Assessment of soil carbon and pH under forest stands and adjacent farmlands in a micro-tropical forest in Machakos County

Abstract

This study evaluated the soil total organic carbon (TOC) and pH within Iveti forest, Machakos Kenya and the adjacent farmlands to determine variability among forest stands and adjacent agricultural fields. A total of 39 sampling points was established in the farmlands and in the forest along a line transect and at intervals of 200 meters and soil pH and carbon determined for each point. Forest soil had higher soil TOC and pH than the adjacent farmlands at 18.8 - 5.1 mg C/ha and 7.78-1.51 mg C/ha respectively. An increased TOC from the northern upper part of the forest to a peak in sampling points near the middle of the forest was observed while pH in soils at the inner parts of the forest was higher than that at the forest edges. Within the forest ecosystem, Pinus patula stands had significantly (P < 0.05) high soil TOC while Cupressus lusitanica stands contained the highest pH. The open forest canopy soils contained significantly higher TOC and pH. In farmlands, soil total organic carbon and pH varied depending on crops or types of trees planted. The Grevillea robusta stands had the highest soil TOC and pH followed by Eucalyptus trees stand, coffee farms and vegetable fields. Introduction of exotic tree species in plantation forests also invariably affected the soil organic carbon and pH. There is a need to foster management of soils both within the micro-forests and in the adjacent farmlands to decrease soil degradation and enhance soil quality.