Soil erosion tolerance rates in contrasting land uses and soil types in the Rwenzori Highlands of Uganda

Abstract

Soil erosion is a primary factor contributing to environmental deterioration in the 21st century. High erosion rates experienced in the Rwenzori highlands undermine the soils' capacity to sustain productivity once the soil loss tolerance threshold is surpassed. Erosion rates differ across different land uses and soil types, and how these variations relate to soil loss tolerance thresholds remains poorly understood in the region. This study, therefore, aimed to assess the soil erosion tolerance thresholds of Ferralsols and Andosols under different agricultural land uses in the upper slopes of the Rwenzori highlands. Two case studies with the quasi-experimental research design were used in this research: one case study done in Kabarole on Andosols and the other in Kasese on Ferralsols, both under annual, woodland, grasslands, and perennial land use at slopes between 25% and 27%. Soil erosion rates were estimated using the RUSLE and measured using closed runoff plots. Runoff samples were collected after every rainfall event for 12 months. Soil loss tolerance thresholds were determined from a method developed by the USDA, which uses favourable rooting depth and bio-physical soil properties (bulk density, soil erodibility, pH, organic carbon, and infiltration rate) linked to erosion. While using runoff plots, Andosols had the highest mean erosion rate in annual land use (14.21 ± 2.15 tha-1yr-1) at 26% slope and the lowest in woodlands (7.06 ± 3.78 tha-1yr-1) at 25% slope. Similarly, Ferralsols had the highest mean erosion rate in annual land use (8.50 ± 1.26 tha-1yr-1) at 26% slope, and the lowest in woodlands (4.49 ± 2.05 tha1yr-1) at 27% slope. Mean erosion rates estimated with the RUSLE were highest in annuals under Andosols at 26% slope (78.33 ± 5.36 tha-1yr-1) and were lowest in woodlands under Ferralsols at 27% slope (2.52 ± 0.26 tha-1yr-1). There are evident disparities between observed soil loss from runoff plots and estimations from the Revised Universal Soil Loss Equation (RUSLE). The average tolerance thresholds of Andosols were highest in grasslands (11.67 ± 1.29 tha-1yr-1) at 27% slope and lowest in annual (8.33 ± 1.29 tha-1yr-1) at 26% slope. For Ferralsols, the tolerance thresholds were highest (8.75 ± 1.37 tha-1yr-1) and lowest (6.25 ± 1.37 tha-1yr-1) in grasslands at 26% slope and perennials at 25% slope, respectively. Although annual land use was the only land use with soil erosion rates exceeding tolerance thresholds, grasslands, woodlands, and perennials are at risk of losing productivity due to the very small margin left for the erosion rates to exceed the tolerance thresholds. Thus, the soils in this study are vulnerable to degradation in the future. Therefore, there is a critical need to implement sustainable land management practices in order to curb degradation and prevent further soil erosion and future threshold exceedance.