Abstract
Eshetu, Z. Forest soils of Ethiopian highlands: Their characteristics in relation to site history, studies based on stable isotopes. Doctor's dissertation. Issn 1401-6230, ISBN 91-576-5881-1.

Isotopic composition of and nutrient contents of soils in forests, pastures and cultivated lands were studied in Menagesha and Wendo-Genet, Ethiopia, in order to determine the effects of land use changes on soil organic matter, the N cycle and the supply of other nutrient.

In the Menagesha forest, which according to historical accounts was planted in the year 1434-1468, 13C values at > 20 cm soil depth of from -23 t0 -17 per mil and in the surface layers of from -27 to -24 per mil suggest that C4 grasses or crops were important components of the past vegetation. At Wendo-Genet, the 13C values in the topsoil of from -23 to -16 per mill and in the > 20 cm of from -16 to -14 per mil indicated more recent land use changes from grassland to forest. At Menagesha, 15N values shifted from -8.8in the litter layer to +6.8 per mil in the >20 cm. The low 15N in the litter layer (-3 per mil) and topsoil (0 per mill) suggested a closed N cycle at Menagesha. At Wendo-Genet, the high 15N (3.4-9.8 per mil) and low total N concentrations suggests a more open N cycle with greater N losses.

At Menagesha, the variation in soils nutrient contents followed the patterns of %C and %N. At the mid-altitudes, where there had been undisturbed forest cover for > 500 years, %N and %C were higher and the surface layers showed high accumulation of Ca and S. The strong relation between %C and total cation exchange capacity (CECt) suggests that organic matter increases the nutrient retention capacity of these soils. Exchangeable and total Ca were strongly related (r^2 + 0.95, p < 0.001). It is suggested that the presence of forests in this otherwise bar landscape leads to interception of base cations in dust, which cal help to sustain a productive forest. The studies show that the approach to combine stable isotopes with nutrient elements is especially useful when studying the chemical properties of forest soils in relation to site history. They also show that productive forests with a high soil organic matter content can be established on fairly steep slopes in the Ethiopian highlands.

Key words: closed and open N cycle, elevational transect, Ethiopia, forest soils, land use, site history, soils chemistry, stable isotopes
Distribution: Swedish University of Agricultural Sciences, Department of Forest Ecology, SE-901 83 Umeå.
Current author's address: Department of Botany University of Cape Town 7701 Rondebosch, South Africa.