Abstract: This study evaluated the particle size distributions of mechanically stable and water stable aggregates in the 0-40 cm soil depth in different categories of desert steppe grassland in the Haba Lake Nature Reserve in Yanchi County, Ningxia. Potentially desertified grassland, lightly desertified grassland, moderately desertified grassland and severely desertified grassland were studied. The stability characteristics of soil aggregates were evaluated using aggregate destruction rate (PAD), mean weight diameter (MWD), geometric mean diameter (GMD), fractal dimension and soil erosion factor data. It was found that the mechanically stable aggregates in the 0-20 cm soil layer and the water stable aggregates in the 0-40 cm soil depth of each grassland desertification category were dominated by <0.25 mm micro-aggregates with the percentage of these being 45.78%-88.36% and 48.05%-97.79% respectively. The <0.25 mm mechanically stable and water stable micro-aggregate contents, PAD, fractal dimension and erosion factor in the 0-40 cm soil depth all increased with increasing grassland desertification, while the MWD and GMD gradually decreased, and the soil structure was variable. The soil stability increased with depth in the soil profile, especially in the potentially and lightly desertified grassland, which indicated that the land desertification in this grassland mainly occurs in the upper 20 cm of the soil profile. The stability of soil aggregates was positively correlated with organic carbon content, total nitrogen content (P<0.01), and silt and clay particles (P<0.05), and negatively correlated with soil bulk density (P<0.01). With the erosion of fine soil particles and transportation of nutrients such as carbon and nitrogen in the process of grassland desertification in Yanchi County, soil structure and physical stability is reduced.

Key words: desertification grassland, mechanical stable aggregates, water stable aggregates, aggregate stability