The effects of raindrop impact and runoff detachment on hillslope soil erosion and soil aggregate loss in the Mollisol region of Northeast China

Soil aggregates profoundly influence soil fertility and soil erosion. A large number of studies have showed that soil aggregate loss was mainly affected by raindrop impact and runoff detachment during hillslope erosion process; however, few attempts have been made to investigate which one plays the dominant role in soil aggregate loss. Therefore, a laboratory study was conducted to quantify the effects of raindrop impact and runoff detachment on soil erosion and soil aggregate loss during hillslope erosion processes. A soil pan (8 m long, 1.5 m wide, and 0.6 m deep and with an adjustable slope gradient of 0–35°) was subjected to rainfall simulation experiments under two soil surface conditions: with and without raindrop impact through placing nylon net over soil pan. Two rainfall intensities (50 and 100 mm h−1) of representative erosive rainfall and two slope gradients (5 and 10°) in the Mollisol region of Northeast China were subjected to two soil surface conditions. The results showed that raindrop impact played the dominant role in hillslope soil erosion and soil aggregate loss. Soil loss caused by raindrop impact was 3.6–19.8 times higher than that caused by runoff detachment. The contributions of raindrop impact to hillslope soil erosion were 78.3% to 95.2%. As rainfall intensity and slope gradient increased, soil loss caused by raindrop impact and runoff detachment both increased. The loss of each size aggregate was greatly reduced by 46.6–99.4% after eliminating raindrop impact. Meanwhile, the contributions of raindrop impact to the >2, 1–2, 0.5–1, 0.25–0.5 and <0.25 mm soil aggregate loss were 79.1% to 89.7%. Eliminating raindrop impact reduced rainfall intensity effect and increased slope gradient impact on aggregate loss.