2.7 Statistical analysis
The statistical analyses were performed by SPSS 18.0 for Windows (SPSS Inc., Chicago, IL, USA). Differences in soil chemical properties, and relative abundances and α-diversity indices, were determined between treatment means using a one-way analysis of variance (ANOVA), and Duncan’s test was performed for multiple comparisons. A two-way ANOVA was employed to evaluate the interaction effects of soil depth and plantation age on relative abundances and α-diversity indices. Correlation analyses were followed using the Pearson correlation method. Microbial community structure and diversity was quantified based on results from T-RFLP. Non-metric multidimensional scaling (NMDS) and tests of Analysis of Similarity (ANOSIM) were used to determine differences in CO2 and N2 fixation bacteria composition among plantation age (Clarke & Green, 1988), and the r values associated to the NMDS were calculated using ANOSIM based on 999 permutations. NMDS and ANOSIM were computed based on the Bray–Curtis distance using the ‘vegan’ package in R. Redundancy analysis (RDA) was carried out to determine the relationship between soil property parameters and CO2 and N2 fixation bacteria composition using Canoco 4.5 for Windows. The statistical significance was defined at the 5% level unless otherwise stated. All results were reported as the means standard errors (SE) for the three replications.