In order to quantitatively analyze the effects of different grazing intensities on the understory soil physical properties of natural Quercus mongolica forest and explore the evaluation system of forest soil physical properties under grazing disturbance in mountain areas of northern Hebei province.

Soil of 0-20 cm in natural Q. mongolica forest under heavy grazing (HG), moderate grazing (MG), and light grazing (LG) were collected as the research object, and the same site conditions and long-term closed treatment were taken as the control group (CK). One-way analysis of variance (ANOVA) and grey correlative degree analysis were conducted to reveal the effects of different grazing intensity on soil physical properties of natural Q. mongolica forest. Principal component analysis method, multidimensional coordinate-synthetic-evaluation method and entropy weight TOPSIS method were used to evaluate the soil physical quality under grazing disturbance.

1) The contents of coarse silt, sand grain and clay in soil increased with the increase of grazing intensity, while the contents of medium silt and fine silt decreased with the increase of grazing intensity. The effect of grazing intensity on soil fractal dimension was not significant. 2) The soil bulk density increased with the grazing intensity, and other physical factors decreased as the grazing intensity increased. There were significant differences in each soil physical factor between the heavy grazing group and other grazing groups. 3) All the three evaluation methods showed consistent ranking results: CK > LG > MG > HG.

In view of the impact of grazing on the physical properties of understory soil, it is concluded that light grazing can promote the decomposition of understory litters and improve the physical quality of understory soil, while overgrazing may cause adverse effects on understory soil. The effects of grazing on the understory soil are multi-level, so it is necessary to strengthen the long-term monitoring and in-depth study.