Abstract: Taking two typical vegetation types after de-farming,i.e., Robinia pseudoacacia plantation and abandoned grassland, in the Yangjuangou catchment of Yan’an City, Shaanxi Province of Northwest China as test objects, this paper studied the seasonal variations of soil microbial biomass carbon and nitrogen and the key affecting factors, aimed to understand the seasonal variation pattern of soil microbial biomass and its main controlling factors on the hill slope in the hilly areas of Loess Plateau. Under the two vegetation types, soil microbial biomass all had an obvious seasonal variation trend. The soil microbial biomass carbon under R. pseudoacacia plantation was higher in summer and spring than in autumn, while that under abandoned grassland peaked in autumn. The soil microbial biomass nitrogen under the two vegetation types was the highest in summer, followed by in autumn, and in spring. The seasonal variation of soil microbial biomass carbon was more affected by soil organic carbon, soil temperature, and air temperature, whereas that of soil microbial biomass nitrogen had significant correlations with soil moisture and air humidity.

Key words: Winter wheat, Dry matter, Accumulation and distribution, Simulation