Publication: Effects of Vegetation Cover and Topographic Positioning on Soil Organic Carbon, Soil Aggregates and Water Infiltration Rates in Laikipia Grasslands, Kenya
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2020-11
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Abstract
Soil degradation activities in semi-arid grasslands have contributed to loss of vegetation
cover, removal of surface soil, reduced soil organic matter and instability of soil
aggregates thus reducing water infiltration rates in the soil. Soil losses are partly
influenced by human-induced practices such as grazing, bush clearing and cultivation.
The aim of this study was to investigate the influence of vegetation cover types and
topographic positioning on soil organic matter, aggregate stability and water infiltration
rates. The study was carried out in two semi-arid grasslands: Mpala and Ilmotiok ranches
in Laikipia County, Kenya. Three vegetation cover types; (Tree, Grass and Bare gounds)
and four topographic positions (Hillslope, Headwater, Riparian, and Plateau) were
evaluated. A reconnaissance survey was done to demarcate a sampling grid of 4*4 km2
area in each site. A stratified sampling design was then used to demarcate four transect
plots (Riparian-RIP, Plateau-PLA, Headwater-HW and Hillslope-HS). Within the four
plots, three subplots comprising of Grass fields, Bare grounds and Tree fields were
identified. Soils were sampled within a 5 m radius at 0-10, 10-20, 20-30, 30-40, 40-50 cm
depths for laboratory determination of organic carbon, bulk density and water stable
aggregate during the dry season of May to August 2016. Organic matter was estimated
from the organic carbon content by multiplying percent organic carbon with a factor of
1.724. Data was subjected to two-way analysis of variance using SAS statistical software.
A multiple comparison test (T-test) using LSD at P=0.05 was done to separate the means
of various soil parameters and to isolate the significant differences between each of the
vegetation cover types, topographic zones and soil depths in both sites. All statistical tests
were considered significant at the level of P < 0.05. The study revealed that soil organic
carbon and organic matter (SOM) differed significantly at (P=0.0001) among the
vegetation cover types, topographic zones and soil depth for both sites. Soil aggregate
stability varied significantly between topographic zones (P=0.0124) but not between the
vegetation cover types and soil depth in Ilmotiok site. Mpala site showed a significant
difference in aggregate stability between the topographic zones (P=0.0152). However, no
significant difference was observed in variation of aggregate stability between the
vegetation cover types and soil depth (P=0.8998; P=0.8284), respectively. In Ilmotiok
site, the highest infiltration rate was recorded in the Tree covered fields (73.3 mm/hr) and
decreased in Grass fields and Bare grounds at 25 and 17 mm/hr, respectively. The
Headwater zones had the highest infiltration rates (73.3 mm/hr) while the lowest
infiltration rates were (0 mm/hr) in the Hillslope zones. The infiltration rates in Mpala
site were highest in Bare grounds (37.8 mm/hr) and lowest in Tree fields with 5.7 mm/hr.
The Headwater zones had the highest infiltration rates followed by Hillslope zones with
(8.9 mm/hr) while the Riparian zones had the lowest infiltration rates (0.00 mm/hr).
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Karatina University