In situ measurements of denitrification in constructed wetlands

Quantitative estimates or denitrification are needed in designing artificial wetlands to optimize nitrate (NO₃^-) removal. Acetylene blockage and ¹⁵N-tracer methods were employed to quantify denitrification in constructed wetlands receiving agricultural tile drainage, using plastic tubes to enclose in situ mesocosms. Estimates were also made through NO₃^- disappearance from mesocosm water columns. The ¹⁵N and C₂H₂ methods yielded comparable rates. At 4 to 25 °C, and with 9 to 20 mg NO₃^--N L^-1 initially in the mesocosm water columns, denitrification rates by the C₂H₂ technique ranged from 2.0 to 11.8 mg N m^-2 h^-1. In the June-August ¹⁵N experiment, when wetland NO₃^- was below detection, a time series of denitrification rates followed a bell-shaped curve after a pulse input of NO₃^- (approximately 15 mg N L^-1, 70 atom% ¹⁵N). The maximal denitrification rate (9.3 mg N m^-2 h^-1) was observed 5.4 d after the pulse. After 33 d, 58% of the ¹⁵NO₃^- had been evolved as N₂, only approximately 0.1% as N₂O; 6 to 10% was recovered in plant shoots and as organic N in the upper 5 cm of sediment. From 32 to 36% of the ¹⁵NO₃^- spike was not recovered, and presumably seeped into the sediments. The NO₃^- disappearance rates in the water column ranged from 12 to 63 mg N m^-2 h^-1 at 11 to 27 °C. Because water infiltration carries NO₃^- through the anaerobic sediment/water interface for denitrification, a subsurface-flow wetland may denitrify more NO₃^- than a surface-flow wetland.