Abstract: Abstract: Soil aggregate composition and stability is closely related to soil quality, soil erosion and agricultural sustainability. In this study, forty-nine soil samples at the 0-10cm surface layer were collected from four soil groups (i.e. Ari-Sandic Primosols, Calci-Orthic Aridosols, Siltigi-Otrthic Anthrosols and Ustic Cambosols) in the marginal farmland of oasis in the middle of Hexi Corridor region. Some selected soil physical and chemical properties were determined, and the composition of dry-seived and wet sieved aggregates, dry mean weight diameter (DMWD) and percentage of aggregate destruction (PAD) were analyzed. Soil particle size distribution is dominant in sand fine sand fraction in most of soils with exception of Ustic Cambosols. Soil organic carbon concentration is 5.88±2.52g kg-1 on average, and range from 4.75 g kg-1 for Ari-Sandic Primosols to 10.51 g kg-1 for Ustic Cambosols. The soils have high Calcium carbonate（CaCO3）concentration and range from 84.7 g kg-1 to 164.8 g kg-1, which increased with soil fine particle and organic carbon content. The percentage of >0.25mm dry aggregates range from 65.2% for Ari-Sandic Primosols to 94.6% for Ustic Cambosols, and large dry aggregates (>5mm) are dominant in all soils. The DMWD averages in 3.2-5.5mm. The mass of >0.25mm water-stable aggregate is 23.8%-45.4%. PAD is 52.4%-66.8%. The distribution and characteristics of soil aggregates are favorable for controlling soil erosion by wind. However, the stability of aggregate for all soils is low and soils are prone to disperse and harden after irrigation. The mass of macro-aggregates and DMWD are significant positively correlated with soil properties, such as soil clay and silt, soil organic carbon (SOC), CaCO3, and oxides of Fe3+ and Al3+. Soil fine silt and clay, SOC and CaCO3 are important agents of aggregation in this region, and the effect of SOC and CaCO3 on aggregate stability are more significant than that of soil silt and clay. Converting cropland to alfalfa forage land can increase SOC concentration, and in turn, enhance the formation of aggregates and stability. For the marginal farmlands in this fragile ecological area, returning cropland to alfalfa grassland or performing crop-grass rotation, and applying organic manure is an effective and basic strategy to improve soil structure and quality, mitigate soil erosion by wind and enhance oasis agricultural sustainability.

Key words: Soil aggregate, Soil properties, Returning cropland to grassland, Marginal oasis, Hexi Corridor region