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Nitrogen (N)-fixing species have a function to enrich N in soil. Mixing N-fixing shrub species into poplar stands can be assumed as a measure to increase productivity while improving soil fertility. To verify this assumption and to understand the temporal influences of N-fixing shrub species mixed into poplar plantations on soil fertility, we investigated selected soil chemical and microbial properties in pure poplar (Populus x xiaozhuanica W. Y. Hsu et Liang) and mixed poplar-seabuckthorn (Hippophae rhamnoides L) stands at ages of five and 15 years in a semi-arid region of Northeast China. Both stands at age of five have similar values of aboveground biomass, total soil organic C concentration, total N concentration, microbial biomass C, and metabolic quotient; however, at age of 15, these values except for soil metabolic quotient are significantly greater in mixed poplar-seabuckthorn stand than in pure poplar stand. The soil metabolic quotient is lower in the former stand than in the latter stand. Our results suggest that, in semi-arid regions, mixing N-fixing shrub species into poplar plantations can improve soil fertility in a long run rather than in a short term; therefore, mixing N-fixing shrub species into poplar stands is an option to improve soil fertility and increase productivity in a long run. (C) 2010 Elsevier Masson SAS. All rights reserved.