Abstract: In order to explore soil grain-size characteristics under the Pinus sylvestris var. mongolica, soil samples,containing seven depths (0-200 cm), under the P. sylvestris var. mongolica were studied in three districts of Hengshan, Yuyang and Shenmu in Yulin City, Shaanxi Province. We measured the physical component of soil grain and analyzed the soil grain size parameters. Results show that:(1) Soil grain size are mainly occupied by fine sand (34.73%-44.57%) and middle sand (40.63%-50.28%), and the coarser sediment content (0.27%-1.34%) is the lowest under P. sylvestris var. mongolica plantations in the southeast Mu Us Sandy Land. (2) The mean grain size of the soil ranges from fine sand to middle sand (1.81-2.20Φ), the sorting coefficient ranges from better sorting to middle sorting (0.63-0.76), the skewness ranges from near symmetrical to positive bias (0.03-0.13), and the kurtosis is medium (0.94-1.07). (3) In space,the mean grain size of the soil at the depth of 0-200 cm becomes fine gradually and shows significant difference from the northeast to the southwest; while for other soil grain size parameters, no uniform rule exist for the soil at the depth of 0-60 cm, but the soil at the depth of 60-200 cm have a uniform and non-significant rule, which contains that the worse sorting, the increased fine particles,and the narrowed grain size frequency curve from the northeast to the southwest. In depth,there is no uniform rule for the soil grain size parameters at the depth of 0~200 cm. However, in comparison to the soil of 0-30 cm, the soil of 30-60 cm has the finer mean grain size, the worse sorting,finer particle and the narrower grain size frequency curve. (4) The interaction between the plant and the soil is significant at the depth of 0-30 cm compared with other depths.

Key words: Mu Us Sandy Land, Pinus sylvestris var.mongolica plantations, grain size