Abstract: Aimed to explore the effects of different vegetations and of the years of vegeta tion restoration on soil microbial biomass carbon and nitrogen, a comparative st udy was conducted, with the 5 year old Robinia pseudoacacia, Hippophae reamno ide and Prunus armeniaca plantations and the 5, 15 and 25 years old R. pseudoacacia plantation in the Yangjuangou catchment of Yanan City of Shaa nxi Province, a typical hilly area of the Loess Plateau, as test objects. The re sults showed that among the three 5-year old plantations, H. reamnoides pl antation had the highest soil organic carbon (SOC) and total nitrogen (TN) conte nts, while R. pseudoacacia plantation had the highest soil microbial biomass carbon (MBC) (9956 mg·kg-1) and nitrogen (MBN) (2881 mg·kg-1) . The MBC was in the order of R. pseudoacacia>H. reamnoides>P. armeniaca, an d that of MBN was of R. pseudoacacia>P. armeniaca>H. reamnoides. The MBC/SOC was in the order of R. pseudoacacia>H.reamnoides>P. armeniaca, and that of MBN/TN was of R. pseudoacacia>P. armeniaca>H. reamnoides, with the differenc es being significant (P<005). With the increasing years of vegetation rest oration, the soil pH in R. pseudoacacia plantation decreased, while the SOC, TN, electricity conductance (EC), MBC, and MBN all had an increasing trend, whi ch illustrated that in the hilly area of Loess Plateau, planting R. pseudoacac ia was more beneficial to the increase of soil MBC and MBN, and, with the incr easing years of this planting, soil MBC, MBN, SOC and TN tended to be increasing.

Key words: nitrogen use efficiency, partial substitution of mineral fertilizers with organic manure, functional microbial community, nitrogen cycle, paddy ecosystem, mechanism of yield increase