Abstract: To determine the optimal thinning width for Pinus sylvestris var. mongolica plantation, we analyzed soil chemical properties, enzyme activities, and microbial biomass in plots with different thinning width (5, 9, 13 and 17 m), and evaluated the improvement effects of soil nutrients six years later after thinning using canonical correspondence analysis and principal component analysis. The results showed that, with the increasing thinning width, soil organic matter, available nitrogen, available phosphorus, invertase, urease and microbial biomass carbon, nitrogen and phosphorus were initially increased and then decreased, while the activities of soil catalase and phosphatase tended to increase. Compared with the control plots, all the soil nutrient factors significantly increased in the plots with thinning width of 9 and 13 m (P<0.05). Available phosphorus, microbial biomass carbon, and microbial biomass nitrogen were closely correlated with other soil nutrient factors. The comprehensive score of soil nutrient factors in the plots with different thinning widths followed an order of 13 m>9 m>17 m>5 m>CK. The thinning width of 9-13 m is optimal for Pinus sylvestris var. mongolica plantation in Horqin sandy land, while soil phosphorus is the key factor for stand improvement.

Key words: Pinus sylvestrisvar. mongolica, soil nutrient, stand transformation, Horqin sandy land.