Abstract:

【Objective】 Soil microbial biomass is an important parameter representing material cycle and energy flow in soil ecosystem. Study on dynamics of soil microbial biomass in the abandoned cropland in loess hilly area so as to understand the change and effect evaluation during process of ecological restoration is of great importance. 【Method】 The research was carried out in Zhifanggou small watershed of typical erosive environment, the croplands abandoned in different times were selected as objects, interchangeablely spece-time method was adopted and slope croplands and the plots with Platycladus orientailis were used as contrasts. Several indexes were analyzed such as soil microbial biomass, respiration and metabolic quotient (qCO2) and involvement characteristics of physical and chemical properties. 【Result】 The research indicates that the content of microbial biomass and physical and chemical properties under erosive slope cropland was low. However, there was a significant improvement on physical and chemical properties after abandoning. Soil microbial biomass C increased dramatically one year abandoned later, rose sharply in the first 7 years of abandonment, then took on wavelike rising and reached maximum 50 years later. Soil microbial biomass N increased slowly at early stage and reached significant level after 40 years. Soil microbial biomass P dropped significantly at early abandoned stage, reached minimum 5-7 years later and then rose gradually, after 20-25 years, there was no significant difference compared with the slope cropland but reached maximum 50 years later. After abandoning 50 years, soil microbial biomass C, N and P increased by 166%, 146%, and 52%, respectively, compared with slope cropland, which were only 43.42%, 45.06%, and 51.47% of that in soil with Platycladus orientailis. Respiration increased sharply at early abandoned stage and tended to be stable, which did not differ significantly compared with Platycladus orientailis. Metabolic quotient (qCO2) increased drastically at early abandoned stage, then took on wavelike dropping and dropped to minimum which was also higher greatly than that of Platycladus orientailis. Correlation analysis showed that there was a significant relationship between soil microbial biomass C, N, P and soil nutrition. Soil microbial biomass C, N, P, organic C, total N, available N and available K presented a linear growth. 【Conclusion】 The results suggest that it is impossible to recovery soil fertility and soil microbial biomass by natural vegetation recovery under erosive slope cropland in the loess hilly area, but much longer time, compared with degradation of soil fertility after deforestation, is required to reach to the climax before vegetation destruction.

Key words: hilly loess plateau, abandoned cropland, microbial biomass