Mid- and long-term effects of biochar on soil improvement and soil erosion control of sloping farmland in a black soil region, China

Biochar has been widely used as a soil conditioner, but research on the mid-and long-term effects of biochar on soil structure and soil erosion is still seriously lacking. To investigate the effects of mid- and long-term applications of biochar on soil improvement and soil erosion, a 4-year experiment was carried out on a field runoff plot of 3° sloping farmland in the black soil region of Northeast China. By examining the influence of different biochar application amounts (0, 25, 50, 75, and 100 t ha−1) on annual runoff, annual soil erosion, soil water retention curves, unsaturated hydraulic conductivity and unsaturated water diffusivity over four consecutive years, the optimal biochar application rate was determined. The results showed that the biochar application amount, year and their interaction imposed significant effects on soil structure, annual runoff, annual soil erosion, and moisture characteristic parameters. When the water content exceeded a certain value (0.28 cm3 cm−3), biochar improved the soil hydraulic conductivity. biochar inhibited the horizontal diffusion of water, while Pearson correlation analysis indicated that soil structure improvement encouraged soil water holding capacity enhancement and prevented soil erosion. Based on the analysis of the soil structure and soil erosion during the four-year period, the best biochar application rate for water and soil improvement of the sloping farmlands in the black soil area was proposed, namely, biochar application at 50 t ha−1 for two consecutive years. The research results provide practical application for combat soil erosion of sloping land.