Abstract: Dissolved organic matter (DOM) is an important component of aquatic ecosystems, playing an indispensable role in material cycling. However, the DOM components and sources in agricultural watershed waters and their key influencing factors largely remain unknown. In the Nongjiang River Basin, a typical agricultural watershed in the Sanjiang Plain of China, we examined the spectral characteristics and composition of DOM in the watershed during the period of agricultural drainage, using UV-visible absorption spectroscopy (UV-vis) and three-dimensional fluorescence spectroscopy (3D-EEMs) combined with the parallel factor method. We analyzed the sources of DOM in the watershed, and investigated the impacts of anthropogenic and natural factors on the DOM of waters in the river-wetland system. The results showed that the water body DOM in the Nongjiang River Basin consisted of four components. The microbial humus component C1 was the dominant fluorescence component. Components C2 and C3 were terrestrial-source humus-like compounds, while component C4 was tryptophanlike compound. The DOM component of the watershed water body was dominated by humus-like substances. The UV and fluorescence parameters indicated that DOM in the watershed water body was dominated by fulvic acid substances in humus-like substances. The DOM in the watershed water bodies generally exhibited a mixed characteristic of endogenous and exogenous sources. Specifically, DOM in paddy water bodies showed a relatively strong endogenous source feature, while the aromaticity and hydrophobicity of DOM in wetland water bodies were higher, presenting a strong humification feature, with more obvious exogenous source feature. The basic physicochemical and optical parameters of water bodies showed different degrees of correlation with DOM, indicating that natural processes (such as carbon burial and microbial metabolism in wetlands) and anthropogenic activities (such as agricultural activities and sewage inputs) jointly influenced the composition and distribution of water body DOM in the Nongjiang River Basin. Our results can provide reference for the future evaluation of water body material cycling, carbon balance, and carbon pool in typical agricultural watersheds in Northeast China.

Key words: dissolved organic matter, drainage period, three-dimensional fluorescence spectroscopy, parallel factor analysis