Abstract :
In Ethiopia, plantation forestry for the purpose of soil conservation and wood production is established mainly on degraded mountain slopes. These forests with a rotation age of 12–34 years consist of mainly exotic tree species; and have great potential to provide maximum wood production and economic incentives at the age of 8–20 years. 15N natural abundance in soils under a 25-year-old forest on Mt. Yegof has been used to determine if the short rotation forests result in significant inputs of N to soils and improve the degraded soils before harvest at the rotation age of 12–34 years. Since I have previously described the fractional contributions of the present forest vegetation to the build-up of soil organic matter (SOM) on Mt. Yegof, patterns of (delta)15N and (delta)13C values were compared to determine if shifts in soil 15N are related to vegetation shifts by afforestation. On Mt. Yegof, soil (delta)15N values were 0.9–3.9‰ in soil at 0–5 cm and>6‰ in soil at 30–50 cm. Plant (delta)15N values ranged from -4.6 to -0.7‰. Despite the negative (delta)15N values of the vegetation cover, the high (delta)15N values in the topsoil indicate that return of N to soils by litter-fall is minimal on Mt. Yegof and, hence, the present forests do not change very much the soil 15N signals at the surface layer. At this site, the high (delta)13C values throughout the soil profiles is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops along the entire elevation gradient. The slight shift towards lower (delta)15N values in soil at 0–5 cm indicate only a trend of 15N depleted SOM build-up through litter-fall from the present forest vegetation. This and previous studies show that the planted forests on Mt. Yegof did not result in significant changes in the inputs of C, N and SOM, and hence, cannot signify the improvement of degraded soil conditions during a 25-year forest growth. Thus, these forests made need a rotation period much longer than 25-year period to signify ecological rehabilitation and to establish a sustainable forest ecosystem with less N losses on the fairly steep slopes at Mt. Yegof. From the points of view of soil conservation and soil fertility management, it could be suggested that forest harvest at the time of maximum wood production may not necessary be considered if ecologically sustainable forests are to be managed on highly degraded mountain slopes in the Ethiopian highlands.
