Soil carbon stocks and turnovers in various vegetation types and arable lands along an elevation gradient in southern Ethiopia

Soil carbon (C) and total N stocks and turnovers were investigated in five vegetation types and following deforestation and conversion of each vegetation types into arable lands along a 37-km elevation transect in southern highlands of Ethiopia. The elevation transect spanned five different eco-climatic zones from semiarid to cool sub-Afroalpine range, each with different vegetation type. Soil C and total N stocks in the upper 0.60 m mineral soil under the natural vegetations varied from 40.3 Mg C ha−1 and 5.3 Mg N ha−1 at the semiarid Acacia woodland (AWL) eco-climatic zone to 234.6 Mg C ha−1 and 20.2 Mg N ha−1 at the humid Podocarpus falcatus forest (PFF) eco-climatic zone, respectively. This trend was directly proportional to the mean annual precipitation and inversely proportional to the mean annual temperature prevailing along the elevation gradient. The soils of the farmlands had significantly lower soil C and total N stocks than the soils under the natural vegetations. Losses of soil C and total N from the upper 0–10 cm soil depth following conversion of the natural vegetations to farmlands were highest at the humid PFF eco-climatic zone and lowest at the semiarid AWL eco-climatic zone. The average rates of soil C losses ranged between 2.0% and 3.0% per annum in the sub-humid to humid eco-climatic zones and 0.5–1.0% per annum in the semiarid lowland or the cool sub-Afroalpine eco-climatic zones. The results revealed the existence of considerable differences, as large as 191.7 Mg C ha−1, in soil C stocks along the elevation gradient, and wide range of differences in the rate and amount of soil C and total N losses following conversion of natural vegetations into arable lands.