Abstract
The lakes distributed in the Tibetan Plateau constitute a lake group with the highest altitude, largest lakes and largest area in the world and are important in global climate and environmental effects. Freshwater lakes in the Tibetan Plateau possess high ecological values and high vulnerability. The migration and transformation of nitrogen in sediments are critical to lake ecosystems, but information on sedimentary nitrogen in the freshwater lakes in the Tibetan Plateau is limited. A case study was conducted in Keluke Lake, China, to reveal the effects of sedimentary nitrogen on water quality in plateau freshwater lakes. Nitrogen speciation, mineralization potential and release flux were analysed through a sequential extraction method, waterlogged incubation experiment and Fick's first diffusion law, respectively. The content of total nitrogen (TN) was 1295.75–6151.69 mg kg−1, and 94.2% of TN was organic nitrogen (ON). The contents of three nitrogen fractions were in the order of hydrolysable nitrogen > residual nitrogen > exchangeable nitrogen. Ammonia nitrogen was the main mineralization product, and hydrolysable ON was the most significant contributor. The sediments showed a great mineralization potential, with a potentially mineralizable nitrogen value of 408.76 mg N kg−1 of sediment, that was mainly affected by hydrolysable ammonium nitrogen. The diffusion flux ranged from 24.14 to 148.75 mg m−2 d−1, and the sediments served as an internal nitrogen source. Nitrogen release from sediments was considerably influenced by exchangeable ammonia nitrogen. The sediments in Keluke Lake pose a potential nitrogen release risk and threaten the water quality of the lake. The total content, speciation, mineralization of ON and the release flux at sediment–water interface should be considered comprehensively to evaluate the effects of nitrogen in sediments to water quality.
Highlights
Nitrogen is a major nutrient in aquatic ecosystems and a driving factor of lake eutrophication associated with energy flow and material circulation in lake ecosystems [1,2]
Numerous studies on the fractionation and bioavailability of nitrogen in lake sediments have shown that the mobility and bioavailability of nitrogen in sediments primarily depend on how nitrogen is bound to other elements rather than the total content alone [13]
The mean value (3709.32 mg kg21) was between the values of the lowest effect level and the severe effect level regulated in the sediment quality evaluation standard set by Ontario Ministry of Environment and Energy [18]
Summary
Nitrogen is a major nutrient in aquatic ecosystems and a driving factor of lake eutrophication associated with energy flow and material circulation in lake ecosystems [1,2]. Large amounts of external pollutants (e.g. nutrients, metals and organic pollutants) are induced into natural water bodies by human activities [6]. These pollutants partly accumulate in sediments after a series of biochemical processes and cause sediment pollution. The nitrogen in sediments can be released into the water column again under certain conditions (e.g. low pH, anaerobic condition and disturbance) [11,12] In such a case, sediments act as a source and support the primary production of the lake ecosystem by nitrogen sediment flux. The impacts of sediments with high nitrogen load on the aquatic ecosystem of plateau freshwater lakes remain unclear
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