Soil microbial properties and soil nutrients content under exotic and native tree species in Southern Rwanda

Erosion has been ranked as the primary cause of soil degradation in Rwanda, a land that has naturally inherited very acidic soils with low contents of organic matter. With its hilly landscape and heavy rainfall, Rwanda loses annually millions of tons of its fertile soil from unprotected slope hills.
Forests plantation seems to be an efficient option, not only for land protection and restoration, but also as a timber and household energy source. Tree species may influence soil quality and soil microbial activity via litter decomposition and root exudation. Although most of these introduced fast growing exotic tree species are now scattered all over the country, their effects on soil microbial processes is unknown. A 200 ha arboretum of Ruhande with various exotic and native tree species was used in this study to assess the impacts of tree species on soil properties. The aims of this study were to investigate (i) the effects of tree species on soil physico-chemical properties, (ii) compare microbial processes under different tree species.

Tree species were selected based on their importance, adaptability throughout the country and relevance to daily use. Soil was sampled in 3 plot replicates per species (grouped into exotic (Exot), agroforestry (Agro), native (Nat) and mixed native species (MNS)) and in each sample 2 horizons (organic and mineral) were taken separately. Samples were analysed for physico-chemical properties (pH, moisture, organic matter content, exchangeable cations) and microbial properties (net N mineralization, potential bacterial and archaeal nitrification, respiration potential, microbial biomass C and N, metabolic quotient).
We observed a higher nutrient content in the thin organic horizon: Ca2+= 5215.3 vs 2396.8 (MNS), 3242.9 vs 507.5 (Exot) mg kg-1 in organic versus mineral respectively. Mean values of pH were 5.1 vs 4.2 (Agro), 5.3 vs 4.7 (MNS), 4.4 vs 3.8 (Exot), and 5.4 vs 4.3 (Nat) in organic versus mineral respectively. Mean values for soil microbial biomass carbon were 1065.2 vs 326.4 (Agro), 1733.4 vs 490.6 (MNS), 1638.7 vs 271.5 (Exot), and 1463.3 vs 267.4 (Nat) µg C g-1 of soil in organic versus mineral horizon respectively. Preliminary analyses indicate higher soil microbial activities and alleviation of soil acidity under native tree species compared to the exotic species.