Fig. 1.

Principal component analysis (PCA) of soil characteristics and secondary metabolites in the roots of licorice plants of different ages. (a) Ordering distribution and interpretation of the first ten principal components. (b) The contribution of the variables to the principal components. The depth of the color of the arrow line represents the degree of contribution. The angle of the arrow line represents correlations. pH, soil pH; SWC, soil water content; SOM, soil organic matter; TC, total carbon; TN, total nitrogen; AN, available nitrogen; C/N, total carbon/nitrogen ratio; TP, total phosphorus; AP, available phosphorus; TK, total potassium; AK, available potassium; Cu, copper; Fe, iron; Zn, zinc; Liq, liquiritin; Isoliq, isoliquiritin; and Acid, glycyrrhizin