Agriculture Reference
In-Depth Information
decline is related to cropping intensity on
shrinking smallholder farms (less than
1
ha) and to the limited use of organic and
inorganic fertilizers (Lekasi
et al
., 2005). In
Kenya's semi-arid regions like the Kiboko
Makindu area in eastern Kenya, farmers do
not apply fertilizers and manure to maize
because of the unreliable rainfall and also
the assumption that their soils are fertile
enough. Nutrient depletion through crop
cultivation and soil erosion in sloping
areas results in substantial decline in SOM
content.
moisture, improves the moisture retention
and release characteristics of soil and pro-
tects the soil against erosion (Batjes and
Sombroek, 1997). Decomposing organic
matter releases nutrients, such as nitrogen
(N), phosphorus (P), sulfur (S) and potas-
sium (K), essential for plant and microbial
growth.
Status of Soil Carbon
Soils in West Africa are poorly endowed
when it comes to soil fertility. Unlike, for
example, the Rift Valley area of East Africa,
West African soils never enjoyed volcanic
rejuvenation (Bationo
et al
., 1998). In the
equatorial forest zone with higher rainfall,
abundant moisture favours high biomass
production, which in turn brings about
higher SOC (~24.5 g kg
-
1
organic C) and ni-
trogen contents. In the Sudan savannah,
organic carbon (~3.3-6.8 g kg
-1
) and total ni-
trogen are very low, because of low biomass
production and high rates of decompos-
ition. With kaolinite being the main clay
type, the cation exchange capacity of the
soils in this region are often less than
1 cmol kg
-
1
,
depending on the level of SOC
(Bationo
et al
., 2005). There is a rapid de-
cline of SOC levels with continuous cultiva-
tion. For sandy soils, the average annual
losses may be as high as 4.7%, whereas with
sandy loam soils, losses are lower, with an
average of 2%.
Estimates of SOC stocks and changes
made for Kenya using the Global Environmen-
tal Facility Soil Organic Carbon (GEFSOC)
Modelling System indicated soil C stocks
of 1.4-2.0 Pg (0-20 cm) (Kamoni
et al
.,
2007), which compared well with a soil and
terrain (SOTER)-based approach that esti-
mated ~1.8-2.0 Pg (0-30 cm) (Batjes, 2004).
In 1990, 48% of the country had SOC stocks
of <
18
t C ha
-
1
and 20% had SOC stocks of
18-30
t C ha
-
1
,
whereas in 2000, 56% of the
country had SOC stocks of <
18
t C ha
-
1
and
31% had SOC stocks of
18-30
t C ha
-
1
.
Con-
version of natural vegetation to annual
crops led to the greatest soil C losses. All
three methods involved in the GEFSOC
Role of Organic Carbon in
Sustainable Land Management
Organic matter is of great importance in
soil, because it impacts on the physical,
chemical and biological properties of soils
(Bationo
et al
., 2013). Physically, it pro-
motes aggregate stability, water infiltration,
percolation and retention. It impacts on soil
chemistry by increasing cation exchange
capacity, soil buffer capacity and nutrient
supply. Biologically, it stimulates the activ-
ity and diversity of organisms in the soil.
The global carbon cycle involves the ex-
change of CO
2
between the atmosphere and
the biosphere, apart from oceans. Plants fix
CO
2
from the atmosphere during photosyn-
thesis to produce organic matter, which is
stored above and below ground. The bulk of
the biomass in above- and belowground
plant parts is eventually transferred to the
dead organic matter pool, or is oxidized or
burnt. Dead organic matter, which consists
of deadwood (standing as well as fallen)
and litter, is either decomposed or oxidized,
or stored for longer periods above or below
the ground as detritus. CO
2
fixed by plants
ends up in soil as organic matter or finer
forms as humus through the decomposition
process. Thus, CO
2
removed from the atmos-
phere is stored as dead and living biomass
or soil carbon in the biosphere. Sustainable
land management practices enhance carbon
sequestration and sustain agricultural prod-
uctivity, thus mitigating against climate
change. SOM has a stabilizing effect on soil
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