| Erhai Lake is the second largest freshwater lake in the Yunnan-Guizhou Plateau in China, although the water quality is well, there also exists eutrophic risk.Nitrogen forms of different regions in the surface and core sediments, temporal and spatial distributions of nitrogen releasing rates and bioavailability of dissolved nitrogen (DON) in lake sediments were studied. It can provide reference for studying eutrophication of Erhai Lake and has important guiding significance for revealing eco-environmental effects of nitrogen in the aquatic ecosystems and assessing eutrophication. The main conclusions are as follows.(1) The contents of total nitrogen (TN), ion exchangeable nitrogen (IEF-N), weak acid extractable nitrogen (WAEF-N), strong alkali extractable nitrogen (SAEF-N), strong oxidant extractable nitrogen (SOEF-N) and Residual-N (Res-N) were4405.14±1928.31mg/kg,75.80±23.23mg/kg,126.43±33.52mg/kg,105.82±57.19mg/kg,1524.45±765.03mg/kg, and2572.64±1220.09mg/kg, respectively. The relative content of different nitrogen form ranked according to the following sequence: SOEF-N>WAEF-N>SAEF-N>IEF-N. IEF-N is homologous with TN, Res-N and SOEF-N. The releasing mechanisms of SOEF-N and SAEF-N are obviously different. High organic matter is the main reason for high contents of nitrogen forms.in the Erhai Lake sediments.The content of TN ranged from (1363.49±313.72) to (6187.09±1874.43)mg/kg with depth increased, whereas IEF-N from(124.72±30.59) to (257.14±52.13) mg/kg, WAEF-N from (29.17±26.91) to (4.52±31.62)mg/kg, SAEF-N from (14.00±6.60) to (114.68±50.37) mg/kg, SOEF-N from (425.97±101.43) to(1995.17±544.88)mg/kg and Res-N from (644.85±158.16) to (3774.76±1274.03)mg/kg.(2) The inorganic nitrogen releasing fluxes across the surface-sediment interface in spring, summer, autumn and winter were6.86±6.01mg/(m2·d),7.71±3.97mg/(m2·d),6.86±2.97mg/(m2·d) and6.01±3.80mg/(m2·d), respectively. The annual releasing fluxes was6.86±4.28mg/(m2·d). Seasonal variation described as the following sequence:summer>autumn>spring>winter. Spatial distribution described as the following sequence:western> eastern. Nitrogen releasing risk is high. Temperature and terrestrial sources are the mian effect factors.(3) After the28d dark bioassay experiment, NH4+-N concentration declined slightly and NO3--N concentration increased a lot, while DON concentration decreased a lot. Bioavailability of DON ranged from21.07%to52.64%, with mean value of34.47%and it was high. After exposed in light for24h, NH4+-N concentration increased slightly and NO3--N concentration increased a lot. The mean percent of DON concentration declined is6.25%. Mineralization of organic and usage of microbes is the main factors for declining in DON. |
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