Abstract:
Isotopic composition and nutrient contents o f soils in forests, pastures and cultivated lands
were studied in Menagesha and Wendo-Genet, Ethiopia, in order to determine the effects
o f land use changes on soil organic matter, the N cycle and the supply o f other nutrients.
In the Menagesha forest, which according to historical accounts was planted in the year
1434-1468, 5 13C values at > 20 cm soil depth o f from -23 to -17%o and in the surface layers
o f from -27 to -24%o suggest that C., grasses or crops were important components o f the
past vegetation. At Wendo-Genet, the 5 I3C values in the topsoils o f from -23 to - \6% o and
in the > 20 cm of from -16 to - 14%o indicated more recent land use changes from grassland to
forest. At Menagesha, 5 I5N values shifted from -8.8%o in the litter to +6.8%o in the > 20 cm.
The low SI?N in the litter (-3%o) and topsoils (0%o) suggest a closed N cycle at Menagesha.
At Wendo-Genet. the high 5 I5N (3.4-9.8%o) and low total N concentrations suggests a more
open N cycle with greater N losses.
At Menagesha, die variation in soil nutrient contents followed the patterns o f %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 o f Ca and S. The strong
relation between %C and CEC, suggests that organic matter increases the nutrient retention
capacity o f 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
o f 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.
