Distribution of Carbon, Nitrogen and Phosphorus in Sediments of Lhalu Wetland in Dry and Wet Seasons and It's Pollution Risk Assessment

Lhalu wetland is the world's highest and largest natural wetland in city. In order to study the changes of its sediment pollution, the sediments from 59 and 48 points of Lhalu wetland were collected in December 2018 (dry season) and May 2019 (wet season), respectively, which were analyzed for the spatial distribution characteristics and stoichiometric ratio of total nitrogen (TN), total phosphorus (TP) and total organic matter (OM). In addition, pollution risk assessment was carried out by the comprehensive pollution index method and organic pollution index. Results show that the content of TN, TP and OM in sediments were higher in wet season than in dry season. The content of TN, TP and OM in sediment in dry season was 0.18-6.35 g·kg^-1, 0.33-2.88 g·kg^-1 and 27.18-268.98 g·kg^-1, respectively. The highest content of TN and OM was found in middle-west and east of Lhalu wetland. However, the highest content of TP was found in west and middle-west of Lhalu wetland. The ratio of C/N in dry season was 15.04-85.31, which was significantly higher in the north part than in other parts of the Lhalu wetland (P < 0.05). The ratio of C/N in wet season was 3.09-97.46, which was significantly lower in the west part than in the other parts of the Lhalu wetland (P < 0.05). The ratio of C/N of the wetland was higher than 10, which suggests that the organic matters in dry and wet seasons were from outside. The OM in the sediments was mineralized during the wet season. The Lhalu wetland is relatively clean in the north, while some degrees of pollution were found in other parts of the wetland.