Abstract: This study determined the effect of Robinia pseudoacacia plantation age (10, 15, 30 and 38 years) on soil properties?in the Zhifanggou Basin, Ansai, Shaanxi Province. Soil samples were collected in the typical hilly region of the Loess Plateau from the 0~10 cm, 10~30 cm and 30~60 cm depths and analyzed for nitrogen (N) content, microbial biomass carbon (Cmic), microbial biomass phosphorus (Pmic), microorganism quotient and basal respiration. Additional physical and chemical properties of soil were also determined. The results showed that soil NH₄⁺-N, NO₃^--N, and organic N contents gradually increased with increasing plantation age and decreased with increasing soil depth. Soil bulk density decreased with increasing plantation age and increased with increasing soil depth. Total soil N in the 0~10 cm depth increased from 0.40 g·kg^-1 in the 10-year-old plantation to 0.87 g·kg^-1 in the 30-year-old plantation. On soil N fertility scale, this represented an increase, from level-7 to level-5.?Soil Cmic and Pmic were significantly higher in R. pseudoacacia plantations than in abandoned lands (CK). Both soil Cmic and Pmic increased with increasing plantation age and decreased with increasing soil depth.?Microorganism quotient for all three soil layers was significantly greater in R. pseudoacacia plantations than in abandoned lands. Furthermore, the microorganism quotient Cmic to TOC (total organic carbon) ratio increased significantly with increasing plantation age.?Soil respiration quotient (basic respiration to Cmic ratio) for all three soil layers decreased significantly with increasing plantation age. Soil microorganism quotient was positively correlated with organic N, total N and NO₃^---N (P < 0.05). R. pseudoacacia plantations also improved some other soil physical and chemical properties.?Organic C and available P increased significantly with increasing plantation age. However, C to N ratio decreased with increasing plantation age. R. pseudoacacia trees not only increased carbon availability to soil microorganisms, but also enhanced soil carbon utilization. Moreover, increase in microbial activity increased soil organic N content and overall soil fertility.