There are over 350 different species of sedum (Sedum spp.) and most of them can tolerate harsh conditions including very cold to hot temperatures, drought, and poor and stony soil. Sedum plants are used in rock gardens and edging flower beds, and for greening the tops of buildings, cottages, and thatched roofs. However, little is known about the effectiveness of sedum as vegetation cover in protecting soil erosion from a road embankment made of acid soil. Acid soil is believed to be vulnerable to soil erosion and is not suitable for plant growth. Liming treatment is required first before revegetation to alleviate the soil acidity; however, lime incorporation may affect the soil physical properties and, consequently, runoff and sediment generation. A rainfall simulation study was conducted to test the effectiveness of hydrated lime and artificial zeolite as amendments and Sedum sediforme (Rupestria group) as vegetation cover in controlling soil erosion from an acid soil taken from mountain cuts in Yamaguchi prefecture, Japan, where it is used for road embankment. The soil was treated with 0.5% lime and 10% zeolite. Two rainfall intensities of 30 and 60 mm/h were tested for 2 and 1 h, respectively, on sedum-growing soil plots measuring 0.50 by 0.30 by 0.05 m. Three levels of vegetation cover (bare soil, 25%, 75%) of sedum plant of 5-month growth under 2-day irrigation intervals were tested.

The incorporation of hydrated lime and artificial zeolite amendments improved wet aggregate stability, which contributed to significant decrease in surface runoff, sediment concentration, and total soil loss by rain splash from the bare soil. Zeolite was more effective in promoting plant growth than the lime treatment; as a result the decrease in sediment generation and soil loss by rain splash, compared with the control, was larger with zeolite than with lime. Under both intensities of simulated rain, the sediment concentration and total soil loss by rain splash decreased significantly (P < 0.05) with increasing surface cover. The correlation between cumulative soil loss (CSL) and cumulative surface runoff was linear and significant (P < 0.001) and the slope coefficient decreased with increasing surface cover. This suggests that the sediment carrying capacity or the erosivity of the surface runoff was constant and it decreased with increasing surface cover. The sedum cover reduced the CSL up to 72 and 79% under 30 and 60 mm/h rainfall intensities, respectively. The mean weight diameter of the soil sediment transported by runoff and soil loss by rain splash were significantly increased, and therefore, the silt and clay proportion of the crust material formed on the soil surface decreased up to 6 and 16% under 25 and 75% vegetation cover, respectively. These results demonstrate that hydrated lime and artificial zeolite could be used as amendments and sedum plant as vegetation covers in controlling soil erosion from an acid soil.