An aerobic denitrification biofilter (ADB) for groundwater remediation was developed with high removal efficiencies (total nitrogen (TN): 82.3-95.8%; NO3-N: 93.2-98.2%). Nitrate (NO3-N) transformation rates stabilized between 21.0 and 23.4 g/(m(3) h), whereas nitrite (NO2-N) and ammonium (NH4+-N) transformation rates remained less than 6.0 g/(m(3) h) as the dissolved oxygen (DO) level increased from 1.0 mg/L to 6.0 mg/L. Nitric oxide (NO) and nitrous oxide (N2O) accumulated with great fluctuations (NO: 0-1.6 x 10(-3) g/(m(3) h); N2O: 0.1-1.1 g/(m(3) h)) throughout the experiment. This study suggested that gene associations reflect quantitative relationships with aerobic denitrification rates and can provide useful information regarding aerobic denitrification processes in groundwater. Especially, the qnorB/nosZ ratio acts as the main driver for NO3-N and NH4+-N transformation, while the qnorB/nosZ ratio followed by the (nirS + nirK)/nosZ ratio serve a dominant role in the accumulation of N2O and NO. (C) 2015 Elsevier Ltd. All rights reserved.