Agriculture Reference
In-Depth Information
Controlled grazing management has been recognized as one of the strategies to
enhance SOC sequestration and preservation; however, empirical evidence is sparse.
Aynekulu et al. (2014) investigated effects of long-term (14-36 years) livestock exclo-
sures on SOC in the semiarid savanna of southern Ethiopia and found no significant
(P > 0.05) differences between exclosures and open-grazed rangelands across three
age categories and two soil depths. The age chronosequence further suggested no
significant changes in SOC content with increasing duration of exclosures (Aynekulu
et al. 2014).
Milchunas and Lauenroth (1993) reviewed reports from 34 studies involving
grazed and ungrazed sites around the world, and reported both decreased (40%) and
increased (60%) SOC as result of grazing exclusion. Other studies of grazed lands
worldwide have also shown both increases (e.g., Schuman et al. 1999; Reid et al.
2004) and decreases (Derner et al. 1997; Yong-Zhong et al. 2005) in SOC storage
and accumulation compared with adjacent ungrazed soils.
The inconclusive evidence indicates the need to further study the effect of live-
stock grazing on SOC stocks before deciding to implement broad-scale SOC seques-
tration strategies in African rangelands. In East African rangelands, semiprivate
exclosures are extensively practiced by pastoralists to put aside a fodder bank for use
during the dry season when grazing resources are in short supply. Here, exclosures
may be defined as an area of land that is enclosed by a fence to prevent grazing and/
or browsing by livestock and restore vegetation resources (Coppock 1994; Aerts et al.
2009). Calves and sick/weak animals are allowed to graze inside exclosures for 3-4
months depending on the length of the dry season.
The Borana rangelands in southern Ethiopia comprise extensive grazing lands
with indigenous knowledge on natural resource management (Homann et al. 2008).
For the last four decades, however, the Borana rangelands have undergone substan-
tial reduction in grassland cover due to bush encroachment, expansion of cultivation,
and increased settlements (Dalle et al. 2006; Solomon et al. 2007; Angassa and Oba,
2008), with negative consequences on the livelihood of the local communities.
The conversion of the savanna rangelands of Borana into bushland, exclosures,
and cropland began in the 1970s, with a peak expansion of bush encroachment and
crop cultivation in the 1980s (Angassa and Oba 2008). During that time, about 40%
of the Borana rangelands directly shifted to bush encroachment (Coppock 1994),
while the communal rangelands were further shrunk by the expansion of cropland,
exclosures, and ranches (Angassa and Oba 2008), leading to large declines in valu-
able perennial grass species and grass biomass (Angassa et al. 2012).
Widespread bush encroachment and increased grazing pressure around water
points and settlements, as well as intensified cultivation, have resulted in a loss of bio-
diversity (Oba et al. 2000), altered nutrient cycle (Angassa et al. 2012), and reduced
resilience of the ecosystem. Although not quantified to date, changes in land use may
greatly affect carbon and GHG emissions, as savanna grasslands are believed to have
the potential to restore carbon and avoid emissions (Guo and Gifford 2002).
In response to the shifts in patterns of land use, many villages in the Borana pla-
teau have set aside some of their lands that have been degraded by high densities of
livestock to manage as enclosures. Angassa et al. (2010) indicated that herbaceous
biomass and grass basal cover were significantly greater in the zones protected
