An in-depth understanding of the succession mechanism of natural vegetation is critical and badly needed.The responses of cultivable soil microorganisms to the vegetation succession in Ziwuling

the Loess Plateau were investigated with the method of "converting spatial series into temporal series".The results demonstrate that the gross amount of microbes

bacteria and fungi in the soil increased in lands as the order of abandoned land→grassland→shrub land→Betula platyphylla forest→Quercus liaot.ungensis forest→Quercus wutaishanica—Betula platyphylla forest(F>0.01

p<0.01).The amount of actinomyces fluctuated and relatively higher values were found in shrub land and Q.liaotungensis forest.The Shannon—Wiener value as a comprehensive index of soil microorganisms fluctuated as well

influenced by the variations of fungi population.As the succession progressed

the population sizes of ammonifying and nitrifying bacteria increased significantly(F>0.01

p<0.01).The nitrogen-fixing bacteria increased slightly in general but not significantly(F>0.05

p<0.05)

with the largest population in Q.liaotungensis—B.platyphylla forest.Comparatively

the highest numbers of cellulose-decomposing microorganisms were found in Q.liaotungensis forest and Q.liaotungensis—B.platyphylla forest.As indicated

the vegetation succession has significant effects on the variations of the three soil microbial groups tested as well as on the main functional groups

promoting the proliferations of soil microorganisms.The changing of soil microorganism diversity was not always in the same pace as of vegetation succession;vegetation types determined the diversity

which varied with vegetation compositions.