Effects of short-rotation Eucalyptus plantations on soil quality attributes in highly acidic soils of the central highlands of Ethiopia

摘要

In recent decades, conversion of agricultural land to short-rotation (5-10 years) Eucalyptus plantations has become a common practice in the highlands of Ethiopia. Yet, we have a poor understanding of the effect of these land conversions on soil quality attributes under acidic soil conditions. Previous studies along the same line but based merely on physico-chemical properties of soils were inconsistent and contradictory. We compared soil physical, chemical and biological properties under 5- and 10-year-old Eucalyptus plantations with adjacent grassland soils. Results revealed that soil bulk density of adjacent grassland was significantly smaller than in the two Eucalyptus plantations. Although land-use change from grassland to short-rotation Eucalyptus did not affect soil texture significantly, values of soil pH, organic carbon, total nitrogen, calcium and cation exchange capacity (CEC) values in adjacent grassland were greater at both 0-10 cm and 10-20 cm depths compared with 5- and 10-year-old Eucalyptus plantations. Available phosphorus, exchangeable potassium and magnesium were not significantly affected under the three land-use systems. Generally, no differences were observed in available phosphorus, potassium, calcium and magnesium concentrations or in CEC between the two sampling depths (0-10 cm and 10-20 cm). The microbial biomass carbon and microbial biomass nitrogen recorded in 5- and 10-year-old Eucalyptus plantations were comparable but significantly smaller than in adjacent grasslands. Kinetics parameters calculated using a first-order equation (C-t = C-o (1-e(-kt))) showed potentially mineralizable carbon (C-o) was significantly larger (P 0.001) under grassland compared with 5- and 10-year-old Eucalyptus plantations. Conversion of grassland to 5-year-old and 10-year-old Eucalyptus reduced the values of C-o by 21 and 43%, respectively. However, soil physical and chemical properties were not adversely affected by age of Eucalyptus over a 5-year period. It is concluded that Eucalyptus plantations degrade soil ecosystem functioning and environmental sustainability compared with grassland.