In recent decades, environmental problems such as water eutrophication, climate change, acid precipitation and nitrogen pollution have become increasingly problematic. This has resulted in the transformation processes of the nitrogen cycle becoming a hot topic within scientific research. This article reviews the mechanism of nitrogen input/output within lake ecosystems and summarizes the migration and transformation of nitrogen within water, sediments, and the interface between the two. The latest research progress on several predominant nitrogen oxide processes in lake ecosystems are elaborated in detail. These processes include traditional nitrification, Archaeal ammonia oxidation, anaerobic ammonia oxidation and heterotrophic nitrification,
The traditional nitrification process predominantly includes the ammonium oxidation process and nitrosation dominated by bacteria. The predominant Archaeal ammonia oxidation process is driven by ammonium oxidation Archaea, which belong to Crenarchaeota. Anaerobic ammonium oxidation is the process where ammonia is oxidized by nitrite under anaerobic or hypoxic conditions, and eight strains have been confirmed. At present, anaerobic ammonium oxidation reactions have been widely observed in fresh water systems. As is commonly known, there are essential differences in the way energy is utilized between heterotrophic nitrification and autotrophic nitrification, but the microbial enzyme systems are similar. The discrepancies between heterotrophic nitrification and bacteria ammonium oxidation during the oxidation of ammonium to nitrate have been compared here. This paper expounds in detail the types of strains, biological diversity, distribution characteristics and activity in the nitrogen transformation processes mentioned above. The main enzymes and genes which participate in the process of ammonia oxidation have also been described and analyzed.
The main biodenitrification reactions in lake ecosystems, namely heterotrophic denitrification and anaerobic ammonium oxidation denitrification, have also been reviewed here. Recent literature shows that the ratio of nitrogen loss by heterotrophic denitrification to total nitrogen input is substantially variable for various aquatic ecological systems. The ratio is about 63% for marine environments, 0%-50% for freshwater lakes, 10%-60% for estuaries and 1%-36% within aquaculture water. The main environmental factors affecting heterotrophic denitrification have been presented, including dissolved oxygen, pH, temperature, nitrate and organic carbon. These factors are also the main reasons for the differences in heterotrophic denitrification effect. Not only has the mechanism of anaerobic ammonium oxidation denitrification been discussed, but also its denitrification effect on aquatic ecological systems. Recent studies reveal that the ratio of nitrogen loss caused by anaerobic ammonium oxidation to total nitrogen loss fluctuates greatly for different habitats with the ratio ranging from 0%-67%. The main environmental factors affecting the denitrification efficiency of anaerobic ammonium oxidation include dissolved oxygen, temperature and nitrate concentration. Until recently, there has been a paucity of research on anaerobic ammonium oxidation in freshwater systems.
Finally, some research prospects have been proposed regarding improving the denitrification efficiency within actual sewage treatment and also small niche anaerobic ammonium oxidation research. The author also proposes to strengthen research on the separation and enrichment of bacterial cultures, gene expression as well as the characterization of protein activation of novel nitrogen transformation bacteria in order to provide more useful information on new nitrogen cycles.