Fig. 6

Fluorescence (excitation-emission matrix, EEM) spectra of water-extracted/labile state (HSLS) (a) and alkali-extracted/complexed state (HSCS) (b) of humic substances from forest, agriculture, grassland and desert dry-affected soils, and detected fluorescence peaks of humic acid-like (HA-like), fulvic acid-like (FA-like) and protein-like substances-like (PLS-like) identified using EEM-PARAFAC modeling. ‘nd’: not detected.