|Other Abstract||In order to find out the key biological factors and physichemical variables that govern and influence nitrification, thereby, have a deep insight into the nitrification mechnism and its role in nitrification, we studied the spatial and temporal variations in different nitrogen species, and the difference in nitrification rates and the population structure of nitrifying bacteria in water and sediment in Chinese shallow lakes with different trophic levels. Among the study lakes, Lake Yuehu, Lake Taihu and Lake Dianchi were great polluted by ammonia, and the concentrations of free ammonia (NH3) in water were 6-34 times higher than the threshold above which biota in aquatic environment will be toxified. The concentrations of ammonia accounted for 80.1％-97.6％ of total inorganic nitrogen in interstitial water and the exchangeable ammonium concentrations accounted for 71.3％-93.8％ of total exchangeable nitrogen. The large amount of ammonia in sediment offer a abundant supply for nitrogen in water. The inorganic nitrogen concentrations in water were liable to the change of season, while sediment was a relatively stable system. The inorganic nitrogen concentrations in winter and spring were higher than those in summer and autumn. There was no significant difference in spatial distribution of nitrogen, which indicates that external nitrogen input was not a crucial factor determining the inorganic nitrogen distribution in Lake Yuehu. The nitrogen exchange between water-sediment interface had a seasonal variation. In winter and spring, the NO3--N concentrations in surface and overlying water were higher than those in interstitial water, which indicated that NO3--N tended to sink from water into sediment. In summer and autumn, the NO3--N concentrations in surface and overlying water were lower than those in sediment, indicating that NO3--N tended to release from sediment to water. NO2--N had the same trend as NO3--N between water-sediment interface. But, NH4+-N concentrations in interstitial water were always higher than them in surface and overlying water in the four seasons, which revealed that NH4+-N diffused up to water from sediment all the time. The subsurface sediment (5-10 cm) was a critical layer in which exchangeable nitrate contents were the highest and exchangeable ammonium and available nitrogen contents were the lowest. Available nitrogen stored mainly in the form of exchangeable ammonium in both surface (0-5 cm) and the deep layers (> 10 cm) where its content was higher, this distribution can be explained with anaerobic conditions. The surface sediment not only showed higher contents of both total and organic nitrogen, rates of N-mineralization and nitrate reductase activities, but also gave the highest ammonium and the lowest nitrate concentrations in interstitial water. Therefore, as a nitrogen cycling mode, it is proposed that organic nitrogen was remineralized to ammonia and nitrate with the former being nitrified into the later, resulting in anaerobic conditions that contributed to ammonia accumulation by the production of its own and nitrate reduction in interstitial water of surface sediment. Shortly, enriched by organic nitrogen, the surface sediment in eutrophic lakes is the most active dimension for the biogeochemical cycling of nitrogen with ammonia being the major and most effective form. The large amount of ammonia in the lake with high trophic level not only accelerate the growth of ammonia-oxidizing bacteria (AOB), in term of significantly enhanced biomass, but also supply enough substrate for nitritation, leading to the high nitritation rates. In the water of lakes with different trophic levels, particle-associated nitritation and nitratation rates were the main part of total nitritation and nitratation rates, which can help nitrifying bacteria to resist the bad environmental factors. In the lake with high trophic level, the proportion of particle-associated nitritation was higher than that of particle-associated nitratation. So, the high pH values and high concentration of ammonia in the more eutrophic lakes had more adverse effects directly on the nitratation activity that was less protected via particle association, which will inhibite nitratation step in concert, leading to a transient NO2--N build-up. And in sediment, due to both the larger amount of AOB relative to nitrite-oxidizing bacteria (NOB) and the abundant ammonia as its substrate, nitritation rates were significantly higher than nitratation rates. Additionally, restriction fragment length polymorphism（RFLP）targeting the amoA and phylogenetic analysis revealed that AOB in the sediment of lakes with high trophic levels had greater diversity of amoA gene than them in the sediment of lakes with low trophic levels, and there were significant difference between the amoA genes in lakes with different trophic levels. Nitrosomonas oligotropha- Nitrosomonas ureae and Nitrosospira were ubiquitous, but they had different distribution in lakes with different trophic levels. N. oligotropha-N. ureae dominated in the sediments of Yujia Basin, while, Nitrosospira dominated in the sediments of Tuanhu Basin. N. communis was only detected in the eutrophic Yujia Basin. For NOB, Nitrospira dominated in the sediments of Tuanhu Basi, while, Nitrobacter dominated in the sediments of Yujia Basi. This characteristic of nitrifiers population structures can be explained by the sharp differences in environmental conditions (e.g. substrate concentrations, pH values and et al.) between the two basins and heterogeneity in physiological characteristics (e.g. substrate affinity, growth rates and et al.) among the dominant nitrifiers species. In the lakes with high trophic level, nitrifying bacteria increase diversity of amoA genes and species to adapt to the complex environment, furthermore, they can chang dominant species to adapt to the varying environment in the lakes with different trophic levels, which facilitate the stabilization of nitrification in different trophic lakes, accelerating the oxidation of NH4+-N and production of NO3--N that supply the substrate for denitrification.
Conclusively, the high NH4+-N concentrations in lakes with high trophic level accelerate the growth of nitrifiers and supply abundant substrate for nitrification. On the other hand, the large amount of NH4+-N increase diversity of nitrifiers species and the species with high growth rates and oxidation activity are the dominant nitrifiers, which lead to high nitrification rate and accelerate the consumation of dissolved oxygen. At the same time, high NH3 concentration has strong toxicity to aquatic organisms and the inhibition to NOB accelerates NO2--N build-up in water. So, it is very important to control ammonia for the improvement and restoration of eutrophic lakes.|