Long-Term Effects of Forest Plantation Species on Chemical Soil Properties in Southern Rwanda

peer reviewedUnderstanding the long-term effects of tree species on soil properties is crucial for the development of forest restoration policies in relation to the choice of species that meet both environmental and local livelihood needs. This study was performed in the Arboretum of Ruhande, Southern Rwanda, where monocultures of 148 deciduous and 56 conifer species have been established in 0.25 ha replicated plots from 1933 onwards. We investigated the effects of six exotic and two native tree species planted in monoculture plots and native species mixed within one self-regenerated plot on soil properties in two layers (0–5 cm and 5–10 cm depth). We measured general soil properties (pH, SOM, exchangeable base cations) and water-soluble C and N as a proxy for soil functioning. Changes in soil properties were observed in the upper soil layer for all tree species. Planting Eucalyptus species caused soil acidification, whereas soil exchangeable cations and pH were higher under native species (Entandrophragma excelsum and Polyschias fulva) and mixed native species. The effects of tree species were more pronounced for hot water-extractable C and N than for other soil properties. Their analyses could be used for detecting changes in soil functioning linked to vegetation types

Effects of forest restoration tree species on soil properties in Southern Rwanda

Abstract Effects of forest restoration tree species on soil properties in Southern Rwanda 1,2 Peter Rwibasira*, 2Francois Xavier Naramabuye, 2Donat Nsabimana, and 1Monique Carnol 1University of Liège, Belgium – Plant and Microbial Ecology; 2University of Rwanda, Rwanda. *Corresponding Author: [email protected] Understanding the effects of tree species on soil properties is primordial for the development of forest restoration policies regarding the choice of species that will meet both environmental and local livelihood needs. We investigated the effects of exotic and native tree species, planted in 27 plots (31-82 years old), on selected soil properties (pH, SOM, water-extractable C and N) and soil exchangeable base cations (EBC) at two soil layers (0-5 cm and 5-10 cm) in the arboretum of Ruhande, Southern Rwanda. Given that trees were planted on one site with similar land-use history, climatic conditions, parent material, and soil type, we expect current differences in soil characteristics to reflect the influence of the tree species. The effects of tree species were most apparent in the upper thin layer (0-5 cm) and the values of parameters were higher in this layer compared to the lower soil layer. The influence of tree species at 5-10 cm depth, was less marked and the values were lower for all measured parameters except for Al3+ and Fe2+ concentration. Eucalyptus species stands had the lowest soil pH (3.7) and mixed native species had the highest soil pH (5.8) as well as the highest exchangeable base cations (EBC) at both 0-5 cm depth (63±1.4 cmol (+)/kg) and 5-10 cm depth (9.3±0.2 cmol (+)/kg). Water extractable C and N fractions were higher at the upper soil layer compared to the lower layer. At 0-5cm, hot and cold water-extractable C and N fractions were positively correlated to soil organic matter (SOM) and negatively correlated to pH, but no significant correlation was observed at 5-10 cm. Our results showed that: (i) the two soil layers (0-5 cm and 5-10 cm depths) which were visibly undistinguishable under most of the species significantly differed in soil properties. This highlights the importance of the upper thin layer (0-5 cm depth) in tropical soils, which are generally poor and rely heavily on internal nutrient cycling through rapid decomposition of above-and belowground litter from vegetation (ii) Eucalyptus had negative consequences on soil pH and EBC, whereas soil under mixed natives had higher pH and EBC (iii) The sensitivity of water-extractable C and N fractions to detect tree species effects and their correlation with SOM indicates that these fractions could be used as substitutes to SOM analysis. Keywords: Eucalyptus; organic matter; water-extractable C and N; exotic tree species; soil nutrients; soil quality; Ruhande ArboretumARES - Rwanda (Result1

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

Harmonic analysis of exponential nonlinear devices and validity of THD in judging nonlinearities

Derivations of generalized closed-form harmonic equations for a family of polynomial-approximated and parameterized exponential nonlinear devices (NLDs) are presented. The application of this research is for nonlinear signal processing based psychoacoustic bass enhancement systems. The derived harmonic equations are used to compute THD scores analytically to show that even though the nonlinear curves are different, which may cause different perceptual effects, the THD scores turn out to be exactly the same for all six exponential NLDs. The insights gained from this mathematical analysis indicate that, even without linking to perceptual attributes such as audio quality or nonlinear distortion perception, THD is not a suitable metric to judge or measure the quantitative degrees of nonlinear curves