Effects of crop rotation and rainfall on water erosion on a gentle slope in the hilly loess area, China

Cultivation on steep land has long been blamed as a major contributor of water erosion in many fragile regions of the world. Soil and water loss from gentle slopes, however, are always subjectively considered less important and are even neglected in practice. In this study, 21 plots including seven crop-rotation types (CRTs) under three different slight gradients (10°, 15°, 20°), were established in Dingxi, a typical semiarid hilly loess area in China. Eight consecutive years of erosion data under different gentle-slope cultivation conditions were compared and analyzed. The most interesting and key finding is that water erosion remained far higher on slopes with gradients of less than 20° than the tolerable criterion, even when some CRT measures and field treatments (e.g., contour cultivation, stiletto, minimum tillage, and crop-shrub intercropping) were implemented. Newfield techniques targeting erosion control on gentle slopes should be developed. Secondly, compared with other crop species, potato cultivation under sloping conditions was confirmed to cause the highest soil and water loss and should be strictly forbidden at large scales. Being a major source of income for local farmers, potato plantation under terracing conditions, rather than on slopes, is strongly recommended as the first choice for achieving the double advantages of erosion control and farmer income. Thirdly, water erosion on gentle slopes was reduced significantly when different CRTs coupled with land-closure treatments (e.g., farmland abandonment, leaving artificial grassland under natural succession, and consecutive fallows) were conducted simultaneously. This result confirms that these measures are effective for conserving soil and water, and are feasible in practice. Finally, water erosion depended significantly on the timing and proportion of rainstorms in certain periods. The sensitivity of water erosion to natural rainfall, however, was also dependent on the specific surface status. In summary, a higher occurrence of rainstorms coupled with crops that have poor resistance to erosion (e.g., potato, flax, and wheat) and up–down cultivation will certainly accelerate runoff and erosion on slopes, whereas natural succession without human disturbance or appropriate CRTs with contour farming practices can markedly reduce water erosion rates.