally
is used to describe changes in soil carbon following application of
a particular practice that stores carbon. Therefore, rates of carbon
change observed during the initial stages of an activity are usually
higher than the average rates (Figure 4-8a).
Idealized saturating curves such as those in Figure
4-8a require two constraining parameters: the rate of inputs and
the time required to reach "saturation." Data for carbon stock increases
reported in the literature sometimes do not cover the entire duration;
hence, the rates reported are higher than the average rate. We have
corrected for this in determining the average rates in Table
4-4. Many management practices are applied over a period of years
before saturation can occur. In such cases, the average rate of carbon
storage underestimates stock changes that will occur using the approach
outlined in Figure 4-8a. In this case, we have
estimated the average change in stock once the management has come to
an approximate steady-state for carbon (Figure
4-8b).
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|
Figure 4-8. Rate of carbon gain.
|
The magnitude of change in carbon stocks for a given practice depends
on three factors: the average rate of carbon stock change per unit area
after the practice has been applied, the time required for saturation
to occur, and the total area over which the activity is applied. Figure
4-8a shows how we have defined these terms for calculation of magnitudes.
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Adoption of a proposed practice seldom depends solely on perceived effects
on the atmosphere; indeed, other benefits or disadvantages of the proposed activities
will usually outweigh any effect on atmospheric CO2 in affecting land-use decisions.
Table 4-4 shows a few of the key associated impacts
for each of the proposed activities.
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