In situ measurements of denitrification in constructed wetlands

Quantitative estimates of denitrification are needed in designing artificial wetlands to optimize nitrate (NO3-) removal. Acetylene blockage and N-15-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 N-15 and C2H2 methods yielded comparable rates, At 4 to 25 degrees 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 N-15 experiment, when wetland NO3- was below detection, a time series of denitrification rates followed a bell-shaped curve after a pulse input of NO3- (similar to 15 mg N L-1, 70 atom% N-15). 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 (NO3-)-N-15 had been evolved as N-2, Only similar to 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 (NO3-)-N-15 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 degrees 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-now wetland.