The natural chinese pine forest is an important vegetation type in vegetation protection and restoration of the Loess Plateau.To further understand its status and functions during its development process

the three main microgroups and the main functional groups of soils were investigated using the method of plate cultivation and MPN method.Besides

the soil microbial biomass carbon( MBC) was determined by the method of chloroform fumigation.Results showed that there was an increasing trend for the amounts of microorganisms and bacteria in the forest with the growth years from 10 to 25 and after that

they begun to reduce.Meanwhile

the amount of bacteria hold an absolute predominance over the total microorganism amount and the rest two types of microorganisms

fungi and actinomycetes

were fewer.The amounts of ammonifying bacteria and cellulose-decomposing microorganisms appeared in the forest with the growth years of 10 and after that they showed a decreased trend.The azotobacter amount showed an increasing trend in the forest with the growth years from 10 to 25

then decreased until 40 years forest

and after that showed a stable trend.There was no significant variation in nitrifying bacteria for the forest with the growth years from 10 to 40

but its amount significantly increased after that.Meanwhile

the comprehensive indexes of soil microorganism (Shannon) Wiener) and the MBC showed an increasing trend from 10 to 70 years.All above results demonstrate that there are a unique principle and the reasons of variation in soil microorganism composition in the natural chinese pine forest.This study may offer a guidance and reference significance to the protection and management of the natural chinese pine forest on the loess plateau based on soil microbiology.