Effects of Biochar on Sediment Transport and Rill Erosion after Two Consecutive Years of Seasonal Freezing and Thawing

This research explored the effects of biochar on slope runoff and sediment transport processes and the hydrodynamic mechanism of rill erosion under the seasonal freeze-thaw climate in the black soil area of Northeast China. The four slopes of 1.8, 3.6, 5.4 and 7.2 degrees were set, corn straw biochar was used, and three biochar contents of 0 kg m(-2) (B0 treatment), 6 kg m(-2) (B6 treatment) and 12 kg m(-2) (B12 treatment) were applied. The experimental plot was placed outdoors to simulate the freeze-thaw cycle of sloping farmland under natural conditions. Three artificial simulated rainfall tests were carried out before the end of seasonal freeze-thaw cycles and spring sowing date (May) in 2018 and 2019. The sediment transport process of runoff and the variation of hydrodynamic parameters in rills were analyzed under one and two seasons of freezing and thawing in natural outdoor conditions. The results show that biochar has a positive effect on reducing rainfall runoff and soil loss after one year and two years of seasonal freezing and thawing. The effect of biochar on the sediment concentration of slope runoff increased with increasing application time; in the second year, the B6 and B12 treatments reduced the sediment concentration by 5.5-14.8% and 3.3-13.6%, respectively, compared with the values of the first year. The Reynolds number (Re) in the rill flow after the B6 and B12 treatments decreased with increasing duration, which effectively reduced the turbulence degree of the flow on the rill of the slope. With the increase in duration, the rill critical erosion power increased; in 2018 and 2019, the critical shear force, critical runoff power and critical unit runoff power were 0.403 Pa, 0.098 m s(-1), and 0.002 N m(-1) and 0.497 Pa, 0.124 m s(-1), and 0.003 N m(-1), respectively. This result indicates that increasing the duration and number of seasonal freeze-thaws can promote the development of biochar control of the runoff and sediment processes on slope and rill development.