Influence of Vegetation Cover and Topographic Position on Water Infiltration, Organic Matter Content and Aggregate Stability of Grassland Soils in Semi-Arid Kenya

Abstract

A study was conducted in Mpala and Ilmotiok ranches in Laikipia County, Kenya, to investigate the influence of vegetation cover and topographic position on soil organic matter, bulk density, aggregate stability and water infiltration rate. Three vegetation cover types; (Tree, Grass and Bare) and four topographic positions (Hillslope, Headwater, Riparian, and Plateau) were evaluated. Soil samples were collected along the topographic positions and within the vegetation cover types at five levels of depth; 0-10, 10-20, 20-30, 30-40, 40-50 cm during the dry season May to August 2016. The samples were analyzed for soil organic matter, bulk density and soil aggregate stability. Water infiltration rate was measured in situ on the soil surface using a mini-disk Infiltrometer. 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). Soil organic matter (SOM) differed significantly at P (<0.0001) among the vegetation cover types, topographic zones and soil depth for both sites. In Ilmotiok site, the mean soil organic matter was highest (1.96%) in Hillslopes zones and lowest in Grass covered fields (0.30%). In Mpala, SOM content was highest in Tree covered fields at 2.28%, and lowest in Grass covered fields at 0.38%, for RIP and PLA zones respectively. Topographic positions and grazing management influenced soil properties in the semi-arid grasslands, hence the need for strategies in grazing management that will promote restoration of these degraded areas.