Excess nitrogen (N) from agriculture runoff is a problematic pollutant in aquatic ecosystems. Free water surface created wetlands are widely used as a buffering system to lower the impacts of extra nutrients from agriculture runoff. The purpose of this paper is to evaluate critical factors for nitrogen removal. In this study, 12 free water flow wetlands with similar size and vegetation type and density were used as experimental wetlands. The effects of water depths, mean depth of 0.4m and 0.6m, and phosphorus, with external phosphorus and without external phosphorus were investigated from July 2017 to October 2017.

Significant higher relative nitrogen removal (%) and first-order area-based rate coefficients (Ka) with modified Arrenhius temperature dependency were found in deep wetland basins. No significant difference in absolute nitrogen removal (g m^-2 d^-1) between the different water depths. As for phosphorus, significant higher absolute nitrogen removal and first-order area-based rate coefficients were found in basins with external phosphorus. No significant difference in relative nitrogen removal between the wetland basins with phosphorus and without phosphorus. Both the nitrogen removal and first-order area-based rate coefficients showed seasonal patterns, with better performance during warm period.