Spectral Characteristics of Dissolved Organic Matter (DOM) in Waters of Typical Agricultural Watershed of Three Gorges Reservoir Areas

Abstract

As a key geochemical factor in earth system, dissolved organic matter (DOM) plays an important role in controlling environmental quality of watersheds. In this study, a typical agricultural watershed of Three Gorges reservoir areas, Wangjiagou watershed in Fuling district of Chongqing, was selected to characterize DOM in waters through fluorescence and UV-Vis spectroscopy, while the effect of land-use types in this watershed was discussed. The results showed large spatial variances of aquatic DOM in this watershed, with significant differences in compositions and sources. After calculation of <i>a</i>g^*(355) for indicating proportion of chromophoric DOM in bulk DOM, the order of DOM was paddy rice field> ditch> pond> well> outlet point. DOM samples from paddy rice field and ditch showed higher SUVA254 suggesting higher aromaticity. DOM from this watershed showed 2 typical types (4 peaks A, C, B and T) of fluorescent components including humic-like and protein-like components. Dual contributions from autochthonous (e.g., microbial or alga production) and allochthonous both heavily affected the DOM characteristics. Besides active microbial activities due to high organic and nutrients inputs from agricultures, discharge of sewage and water used in agricultural production could contribute proteins possibly inducing ascending proportion of protein-like component as shown in fluorescence analysis. DOM samples from the same sampling points but in different crop plantation seasons were collected to compare, for understanding the differences between two planting seasons. It clearly suggested protein-like component was the key variable to control the DOM dynamics. After land-use changing from rice/corn into mustard plantation, all of DOC, CDOM and <i>r</i>(T/C)showed significant differences, but no such observations were observed in FI, BIX and <i>r</i>(A/C).