Soil and nutrient losses from Mabira Forest

Abstract

Little information is available on the effect of forest degradation and restoration of Mabira Forest Reserve on the soil physical and chemical parameters. Earlier studies concentrated on biodiversity with limited attention to soil properties. This study was conducted in Mabira Forest Reserve to establish the effect of forest degradation and restoration on; runoff, soil and nutrient losses. Six forest regimes at different ages of restoration were selected for the study depending on period of last disturbance namely: Recently restoring, 0-3 years; Young secondary forest, 10-20 years; Old secondary forest, 20-30 years; Very old secondary, 30-40 years ago; Almost intact secondary forest, 40-50 years; and Old forest, over 55 years ago. In each forest regime, three erosion trap plots each measuring 2 x 20 m located 50 m apart were installed for the determination of runoff, soil and nutrient losses. Runoff was measured per rainfall event using a measuring cylinder. Event runoff, soil and nutrient (nitrogen, phosphorus and potassium) losses were seasonally and annually aggregated. Other parameters measured were; infiltration rate, bulky density, hydraulic conductivity, rainfall and soil profile properties. Infiltration was determined using the double ring method at locations 2 m besides the erosion plots while bulk density and hydraulic conductivity were determined from core samples taken 2 m away from the erosion plots at two soil depth of 0-15 and 15-30 cm. Chemical site characterization was done from soil auger samples which were taken 2 m away from the upper and lower ends of the erosion plots. Annual runoff and soil loss were very low ranging between 20-160 m3ha-1and 10 -380 kg ha-1 respectively. Annual nitrogen, phosphorus and potassium losses varied from 0.11-4.26, 0.01-0.18 and 0.08-6.63 kg ha-1 respectively. Excluding the 0-3 year forest regime which had the highest runoff, soil loss increased with forest age. Runoff, soil, NPK nutrient losses and infiltration rate were highest in 0-3 yr regime and least in the 20-30 yr regime. Bulk density and hydraulic conductivity were highest in the 30-40 yr regime. The textural classes ranged between sandy clay loams to sandy clay. Chemical properties varied widely between the regimes. Runoff (20-170 cum/ha), soil (0.01 – 0.55 ton/ha) and nutrient losses were low compared to values reported under agricultural land. Generally, Mabira Forest has favourable hydrological properties favouring vertical water flow. Longer term and watershed studies are recommended to assess the hydrological importance of Mabira Forest ecosystem.