Abstract: By the method of spatiotemporal substitution and taking a slope farmland and a natural Platycladus orientailis L. forest as the controls, this paper studied the dynamic changes of soil microbial biomass, microbial respiration, metabolic quotient (qCO₂) and physicochemical properties under the Caragana korshinkii and Hippophae rhamnoides plantations with different restoration age in loess hilly area. The results showed that with the increasing restoration age of the shrubs, soil physicochemical properties improved obviously, and soil microbial biomass had a significant increase. After 7 years restoration of C. korshinkii plantation, soil microbial biomass C increased apparently, compared with that in farmland, and the increments after each 5-7 years were all significant. Microbial biomass N and P had no significant increase in the first 13 years but kept relatively stable in the 20-30 years restoration of C. korshinkii plantation, and were significantly higher than those in farmland but lower than those under P. orientailis plantation after 30 years restoration of C. korshinkii plantation. Soil microbial respiration was enhanced with the increasing restoration age of the shrubs,with the peak in the 20-25 years restoration. After then,it decreased rapidly,and bottomed out in the 30 years restoration.qCO₂ was significantly higher underP. orientailis plantation than in farmland in the early periods of shrubs restoration, and then decreased rapidly. After 30 years restoration of C. korshinkiiplantation, qCO₂ was lower than that in farmland but still much higher than that under P. orientailis plantation. Different shrub plantations had different effects on soil properties. H. rhamnoides with the same restoration age of C. korshinkii contributed more to the increase of soil microbial biomass and respiration. There were significant correlations between the restoration age of test shrub plantations and the microbial biomass, qCO₂, and hysicochemical properties of soil. It was considered that vegetation restoration could be a feasible way in improving the eco-environment and soil quality in loess hilly area, but a longer period should be required to reach to the climax before vegetation destruction. It is necessary to strengthen and improve forest management to interfere and accelerate plant succession for a sustainable and healthy ecosystem.

Key words: spatial and temporal distribution, mycorrhizal symbiosis, biodiversity, sustainable agriculture