Abstract: This research studied soil properties in different stages of grassland ecological restoration in a karst area. Specifically, the size distribution of soil particles, and the fractal dimension of soil aggregates and micro-aggregates were evaluated for farm cropland (FC), land in conversion from cropland to grassland (CCG), grassland (G), and forest and grass intercropped land (FGI), in a karst mountainous. These four spatially separated land categories were considered to represent a land restoration time series. Results were obtained from a combination of field sampling and laboratory analysis, and the relationship between fractal dimension of soil aggregates and soil physical and chemical properties was also analyzed. It was found that during the process of ‘rocky desertification restoration', the proportion of larger-diameter micro-aggregates and aggregates increased across the ecological restoration time series, while the proportion of smaller-diameter soil peds decreased. However, the mechanical composition of soil showed an opposite trend. The mean weight diameter (MWD) ranking of soil micro-aggregates in 0-10 cm soil layer was: FGI=G>FC>CCG. Quantitatively, the MWD of soil micro-aggregates in CCG was 0.15, 0.15 and 0.11 lower than other grassland types, which was 0.15, 0.15 and 0.11 lower (P<0.05), respectively, than that of FGI, G, and FC land. Soil micro-aggregate MWD in 10-20 cm soil layer ranked G>FGI>FC>CCG, and values for CCG were 8.26% and 13.18% lower (P<0.05), respectively, than those for FGI and G. The soil structure characteristics were weakest in CGC land. Across the ecological restoration time series, the fractal dimensions of soil micro-aggregates and aggregates ranked in descending order: CCG>FC>FGI>G, while the fractal dimensions of soil particles ranked in descending order: G>FGI>FC>CCG. These results indicate that soil structure in this karst mountain area is gradually improved with ecological restoration, and the soil fractal dimensions reflect soil restoration status in the karst rocky desertification area to some extent. At the same time, the fractal dimension of these soils correlates well with their physical and chemical properties. The fractal dimension of micro-aggregates and aggregates can better represent the strength of soil structure and erosion-resistance. In the process of ecological restoration and control of rocky desertification, human input that shortens the time needed to return farmland to grassland or accelerates increase of vegetation coverage, can effectively reduce soil erosion and improve the ecological environment of the region.

Key words: karst mountain area, rocky desertification area, grassland, ecological restoration, soil structure