Soil and water conservation management on hill slopes in Southwest Ethiopia. I. Effects of soil bunds on surface runoff, erosion and loss of nutrients

On the steep hill slopes of southwest Ethiopia, soil erosion may cause significant declines in soil organic carbon (SOC) and nutrients, negatively affecting cropland productivity. Soil bunds are advised as an effective means to reduce surface runoff and soil erosion. However, the effects on SOC and nutrients are rarely quantified. The objective of this study was to assess the quantitative effect of soil bunds on surface runoff as well as soil and nutrients losses from cropland in the region. Data was collected from experimental fields on three farms (fields 1, 2 and 3) in the Omo-Gibe River basin in southwest Ethiopia. On each farm, effects of soil bunds on runoff and erosion were investigated and compared with adjacent plots without soil bunds in the 2018 and 2019 growing seasons. Soil bunds effectively reduced surface runoff (by 80–92%). Without soil bunds, soil losses in the growing season were 5–22 t ha−1 in 2018 and 15–43 t ha−1 in 2019, on average removing 1.3–4 mm soil per year. Soil bunds decreased soil losses by about 96%. Observed soil losses from fields without soil bunds were well described by the Universal Soil Loss Equation (USLE; R2 = 0.92; p < 0.01). Of the total soil loss, 47–69% was removed in suspended form. Suspended material had significantly larger (p < 0.05) SOC, and plant available potassium (K) and phosphorus (P) concentrations than coarser, rapidly settling sediment and bulk soil. In 2019, up to 733 kg SOC ha−1, 77 kg total nitrogen ha−1 and 21 kg K ha−1 were lost per season from plots without soil bunds. For SOC this amounts to 6% of its stocks in the topsoil. Soil bunds are important controls on surface runoff, strongly limiting losses of SOC and nutrients in sloping croplands of southwest Ethiopia.