Impacts of influent COD/N ratio on floc physicochemical characteristics and microbial community of nitrifying sludge under high-strength ammonia conditions

•Impact of COD/N ratio on NS under high-strength ammonia condition was assessed.•The insufficient and excessive organics adversely influenced the stability of NS.•The PS contents increased to exceed that of PN with the increase of COD/N ratios.•The humic acid-like substrates increased at extremely low and high COD/N ratios.•A certain amount of organic matter was beneficial to proliferation of Nitrospira.