Determining Ecological Equivalence of Biodiversity Offset for Tropical Forests using Soil Physical and Chemical Properties

Abstract

Ecological restoration of degraded forest ecosystems is critical for maintaining environmental quality, forest productivity, biodiversity conservation, and provision of ecosystem services. Soil chemical and physical properties are key indicators of forest ecosystem performance during restoration. However, the impacts of forest restoration on soil physical and chemical properties remain under-investigated. We assessed soil physical and chemical properties at the impact and biodiversity offset sites in Gangu and Mabira Forest Reserves in central Uganda following the establishment of a high-voltage power transmission line that involved forest clearance and setting up of a biodiversity offset site. Soil samples were collected from the sample plots at the impact and offset sites in Gangu and Mabira Forest Reserves at 0-15 cm and at 15-30 cm. Surface litter at each sampling spot was removed, and soil was extracted from 0-15 cm and 15-30 cm depths using the augur. The soil samples were obtained from four study sites and analysed for pH, organic matter (OM), total nitrogen, available phosphorus, exchangeable potassium, calcium, magnesium, soil moisture, and soil texture (sand, clay and silt). Data on soil properties were tested for homogeneity of variance and normality assumptions using Levene’s test and the Shapiro-Wilk test, respectively and subjected to analysis of variance (ANOVA). In addition, data were subjected to an independent-samples t-test to compare soil properties at the impact and offset sites and visualised in R software. The following hypotheses were tested: Ho1: There is no significant difference in the soil pH between the impact and offset sites; Ho2: There is no significant difference in the soil organic matter between the impact and offset sites; Ho3: There is no significant difference in the soil nutrient composition between the impact and offset site; Ho4: There is no significant difference in the soil texture between the impact and offset sites; Ho5: There is no significant difference in the soil moisture content between the impact and offset sites. Results revealed significant differences in the soil physical and chemical properties between the offset and impact sites, suggesting that the offset and impact sites were ecologically different from a soil perspective. The pH of soils at the impact and offset sites in Mabira Forest Reserve was significantly different (ρ < 0.001). There was a significant difference in soil organic matter at the impact and offset sites in Gangu Forest Reserve (ρ = 0.034). There was significant difference in phosphorus (ρ =0.033), potassium (ρ =0.04), magnesium (ρ = 0.044), sand (ρ =0.001), clay (ρ =0.02) and silt (ρ =0.017) between the impact and offset sites in Gangu Forest Reserve while in Mabira Forest Reserve there was significant difference in potassium (ρ =0.002), calcium (ρ =0.04), magnesium (ρ =0.04), sand (ρ =0.01) and clay (ρ =0.049) between the impact and offset sites. The soil properties reported in this study provide indicators that can deepen understanding of the ecological equivalence of biodiversity offset. Future research should examine soil biological properties in order to broaden practical knowledge of the concept of ecological equivalence for biodiversity offset based on soil characteristics of the impact and offset sites for ecological restoration of degraded sites.