Abstract: In order to understand the relationship between nitrogen transformation rate and microorganisms of three different grasslands (natural grassland, artificial grassland and abandoned land) in the mountainous area of southern Ningxia, northwest China, microflora, nitrogen physiological groups and net N mineralization rate, net ammonification rate and net nitrification rate as well as the microbial immobilization were examined with the method of in stiu closed-top PVC tube incubation for a period of 360 days. The results showed that:1) The average number of bacteria, fungi and actinomycetes among the three grasslands was 9.6×10⁵,3.8×10⁴ and 4.0×10¹ CFU/g (dry soil), respectively. The highest amount of microbes was found in the artificial grassland among the three grasslands. Soil nitrogen bacteria groups decreased with an order of ammonifier (4.5×10⁴ CFU/g) > azotobacter (4.3×10³ CFU/g)>nitrifier (6.5×10² CFU/g) > denitrifier (3.9×10² CFU/g)>nitrite bacteria (1.7×10² CFU/g). 2) The time of microbes that reached the peak varied, being 60 days for nitrifier, 120 days for bacteria, fungi, nitrite bacteria and denitrifier, 240 days for actinomycetes and azotobater; however, there was no unique pattern for ammonifying bacteria. 3) The lowest net N mineralization rate, net ammonification rate and net nitrification rate were similarly distributed at 61-120 days among the three grasslands. Net N mineralization rate and nitrification rate were all highest during the time span of 181-240 days, while net ammonification rate was delayed to 241-360 days. The microbially N-fixing rate was maximized at 121-180 days but minimized at 241-360 days, with an overall higher value in the abandoned land than in the artificial grassland. Soil nitrogen transformation rate was closely related to microbial numbers; the soil temperature and moisture were the main factors affecting the soil nitrogen mineralization by affecting the number of microorganisms. Immobilization process of soil nitrogen was more active than organic nitrogen mineralization; artificial grassland planted with alfalfa was more advantageous to the breeding of microorganism and soil nitrogen mineralization than natural grassland and abandoned land.