Abstract: Taking the soil dissolved organic matter (DOM) in the typical small watershed of the Piliqinghe River as the research object, parallel factor technology was used combined with fluorescence quenching and titration technology to analyze and discuss fluorescence characteristics and soil DOM contents under different land use types (including woodland, farmland, grassland) in the watershed coordinated with Hg²⁺. The results show that different land use types had significant differences in soil DOM characteristics. The soil DOM contents were in the order of grassland > woodland > farmland, and the chromophoric dissolved organic matter (CDOM) contents were in the order of grassland > farmland > forest land. Under different land uses, soil DOM contains 3 kinds of fluorescence peaks: humus-like fluorescence peak A in the ultraviolet region, fulvic acid-like fluorescence peak C in the visible region, and marine or terrestrial humus-like fluorescence peak M. The parallel factors identified two components, C1 as humic acid and C2 as fulvic acid. In addition, the fluorescence intensity of the DOM components in the three different land-use types showed different degrees of fluorescence quenching with the increase of Hg²⁺ concentration. There were differences in the complexation constants of various fluorescent components and Hg²⁺ in DOM, which indicates that the type and number of functional groups participating in the reaction of DOM did affect the complexation ability of Hg²⁺ due to different land-use pollution sources, resulting in different complexation constants.

Key words: Piliqing River Watershed, dissolved organic matter, parallel factor analysis, fluorescence quenching titration, coordination