Abstract: [Objective] This study was carried out to reveal the effect of girdling on forest soil microbial community composition.[Method] Six plots (6 m × 6 m) were established in Cunninghamia lanceolata and Pinus massoniana stands respectively in June 2012, and each plot contained at least 5 trees. The mean heights of C. lanceolata and P. massoniana were 16.3 and 15.3 m respectively, and the mean diameters at breast height were 18.5 and 17.2 cm respectively. Three plots were randomly selected as girdling treatment, and trees were girdled by removing 10 cm of bark and phloem over the entire circumference of the stem at breast height, and the others as control treatment. Soil physicochemical properties were measured after one month of girdling, and microbial community composition was measured by phospholipid fatty acid after one month and one year of girdling respectively.[Result] The content of dissolved organic carbon was decreased by 18.3% (P< 0.01), and the content of NO₃^--N was increased by 182.1% (P< 0.01), and available phosphorus content and soil water content were increased by 66.9% and 10.6% respectively (P< 0.05) after one month of girdling in the C. lanceolata plantation. In the P. massoniana plantation, the contents of dissolved organic carbon, NO₃^--N and available phosphorus were increased by 11.3%, 72.7% and 235.5% respectively (P< 0.05), and soil water content was increased by 12.1% (P< 0.01) after one month of girdling. Girdling decreased soil microbial biomass and altered the microbial community composition in both plantations, while the effects of girdling on soil microbes were different between the two tree species at different sampling time. In the C. lanceolata plantation, the contents of total PLFAs, bacteria, fungi, the ratio of fungi to bacteria and Gram-negative bacteria were significantly decreased by 10.3%, 10.9%, 20.0%, 10.1%, and 13.4% respectively (P< 0.05) after one month of girdling, and girdling decreased the contents of bacteria and Gram-negative bacteria by 20.3% and 22.1% respectively (P< 0.05) one year later. In the P. massoniana plantation, the contents of fungi and Gram-positive bacteria decreased by 21.9% and 14.5% respectively (P< 0.05) after one month of girdling, and the contents of total PLFAs, bacteria and actinomycetes were decreased by 17.8%, 15.9%, and 27.4% respectively (P< 0.05) after one year of girdling. Additionally, the ratio of cy17∶0 to 16∶1ω7c was significantly increased (P< 0.05) after one month of girdling in the C. lanceolata plantation, and that occurred after one year of girdling in the P. massoniana plantation (P< 0.05), indicating that the microbial growth was limited by the lack of available carbon. Correlation analysis result showed that the content of dissolve organic carbon was significantly positively correlated with the contents of total PLFAs, bacteria, fungi, Gram-positive bacteria and Gramnegative bacteria (P< 0.01); the contents of NO₃^--N and total nitrogen were significantly positively related to the contents of bacteria and Gram-positive bacteria (P< 0.05); soil available phosphorus content was significantly positively correlated with Gram-positive bacteria content (P< 0.05); the content of NH₄⁺-N was significantly negatively correlated with the contents of total PLFAs, bacteria, Gram-positive bacteria and Gram-negative bacteria (P< 0.01), and fungi (P< 0.05); soil pH was significantly positively correlated with the contents of total PLFAs, bacteria, fungi and Gram-positive bacteria (P< 0.01), and Gram-negative bacteria (P< 0.05); soil organic carbon content had significantly positive correlation with the contents of total PLFAs, bacteria, fungi and Gram-positive bacteria (P< 0.05); total phosphorus content had significantly positive correlation with the contents of total PLFAs, fungi and Gram-negative bacteria (P< 0.05), and bacteria and Gram-positive bacteria (P< 0.01); there was no relationship between soil water content and microbial biomass (P> 0.05).[Conclusion] Girdling altered the soil carbon and nutrient availability by altering plant photosynthate supplying and soil nutrient uptake, and thereby decreased soil microbial biomass and altered microbial community composition. The effect of girdling on fungi was more intense than that on bacteria, and the effects of girdling on microbial biomass and community composition were different in the two plantations.

Key words: Cunninghamia lanceolata, Pinus massoniana, plantation, phospholipid fatty acid, girdling, microbial community composition