*2.5. Statistical Analyses*

The data were subjected to conventional analysis of variance using STATISTICA® 10 (StatSoft, Krakow, Poland). The distribution of the data (normality) was checked using the Shapiro–Wilk test. The homogeneity of variance was checked by Levene's test. The differences between treatments were evaluated with Tukey's test. In the second step principal component analysis (PCA) was used to illustrate the dependence between amounts of N in a particular WOSR part, or in soil and yield and its components. In the third step of the diagnostic procedure, stepwise regression was applied to define an optimal set of variables for a given N characteristic. In the computational procedure, a consecutive variable was removed from the multiple linear regressions in a step-by-step manner. The best regression model was chosen based on the highest F-value for the model and the significance of all independent variables.
