Thesis
Effects of land use changes on soil quality and native flora degradation and restoration in the highlands of Ethiopia [2004]
Lemenih, Mulugeta;
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Land degradation is threatening biological resources and agricultural productivity, the mainstay of the economy in Ethiopia. Ensuring sustainable food and biomass supply while maintaining ecological integrity in Ethiopia requires two imperative efforts: (i) the sustainable use of productive land resources, and (ii) effective regeneration of degraded ecosystems. This thesis aims to (i) identify trends in soil quality and native flora degradation due to deforestation and subsequent cultivation using a chronosequence of farm fields converted from a tropical dry Afromontane forest; and (ii) investigate the possibilities for restoration of soil quality and native flora on degraded sites with the help of reforestation. The studies were conducted near and in the Munessa-Shashamane forest, which is located on the eastern escarpment of the Central Ethiopian Rift Valley. The results showed that following deforestation and subsequent cultivation, soil organic carbon (SOC) and total N declined exponentially in the 0-10 cm layer of the soil. In the same soil layer, analysis based on 13C natural abundance revealed that SOC of forest origin was declining by 740 kg C ha-1 yr-1, while addition to the SOC from agricultural crops was about 240 kg C ha-1 yr-1. The imbalance in SOC addition from the crops and loss of SOC of forest origin has led to the continuous decline of SOC in the bulk soil by 500 kg C ha-1 yr-1. The loss of N from the surface soil of the farm fields was 6
6 kg N ha-1 yr-1 as compared with the fertilizer application rate of 35 kg N ha-1 yr-1 in the area, which, however, is seldom applied due to economic constraints for the farmers. Soil bulk density increased and pore space decreased progressively in the 0-10 and 10-20 cm soil layers with increasing cultivation period after deforestation. Other soil proprieties such as available P and K, exchangeable K, Ca and Mg, BS and CEC also changed significantly but at a slow rate. Most of the significant changes were limited to the top 0-10 cm layer. At the present level of management, the soils of the study area can be used for 25-30 years without loss of productivity. This 25-30 year period of sustainable use is much longer than most reports for tropical soils subject to similar land use changes, and this was attributed to the volcanic nature of the soils and the traditional low intensity tillage practice coupled with the parkland agroforestry used in the farming systems investigated. It was also observed that as tillage intensity shifts from the traditional low tillage to high intensity mechanized tillage the rate of soil degradation increases, which may reduce the period of sustainable use of the deforested sites. Deforestation and subsequent cultivation of the tropical dry Afromontane forest investigated also endangered the native forest biodiversity, not only through the outright loss of habitat but also by deteriorating the soil seed banks. The results showed that the contribution of woody species to the soil seed flora declined from 5.7% after 7 years to nil after 53 years of continuous cultivation. However, soil quality and native flora degradation are reversible through reforestation. Reforestation of bandoned farm fields with fastgrowing tree species was shown to restore soil quality. Tree plantations established on degraded sites also fostered the recolonization of diverse native forest flora under their canopies. An important result from studying the effects of reforestation is that good silviculture, particularly selection of appropriate tree species, can significantly affect the rate and magnitude of both soil quality and biodiversity restoration processes.
