Abstract: Mixed afforestation plays an important role in improving soil structure and soil fertility. In this study, we investigated soil stabilization and nutrient contents in Pinus yunnanensis forests with different mixing ratios in central Yunnan. This would provide theoretical supports for optimizing the mixed forestation patterns of Pinus yunnanensis and better evaluation for the ecological benefits of non-commercial forests. Based on the proportion of Pinus yunnanensis in community timber volume, three mixed proportions were selected: 20% of Pinus yunnanensis and 80% of broadleaved species (Y2K8), 50% of Pinus yunnanensis and 50% broadleaved species (Y5K5), and 80% of Pinus yunnanensis and 20% of broadleaved species (Y8K2). 100% Pinus yunnanensis (Y10) was used as the control. Soil aggregate stability was analyzed using screening method, while Nemerow index was used as nutrient index. The correlations between soil aggregate stability and nutrient index were examined. The results showed that soil aggregate stability of the three mixed forests was better than that of the 100% Pinus yunnanensis forest, whose deep soil showed the poorest stability. The sensitivity of the three mixed forests to soil erosion was low, while that of the 100% Pinus yunnanensis forest was at moderate level. The contents of soil organic matter (SOM), total nitrogen (TN), hydrolyzed nitrogen (AN), total phosphorus (TP), and available phosphorus (AP) were the highest in Y2K8 and the lowest in Y10. Soil fertility indices of the four forest types ranged in 0.84-1.53, indicating that soil fertility was at an average level. Specifically, Y2K8 was the highest in soil fertility while Y10 was the lowest. Besides, AP was the limiting factor for soil fertility across all the four forest types. Mixed forestation can improve soil AN content and reduce nutrient supply limitations, and thereby can improve soil fertility. The mean weight diameter (MWD) of aggregate was significantly positively correlated with soil SOM (R=0.85), TN (R=0.88), TP (R=0.75), AN (R=0.68), and AP (R=0.70). The geometric mean diameter (GMD) of aggregate was significantly positively correlated with soil SOM (R=0.83), TN (R=0.88), TP (R=0.75), AN (R=0.68), and AP (R=0.66). Our results indicated that soil aggregate stability and soil fertility of the three mixed forests were better than that of Pinus yunnanensis monoculture, with 20% of Pinus yunnanensis and 80% of broadleaved species being the optimal ratio. It is suggested that the proportion of broad-leaved tree species should be appropriately increased in forest establishment and low-value and low-efficiency forest transformation in central Yunnan, with 20% of Pinus yunnanensis and 80% of broadleaved species as a guide.

Key words: soil aggregate, soil nutrient, Pinus yunnanensis forest, mixed proportion