This study aimed to investigate nitrogen mass removal rates and microorganisms in planted columns that simulate vertical flow constructed wetlands. Two synthetic wastewaters, with mean NH4-N concentrations of 471 +/- 19 and 475 +/- 17 mg/L, were treated in six individual columns. Under steady hydraulic and pollutant loading, average NH4-N removal rate was in the range of 21-47 g/m(2)d, and average TN removal was 0-27 g/m(2)d. Higher redox potential values benefitted ammonia removal but limited TN removal. The supply of organic carbon, by adding glucose into the synthetic wastewater. slightly reduced ammonia removal rate, but significantly enhanced TN removal. The variation of temperature between 14 degrees C and 20 degrees C had no effect on nitrogen removal. The seeding of microorganisms using diluted activated sludge, and submerging the columns with treated effluent for three days per week, appeared to intensify aerobic microbial degradation activities; oxygen consumption reached 53-363 g O-2/m(2)d, which was equivalent to upward air flux of 0.18-1.21 m(3)/m(2)d in individual columns. Fluorescence in situ hybridization analysis of bacterial mass revealed the population densities of nitrifying bacteria, and specific denitrifying bacteria (Azoarcus-Thauera-cluster, genus Hyphomicrobium, genus Paracoccus, and family Saprospiraceae). Denitrifier Azoarcus-Thauera-cluster was the dominant bacterial group (58% of all cells) when organic carbon is available. Without organic carbon, ammonium oxidizing bacteria (AOB) dominate microbial populations in the columns. (C) 2012 Elsevier B.V. All rights reserved.