Effects of different backfill soils on artificial soil quality for cut slope revegetation: Soil structure, soil erosion, moisture retention and soil C stock

• The type of backfill soil affects the quality of the artificial soil significantly.
• Rock fragments result in poorer soil structure and hydrological properties.
• Rock fragments increase the risk of surface runoff and soil erosion.
• Agricultural soil is more appropriate backfill soil for hydroseeding.

Large-scale railway/highway construction in hilly areas in China has created a number of bare rock cut slopes. Artificial soil is often sprayed onto those slopes to promote revegetation. The artificial soil used is a mixture of backfill soil, humus, straws, soluble chemical fertilizer, composite material and plant seeds. The backfill soil is the most important component of the artificial soil. Knowledge of the changes in the artificial soil quality induced by backfill soil is important for the management of the revegetation of rock cut slopes. This study was conducted to assess the effects of different backfill soils on soil quality parameters and slope restoration parameters, including the particle size distribution, water-stable aggregates, soil organic carbon (SOC), organic carbon fractions, soil water characteristics (SWC), plant coverage, Margalef index and Shannon–Wiener diversity index. In experiment one, the treatments included slopes backfilled with rock fragment (RF), slopes backfilled with agricultural soil (AS) and a natural slope (NS). In experiment two, the treatments were experiment plots with different proportions of agricultural soil to rock fragments. Soils were sampled from the surface layer (0–5 cm) of the slopes. The results indicated that the backfill soil significantly affected the soil structure. RF increased the percentage of large particles (>0.25 mm) and decreased that of micro-particles (<0.25 mm). The fractal dimension and the water-stable aggregate content of RF were smaller than those of AS. Moreover, RF exhibited the highest soil destruction rate of soil aggregates among the three treatments. Our findings also indicate that RF conserved a smaller percentage of SOC in less degraded forms (coarse and fine particulate organic matter (POM) fractions) than AS management. At almost all of the soil suctions studied, AS retained more volumetric water content (θv) than RF or NS. The AS soil contained 17.3% more plant available water (PAW) than the RF soil. The plant coverage, Margalef index and Shannon–Winner were investigated to evaluate the restoration of the rock cut slopes compared with natural slopes. The three parameters of the RF were all smaller than those of the AS and NS. Overall, RF exhibited poorer soil structure and hydrological properties, increasing the risk of surface runoff and soil erosion because it had not undergone sufficient soil-forming processes and lacked SOC. Therefore, the use of rock fragments as backfill soil was not helpful for maintaining artificial soil with sufficiently high quality for the slope revegetation.