Abstract

Tree and soil interactions may result in changes in soil carbon and nutrient contents. Forest plantations made up of monodominant stands of 17 different species, some native and some exotic to Rwanda, as well as a stand with mixed native tree species were investigated. Biosequential sampling was used followed by basic soil chemical analyses. Results revealed that the plantation species composition influenced the soil chemical properties. Total soil C and N, C: N ratio, available P, pH, and cation exchange capacity (CEC) differed significantly between plantation stands of different species (P < 0.001, N = 54).  Increases in the levels of soil C, total N, CEC and base saturation (BS) were observed mainly in mixed native species (MNS), Polyscia fulva, Casuarina equisetifolia andEucalyptus saligna. The pH declined slightly in soil beneath some Eucalyptusspecies treatments and increased in others. The high nutrient uptake by fast-growing trees and the acidic parent material were involved in the acidification process. The findings suggest that the species used in afforestation maintain soil fertility and protect the environment. It is recommended that afforestation of abandoned and less productive lands in Rwanda should utilize fast growingEucalyptus species in combination with agroforestry and native species in order to maintain or improve soil chemical properties.

Key words: Afforestation, base saturation (BS), cation exchange capacity (CEC)forest plantation, soil carbon, total N, Rwanda