Abstract: The mountainous region of eastern Liaoning Province is an important ecological barrier in Northeast China, mainly dominated by secondary forests and plantations. Compared with primary forests, soil available nitrogen and phosphorus contents of those forests are relatively lower. Here, we compared microbial properties in rhizosphere soil of seedlings of main tree species and their response to nitrogen and phosphorus addition, aiming to provide an insight into rational selection of tree species for forest management. We selected seedlings (2-year-old) of six dominated forest species, including Fraxinus mandshurica (FM), Juglans mandshurica(JM), Acer mono(AM), Quercus mongolica(QM), Larix gmelinii(LG) and Pinus koraiensis(PK). A pot experiment with nitrogen and phosphorus addition was conducted to compare microbial biomass carbon (MBC), nitrogen (MBN) and enzymeactivities in the rhizosphere. The results showed that there were significant differences in both soil MBC and MBN of seedlings of six tree species, which arranged in a sequence of FM > LG, JM, AM > QM, PK. Specifically, the MBC and MBN of FM seedlings were 27.2%-482.9% and 15.7%-333.1% higher than those of other tree species seedlings, respectively. The enzyme activities of rhizosphere soil varied with tree species and enzyme types. The activities of β-glucosidase of JM and N-acetyl-β-glucosaminidase of LG were 25.0%-72.4% and 50.0%-200.0% higher than those of other tree species, respectively. Nitrogen and phosphorus additions affected the activities of protease and acid phosphatase. Nitrogen addition decreased acid phosphatase activity in the rhizosphere soil of AM and PK seedlings. In contrast, phosphorus addition increased protease activity of all tree species rhizosphere soil. The results demonstrated that MBC, MBN and enzyme activities were higher in the rhizosphere soil of FM seedlings  than those of other tree species, which would be beneficial to nutrient acquisition during growth. In addition, nitrogen addition inhibited the activity of acid phosphatase and reduced the availability of phosphorus, while phosphorus addition promoted the activity of protease and increased nitrogen availability. Our results indicated that nitrogen and phosphorus addition had different effects on the supply of available nutrients, and that nitrogen addition may limit the supply of available phosphorus in the soil.

Key words: seedling, rhizosphere soil, microbial biomass, enzyme activity, nitrogen and phosphorus addition.