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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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).