Populations and distribution of soil nitrogen bacteria groups(ammonifying bacteria,nitrobacteria,free nitrogen fixing bacteria and denitrobacteria)in alpine steppe of different disturbed habitats in Tianzhu were investigated and analyzed with the plate count and MPN methods.Results show 1)Populations of soil nitrogen bacteria groups varied greatly with soil depth. They were much greater(1.44 to 7.32 times)in 0～20 cm soil layer than in 20～40 cm one;and they also varied sharply from group to group,forming a decreasing order of ammonifying bacteria>free azotobacteria>denitrobacteria>nitrobacteria; 2)They differed sharply from habitat to habitat in the range of 1.27～9.29 times,and moreover the effect of habitat on population of nitrobacteria was much stronger than on those of denitrobacteria and ammonifying bacteria;3)The total population of the nitrogen bacteria groups was the highest in the annual artificial grassland,which was followed by perennial grassland,fenced grassland,rodent hill and un-fenced grassland in a decreasing order;and 4)Regardless of difference in habitat,spatial distribution and groups,the populations of soil nitrogen bacteria groups showed a general saddle-shaped tend,i.e.within the 0～20 cm soil layer,they were low in J uly,rose up the peak in August and declined in September,and within the 20～40 cm soil layer, they show the same but smaller in amplitude.