In Situ Measurements of Denitrification in Constructed Wetlands

Abstract

Abstract Quantitative estimates of denitrification are needed in designing artificial wetlands to optimize nitrate (NO − 3 ) removal. Acetylene blockage and 15 N‐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 NO − 3 disappearance from mesocosm water columns. The 15 N and C 2 H 2 methods yielded comparable rates. At 4 to 25°C, and with 9 to 20 mg NO − 3 ‐N L −1 initially in the mesocosm water columns, denitrification rates by the C 2 H 2 technique ranged from 2.0 to 11.8 mg N m −2 h −1 . In the June–August 15 N experiment, when wetland NO − 3 was below detection, a time series of denitrification rates followed a bell‐shaped curve after a pulse input of NO − 3 (∼15 mg N L −1 , 70 atom% 15 N). The maximal denitrification rate (9.3 mg N m −1 h −1 ) was observed 5.4 d after the pulse. After 33 d, 58% of the 15 NO − 3 had been evolved as N 2 , only ∼0.1% as N 2 O; 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 15 NO − 3 spike was not recovered, and presumably seeped into the sediments. The NO 3 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 NO − 3 through the anaerobic sediment/water interface for denitrification, a subsurface‐flow wetland may denitrify more NO − 3 than a surface‐flow wetland.