Background Soil and water loss is serious in the Loess Plateau. Returning Farmland to Forest (Grass) and restoring vegetation is an effective measure to prevent soil and water loss. Improper planting will increase soil moisture depletion and even dry soil layer. Alfalfa (Medicago sativa)is an important vegetation for returning farmland to grassland on the Loess Plateau. The comparison of soil moisture in six plots with different alfalfa allocation ratio provides theoretical basis for alfalfa planting rational allocation on the loess slope, and makes the soil moisture on the loess slope to be used reasonably.

Methods The experimental plots were six slope plots with different ratios of alfalfa in the loess region. From June to mid-October 2019, a neutron meter was used to measure the soil moisture content in the upper, middle and lower parts of the plots at depths of 0-300, 10 cm apart from 0-100 and 20 cm apart from 100-300 cm. Three to five days after the rain, the infiltration degree of precipitation reached the deeper soil layer, and an additional measurement was conducted. A standard rain gauge was set up in the experimental area to measure precipitation with rain gauge after rain. Excel and Origin software for data processing.

Results 1) The infiltration depth of precipitation in the artificial alfalfa field was 50-60 cm in each slope position after the precipitation was of replenishment in rainy season. 2) The water storage of the alfalfa field varied with different slope positions. The water storage on the upper slope increased by 12.40 mm, in the middle slope increased by 8.10 mm, and in the lower slope decreased by 19.40-66.11 mm. 3) Due to the influence of surface soil density, precipitation infiltration of 0-100 cm soil layer in the abandoned land gradually increased from the upper slope to the lower slope. 3) The precipitation infiltration depth of the abandoned land on the upper slope was 100-220 cm, and the water storage increased by 24.3-68.3 mm. The precipitation infiltration depth of the abandoned land in the slope was 60-120 cm, and the water storage increased by 23.9-36.9 mm. The precipitation infiltration depth of the abandoned land under the middle slope was 50-100 cm, and the water storage increased by 23.9 mm. With the increase of planting proportion of artificial alfalfa, the total soil water storage in slope plot decreased.

Conculsions With the increase of the proportion of the artificial alfalfa on the sloping land, the total water storage in the plot decreased, and the soil water could not recover under the effect of natural precipitation. Slope vegetation configuration should reduce the proportion of vegetation configuration with high water consumption. When the ratio of abandoned land to alfalfa land was 13:2, soil erosion was controlled and soil moisture was in good situation.