Abstract: This study examined the soil aggregate features and fractal dimension (D) of 5-and 10-year de-farmed Bambusa pervariabilis × Dendrocalamopsis daii 3 plantation forests and a Bambusa rigida farmland in southern Sichuan Province, China. The relationships between D and the basic soil properties were also analyzed. Results showed that the bulk density (BD) increased with the increase in soil depth. A sharp increase in water-stable aggregate content between the de-farmed forests and tillage was observed. Both de-farmed for 10 years, B. pervariabilis × D. daii 3 and B. rigida had the maximum water-stable aggregate level, which improved by 63.27% and 58.36%, respectively, compared with tillage. The mean weight diameter ranged from 0.84 mm to 2.50 mm, whereas the geometric mean diameter ranged from 1.04 mm to 1.98 mm. They also showed the same trend under different plots: surface layers > middle layers > lower layers. The micro-aggregate fractal dimension was 2.308 to 2.526, and the D value increased from the surface to the middle layers; however, no significant pattern was observed when the lower layer was reached. Regression analysis indicated a significant positive correlation between D and the clay content, but no relationships were observed between D and the remaining fraction contents such as BD and OM. D could not represent the BD and OM levels in this area. Thus, reversion of farmland to bamboo forest does not only reduce soil compaction but also increases the amount of soil aggregate. Both B. pervariabilis × D. daii 3 and B. rigida can also reduce soil fractal dimension, improve soil permeability, and increase soil structure stability. Thus, the soil anti-erosion ability and soil-water conservation in the study area can be enhanced.

Key words: Bambusa pervariabilis×Dendrocalamopsis daii, Bambusa rigida, returning farmland to forest, soil aggregate, fractal dimension