Agriculture Reference
In-Depth Information
trees. This is followed by a 2-3-year tree fallow period during which little or no
management is required to maintain the trees. After this, the woodlot is cleared to
supply wood for household use, such as building poles, firewood, and tobacco curing.
Subsequently, crops are grown between tree stumps to benefit from the ameliorated
soil conditions (Kimaro 2009).
In Kimaro et al. (2011), two Australian acacias ( A. crassicarpa , A. mangium ), two
African acacias ( Acacia nilotica , A. polyacantha ), and one fertilizer tree ( Gliricidia )
were compared at Morogoro, Tanzania. After 5 years, all of the tree fallows devel-
oped more total SOC than the 13.0 Mg ha −1 measured under continuous maize, and
A. mangium had double the SOC of the continuous maize, probably because of
declining C in the control. The natural grass fallow comparison had 17.8 Mg ha −1
SOC, and the SOC under the tree fallows ranged from 0.89 to 1.4 times the SOC
compared with the grass fallow.
11.3.2.4 Connection with Carbon Markets
The studies cited in Table 11.4 fall broadly within the area of southern and eastern
Africa dominated by open, dry miombo woodlands, which occupy about 10% of
the African land mass (Malmer and Nyberg 2008). According to Williams et al.
(2008), soil C stocks in a Mozambican site had a narrower range (21-74 Mg C ha −1 )
in the top 0.3 m on abandoned land than in the miombo woodland soils (18-140 Mg
C ha −1 ). A growing proportion of miombo woodlands have been cut for fuelwood
and converted to smallholder agriculture, which in very few years of extractive
cropping reduces SOM and nutrients such that maize production becomes unsus-
tainable. Each of the agroforestry interventions described increases SOC, allowing
for production of food crops and fuelwood with less soil degradation. These tech-
nologies allow the restoration of some of the ecosystem services formerly provided
by the miombo forest, while increasing the provisioning services to provide for the
increasing human population resident on the land. Although these practices hold
great promise, their use in smallholder settings to generate C credits comes with
several constraints. SOM is generally lower and more variable on smallholder land
than on larger landholdings or at research sites, decreasing payments and increas-
ing monitoring costs among smallholder households. The residence time of SOC is
controversial (Davidson and Janssens 2006; Lal 2004), and studies are especially
needed in the area of belowground C cycling and GHG evolution within different
cropping systems.
Two ongoing projects operating in Kenya show the difficulties in establishing a
price at the farm level. The TIST program (TIST 2011) is paying 1 shilling per sur-
viving tree per year (similar to about $12 per year per hectare) but hopes to be able
to have a larger payment in later years. The Vi Agroforestry soil C program plans
to pay $11 per Mg of C sequestered, which it estimates may be about 2.25 Mg ha −1
during a 20-year period (which, therefore, is just over $1 per hectare per year) (Vi
Agroforestry Strategy 2013-2015). The actual price will, however, depend on the
actual C sequestered. Given these limitations, the major impetus for the promotion
and adoption of agroforestry practices remains the potential to increase food and
fuelwood productivity for smallholder households, and reduce land, watershed, and
ecosystem degradation.
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