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Abstract

Isotopic composition 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 nutrients.
In the Menagesha forest, which according to historical accounts was planted in the year 1434-1468, 5I3C values at > 20 cm soil depth of from -23 to -17%o and in the surface layers of from -27 to -24%o suggest that C4 grasses or crops were important components of the past vegetation. At Wendo-Genet, the 5'3C values in the topsoils of from -23 to -16%o and in the > 20 cm of from -16 to -14%o indicated more recent land use changes from grassland to forest. At Menagesha, 5I5N values shifted from -8.8%o in the litter to +6.8%o in the > 20 cm. The low 5I5N in the litter (-3%o) and topsoils (0%o) suggest a closed N cycle at Menagesha. At Wendo-Genet, the high 5I5N (3.4-9.8%o) and low total N concentrations suggests a more open N cycle with greater N losses.
At Menagesha, the variation in soil 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 CEC, suggests that organic matter increases the nutrient retention capacity of these soils. Exchangeable and total Ca were strongly related (r2 = 0.95, P < 0.001). It is suggested that the presence of forests in this otherwise bare landscape leads to interception of base cations in dust, which can 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.