Statistical analyses
The analysis of variance (ANOVA) was used to determine the effects of three factors (N, P, W), their interactions (N × P, N × W, and P × W) and C on seven root traits. Means were separated using Tukey’s test atP ≤ 0.05. Besides, the variance contribution rate (VCR) was calculated to determine the magnitude of the influence of the particular factor in terms of percentage on six root traits. The bigger the VCR value, the greater the influence of the particular factor to root traits (Song and Chen 2021). In our study, the VCR is the ratio of the sum of the squares (SSM) of the factor to the total sum of the square (SST) of all the factors. Meanwhile, factor analysis was used to determine the dependence relationship between seven root traits of root morphological traits and root biomass. Factor analysis is a type of multivariate analysis that can be used to reduce a large number of correlated variables to a smaller number of main factors (Kaiser 1958). Principal factor analysis was used for factor extraction and varimax rotation was used to define factors (Kaiser 1958). Furthermore, one-way ANOVA was used to identify the effects of N, P, or W on six root traits (total root length, root surface areas, root volume, root tips, root dry biomass and specific root length) under cluster fragmentation. Means were separated using Tukey’s test atP ≤ 0.05.