The objective of this study was to explore nitrogen removal, especially effluent dissolved organic nitrogen (DON) composition, relative genes and microbial community structures with four individual nitrogen sources at 5 degrees C. Results show that effluent DON did not have dependent relationship with the TN removal rate (urea > ammonia chloride > L-Alanine > D-Alanine). With the same influent TN, the highest effluent DON was formed with urea; the lowest DON was fed with ammonia chloride. The main DON composition was the product of cell metabolism excluding urea, rather than the original substrate. Glutamic acid synthesizing process was of great importance to DON accumulation at 5 degrees C. The nitrogen source type was important to the diversity and heterogeneity of the nitrogen removal genes. Bacterial population structure using redundancy analysis (RDA) showed Simplicispira occupied a higher abundance remarkably in the reactors feeding with urea, and Dyadobacter occupied higher feeding with L-Alanine.