Hybrid treatment wetlands (HTWs) are composed of two or more filters with different modes of flow, allowing the benefits of both types of bed to be combined, resulting in better effluent quality (nitrogen and organic compound removal). Such a heterogeneous environment creates possibilities for different mechanisms of nitrogen removal. The objective of the present study was to investigate the removal of nitrogen versus a range of parameters such as hydraulic and organic loading, redox potential, pH, alkalinity, availability of easily decomposable organic compounds, and theoretical nitrogenous biological oxygen demand, in order to evaluate nitrogen conversion processes. The studies were carried out in a full-scale HTW designed for 200pe, over a two-year monitoring period. The HTW achieved stable and effective removal of all analyzed pollutants: BOD₅ was 97.9% for average load of 3.4 gm^-2·d^-1 and 95.1% COD removal for average load of 6.9 gm^-2·d^-1 (for the entire HTW). Unexpectedly efficient N removal was observed in the first stage of the treatment plant. The removal efficiency of TKN was over 46% in this horizontal sub-surface flow bed and was coupled with 80.5% BOD removal, suggesting heterotrophic competition for oxygen, the supply of which was strongly limited in this stage of treatment.