ABSTRACT
Alpine wetlands and valleys of northwestern Sichuan are the main catchment areas of Minjiang River, where dissolved organic matter (DOM) in surface waters comes mainly from the natural background environment. Sources and structure parameters of DOM are important for calibrating the flux and pattern of organic carbon exports from plateau wetlands and alpine rivers. In this study, surface water samples along the upstream Minjiang River were collected at the end of dry season (April) and rainy season (October). Excitation emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) was used to characterize seasonal variations of DOM along Minjiang River. Results showed fluorescence peaks (humic-like peaks A and C, protein-like peaks B and T) were different along the river. Peak A and peak C were more obvious at the end of dry season, while peak B and peak T were more obvious at the end of rainy season. PARAFAC produced a three-component model including two humic-like components[C1 (250-260/380-480 nm) and C2 (300-330/380-480 nm)] and one protein-like component[C3 (270-280/300-350 nm)], accounting relative intensity 48.68%-65.02% for C1, 23.17%-29.83% for C2, and 11.83%-21.53% for C3. Fluorescence components showed variations along the river more prominently in April than October, in which the most significant one was C1. Average fluorescence index (FI) values ranged from 1.4 to 1.9, indicating that DOM consisted of both autochthonous and allochthonous components. Moreover, higher degrees of humification, aromaticity and hydrophobicity were found in April than those in October, suggesting more terrigenous sources at the end of dry season and more biological sources at the end of rainy season. Additionally, chromophoric dissolved organic matter (CDOM)[a(355)] correlated significantly with humic-like substance[Fn(355)], which also indicated that DOM components originated from terrigenous input in the upstream Minjiang River. The results also showed significant positive correlation between C1 and C2 in April, with no significant correlation in October, which further proved that exogenous input and seasonal variations characterized DOM sources in the upstream Minjiang River.
ABSTRACT
Alpine meadows and wetlands of western Sichuan plateau are essential organic carbon pools for Tibetan plateau; thus, a thorough understanding of the characteristics of dissolved organic carbon (DOC) and its association with soil carbon storage pool helps to reveal the flux and intensity of DOC export in the area. Surface water samples were collected from three rivers (the upper reaches of Min River, Zagunao River, and Fubian River) in the alpine-gorge region and Bai River in the plateau planation surface distributed among the watersheds in western Sichuan plateau, Southwest China. UV absorbance and EEM fluorescence spectroscopy with parallel factor analysis (PARAFAC) was used to characterize chromophoric dissolved organic matter (CDOM). PARAFAC produced a three-component model:C1(260/480) and C2(310/420) represented terrestrial humic-like fluorophores, and C3(280/370) belonged to tyrosine-like substances. The total fluorescence intensity of CDOM in the alpine-gorge region showed fewer changes along the rivers and was lower than that of the Bai River in the hilly plateau. The Bai River had much higher concentrations of humic-like substances (C1,C2) compared to the other three rivers, indicating its terrestrial sources with high humification degree originated from meadows and watersheds along the river. The calculated fluorescence indices (FI, BIX, HIX, ß:α) showed that CDOM in the alpine-gorge region was a mixture with both autochthonous and allochthonous origins with low humification degree, while CDOM in the plateau planation surface had a higher degree of humification and lower extent of degradation. Statistical analysis showed that the C1 and C2 components in four rivers were significantly positively correlated, and C1, C2 and C3 components in Bai River were significantly positively correlated. ß:α and BIX were significantly positively correlated in four rivers, but there was no significant correlation between DOC and CDOM[a(355)].
