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It is generally considered that nitrogen is one of the major factors regulating the trophic status of lakes,and the biogeochemical cycling of nitrogen serves the most important function in lake systems.Lakes often receive large anthropogenic inputs of nutrients such as nitrogen and phosphorus that cause lake eutrophication,and the exchange of nitrogen-compounds between sediment and water has a profound effect in all kinds of lakes.Such increased nutrient loading promotes the growth of phytoplankton while rooted plants and benthic are snppressed due to reduced light availability,and the transfer of the nitrogen through the food chains in algal lake and macrophytic lake is different accordingly.In order to probe the nitrogen cycling in different lake bays of Lake Taihn,DIN ( Dissolved Inorganic Nitrogen) transformation rates of algae-dominated (Meiliang Bay) and macrophyte-dominated (East Lake Taihu) lake zones were measured in laboratory for 20 days.The results demonstrated that rapid nitrifications existed in the sediments and water of the two lake zones,and there were remarkable negative correlations between NH4+-N and NO3--N (p ＜ 0.05 or p ＜0.01 ).Nitrification rates of algae-dominated zone were little higher than that of macrophyte-dominated zone in the microcosm under the same condition,and the higher the initial NH4 +-N concentration,the higher the nitrification rate.Nitrification rates decreased gradually with time when no external nitrogen added.Denitrification proceeded strongerly in Meiliang Bay than that in East Lake Taihu.The decline of TN in the sediment after the experiment was not notable (3.7％),however,the decreases of TN were 26.7％ and 42.2％ in the water body of macrophytic and algal lake zones,respectively.Sediment was the most important place for denitrification.Transformation potential of DIN in short time reflected both the velocity of nitrogen transformation and the effect of different zones on lake nitrogen cycling.Thus,nitrogen cycling processes such as nitrification,denitrification,nitrogen fixation and anammox ( anaerobic ammonium oxidation) need be emphasized in Lake Taihu,an eutrophic shallow lake.