Abstract: The study of soil surface grain size in the agro-pastoral ecotone is of scientific significance to the evaluation of soil quality and the desertification degree. Through the soil surface samples analysis in the study area, we discussed the grain size characteristics of different types of sediments and different land use types. The results showed that: (1)Sand loess, sand covered loess residues and weathering residues have the highest silt content, followed by very fine sand and fine sand. The sum of the contents of the three grain grades is shown as sand loess (96.51%) > sand covered loess residues (88.29%) > weathering residues (77.58%). The surface soil grain components of aeolian sand are mainly fine sand (53.85%) and middle sand (26.13%). (2)Soil grain size parameter results show that the average grain size from fine to coarse is sand loess 4.78Φ, sand covered loess residue 4.62Φ, weathering residue 3.80Φ, and aeolian sand 2.46Φ; the order of sorting coefficient is weathering residual, sand covered loess, sand loess, and aeolian sand; the degree of skewness is positive or very positive; the kurtosis value shows that the grain size distribution of aeolian sand (2.30) is the most concentrated. (3) In the same surface sediment, there are more coarse grains such as gravel and very coarse sand in farmland, more silt in grassland and more fine sand in woodland. In the aeolian sand, there are a little more fixed sand dunes with powder content, more semi-fixed sand dunes with fine sand content, and only coarse grains such as gravel and very coarse sand distributed in the mobile sand dunes.There were more coarse grains (0.56%-9.82%) such as gravel and very coarse sand in farmland, more silt (38.65%-60.33%) in grassland, and more very fine sand (22.46%-39.93%) in forest land. Fixed dunes have the more silt content (12.76%), and semi-fixed dunes have the most fine sand content (57.73%); the coarse particles, such as gravel and very coarse sand, are distributed only in the moving sand dunes (3.12%). (4)The fractal dimension of soil grain is as follows: sand loess (2.5242) > sand covered loess residues (2.4373) > weathering residues (2.3554) > aeolian sand (2.2815). The correlation between the fractal dimension of surface soil and grain content was significance different from that of surface sediment.

Key words: soil grain size, fractal dimension, sediment type, land use type, agro-pastoral ecotone