Using multiple combined analytical techniques to characterize water extractable organic nitrogen from Lake Erhai sediment.

In this study, UV-vis absorbance, fluorescence, and FT-IR spectroscopy were combined to characterize the components and structure of the water extractable organic nitrogen (WEON) in Lake Erhai sediment. Lake Erhai sediment WEON comprised predominantly high molecular weight WEON, with the fraction with a molecular weight>1kDa accounting for 87.7% of the total. It was mainly composed of humic acid-like substances, with fewer simple aromatic proteins. Large amounts of aliphatic and amide compounds were detected by IR in the sediments. There were more polymerizable aromatic rings and carbonyl, carboxyl, hydroxyl, and ester compounds in the high molecular weight WEON than in the low molecular weight WEON. Additionally, fluorescence regional integration results implied that the ratio PIII+V,n/PI+II+IV,n can be indirectly taken as an indicator for WEON content in Erhai sediments. Furthermore, the composition and structural characteristics of the WEON were found to be closely related with their properties in the sediment. The large amount of aliphatic compounds in the sediment as well as the relatively high humification and aromatic degree in high molecular weight WEON, stabilizes the WEON in Lake Erhai sediment. Compared with other lake sediments of different trophic statues (such as Lake Dianchi, Lake Poyang, Lake Taihu and Lake Donghu), Erhai sediment exhibited a higher degree of humification, which benefited for reducing sediment WEON releasing risk. And it can be regarded as the reason why the nutrient content in Erhai sediment is very high, but its water quality is still good.