2.4. Statistical analysis
Data Processing System (DPS) was applied to perform statistical analyses. A three-way analysis of variance (ANOVA) together with Tukey’s test was carried out to analyze the effects of experimental years (four levels: 2015, 2016, 2017 and 2018), soil depths (four levels: 0-20, 20-40, 40-60, and 60-80 cm), and straw interlayers (four levels: 0, 6, 12, and 18 Mg ha-1) on SOC and TN contents, and soil C:N ratio. SOC content was significantly influenced by the interaction among these three factors. Therefore, a two-way ANOVA was performed to compare the effects of soil depths and straw interlayers on soil parameters in each year, and another two-way ANOVA was performed to examine the influences of experimental years and straw interlayers on soil parameters in each soil layer. When significant interactions were observed between experimental years / soil depths and straw interlayers, one-way ANOVA was conducted between straw interlayers in the same experimental year / soil depth. The Pearson’s correlation coefficients between SOC / TN and soil salinity / crop yield were calculated using IBM SPSS Statistics 20.0 (SPSS Inc., ILZZ, USA). Figures were plotted with Origin 2020.