Soil microorganisms play a key role in soil organic matter dynamics,nutrient cycling,and soil fertility maintenance in forest ecosystems,and they are influenced by stand age and soil depth.However,few studies have simultaneously considered these two factors.In this study,we measured soil microbial biomass carbon (SMBC),soil microbial biomass nitrogen (SMBN),soil basal respiration (SBR) rate,and potential extracellular enzyme activity (EEA) in soil to a depth of 60 cm under 10-,30-,and 40-year-old Scots pine (Pinus sylvestris var.mongolica) stands (Y10,Y30,and Y40,respectively) in plantations in north China in 2011.Soil water content (SWC),soil pH,soil organic carbon (SOC),and soil total nitrogen (STN) were also measured to explore their effects on soil microbial indices across different stand ages and soil depths.Our results showed that SMBC,SMBN,and the SBR rate were generally higher for the Y30 stand than for the Y 10 and Y40 stands.Potential EEA,except for α-glucosidase,decreased significantly with increasing stand age.Soil organic carbon,STN,SWC,and soil pH explained 67％ of the variation in soil microbial attributes among the three stand ages.For the same stand age,soil microbial biomass and the SBR rate decreased with soil depth.Lower microbial biomass,lower SBR rate,and lower EEA for the mature Y40 stand indicate lower substrate availability for soil microorganisms,lower soil quality,and lower microbial adaptability to the environment.Our results suggest that changes in soil quality with stand age should be considered when determining the optimum rotation length of plantations and the best management practices for afforestation programs.