*2.4. Statistical Analyses*

The effect of individual research factors (year, N fertilization system, and N rate) for each soil layer and the interaction between them was assessed by two-way ANOVA. Differences between the mean values were compared using the HSD – honestly significant difference test, according to the Tukey method, where the significance level was assessed at 0.05. STATISTICA 13® software was used to conduct all statistical analyses. In the second step of the diagnostic procedure, the relationships between variables representing soil properties were analyzed by principal component analysis (PCA) (StatSoft, Inc., Krakow, Poland 2013). 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.
