Effects of tree species on soil enzyme activities in natural mixed forest and monoculture plantations in Ethiopia

Soil extracellular enzymes are crucial for belowground functioning and are sensitive to anthropogenic land use change. The potential effects of tree species on soil microbial and biochemical properties provide crucial feedbacks on mineralization, a key ecosystem function beneath the tree canopy. In the highlands of northern Ethiopia, remnants of the original Afromontane forests are largely restricted to church forests with indigenous tree species. However the impacts on potential soil enzymatic activity by conversion of those forests to monocultures for wood production is largely unknown. We investigated potential soil enzyme activities under four indigenous tree species and adjacent Eucalyptus globulus and Cupressus lusitanica plantations in Gelawdios, Amhara Regional State, Ethiopia. The potential activities of six enzymes associated with soil C, N and P cycling were measured following the fluorometrically labelled substrates techniques. All enzymes exhibited significantly higher activities in soils under the indigenous trees than the plantation species except, N-acetylglucosaminidase, that was the highest in Eucalyptus globulus soil due to the ectomycorrhizae, associated with the Eucalyptus root systems. Among the four indigenous species Apodytes dimidiata showed the lowest activitie for most of the enzymes. A stronger positive correlation was observed between enzyme activity and total N than with total C in the soil. Acid phosphatase had the highest activity followed by  β-Glucosidase (482 and 167 nmol mg^-1 microbial biomass respectively). The activities of leucine aminopeptidase, β-xylosidase, N-Acetylglucosaminidase and cellobiohydrolase in soils under indigenous trees ranged between 63-23 nmol mg^-1 microbial biomass. The species specific effects of trees on soil enzyme activities indicate strong influence of tree traits on mineralization processes.