In Situ Measurements of Denitrification in Constructed Wetlands

Quantitative estimates of denitrification are needed in designing artificial wetlands to optimize nitrate (NOj-) removal. Acetylene blockage and ~SN-tracer methods were employed to quantify denitriflcation in constructed wetlands receiving agricultural tile drainage, using plastic tubes to enclose in situ mesocosms. Estimates were also made through NO;disappearance from mesocosm water columns. The ~SN and CzHz methods yielded comparable rates. At 4 to 25°C, and with 9 to 20 mg NO~--N L-1 initially in the mesocosm water columns, denitrification rates by the CzHz technique ranged from 2.0 to 11.8 mg N m-z h-1. In the June-August lSN experiment, when wetland NOjwas below detection, a time series of denitrification rates followed a bell-shaped curve after a pulse input of NOj(-15 mg N L-~, 70 atom% ~SN). The maximal denitrification rate (9.3 mg N m-~ h-1) was observed 5.4 d after the pulse. After 33 d, 58% of the ~SNOjhad been evolved as N2, only ~0.1% as N~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 ~NO~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-~ h-~ at 11 to 2T’C. Because water infiltration carries NO~through the anaerobic sedimenl/water interface for denitrification, a subsurface-flow wetland may denitrify more NO~than a surface-flow wetland. W~ have recently been recognized as having xtremely high ecological and economic values (Costanza et al., 1997). Because most natural wetlands have been drained for agriculture and other development (Mitsch, 1994), constructed wetlands are receiving increasing attention (Kadlec and Knight, 1996). One use of constructed wetlands is to remove NO~from agricultural runoff to reduce pollution of drinking water supplies and perhaps also alleviate eutrophication of coastal waters (Anonymous, 1996). A significant proportion of agricultural land in the Midwest is artificially drained with subterranean tiles (clay or perforated plastic pipe) to allow farming to practical and economically viable. Many studies have linked agricultural tile drainage to surface water N concentrations (Logan et al., 1994; Fausey et al., 1996; Drury et al., 1996; David et al., 1997b). More specifically, Gentry et al. (1998) showed a strong relationship between high inorganic soil N pools associated with corn (Zea mays L.) production and high tile and river NO~concentrations, especially after a poor growing season. All of these studies found NO~concentrations greater than the USEPA drinking water standard of 10 mg N L-1 in agricultural tiles that directly drain into surface waters. Y. Xue, M.B. David, L.E. Gentry, R.L. Mulvaney, Dep. of Natural Resources & Environmental Sciences, Univ. of Illinois, W503 Turner Hall, 1102 S. Goodwin Av., Urbana, IL 61801; D.A. Kovacic, Dep. of Landscape Architecture, 101 Temple Buell Hall, 611 E. Lorado Taft Dr., Champaign, IL 61820; and C.W. Lindau, Nuclear Science Center, Louisiana State Univ., Baton Rouge, LA 70803. Y. Xue, current address: Dep. of Agronomy, 119 Keim Hall, East Campus, University of Nebraska, Lincoln, NE 68583-0915. Received 13 Feb. 1998. *Corresponding author ([email protected]). Published in J. Environ. Qual. 28:263-269 (1999). Lateral wetlands (Osborne and Kovacic, 1993) may be constructed on the economically marginal riparian zone to receive tile drainage water. The NO~carried in the tile drainage may be removed in these artificial lateral wetlands through biochemical processes. Lateral wetlands are built by excavating surface soil to make a pond surrounded by artificial berms made of excavated soil. Little maintenance on the wetlands is required after their construction. A substantial proportion of the NO~entering a wetland is believed to undergo denitrification. This view is based largely on the finding that NO~disappears from treatment wetlands, often with little direct evidence that the disappearance is due to denitrification (Hsieh and Coultas, 1989; Kadlec and Knight, 1996). There is a need for more field data to directly and quantitatively verify the importance of denitrification as a pathway for NO~removal in constructed wetlands (Groffman, 1994). Some researchers have studied denitrification in the riparian soils. For example, Hanson et al. (1994) estimated that denitrification in riparian wetlands receiving groundwater NO~inputs may remove up to 50% of the NO~loads. Denitrification rate, in units of mg N m-2 h-1, is directly useful for wetland design. By multiplying the rate with wetland area and retention time of water in the wetland, one can calculate the amount of NO~that may be removed from the tile drainage water by the wetland. Given NO~loading in a tile, one can calculate the size of the wetland that will be needed to achieve certain NO~load reduction in the tile drainage water. Denitrification may be described by the following scheme: NO~~ NO~~ N20 ~ Nz (Weeg-Aerssens et al., 1987; Tortora et al., 1995). Acetylene (C2H2) inhibition and ~SN isotope tracing are two common techniques to quantify denitrification (Mosier and Heinemeyer, 1985). The first method uses C2H2 to prevent N20 from reducing to N2; thus, accumulated N20 may be measured to quantify denitrification. Nitrogen-15 is used in the second method to measure N20 and N2 evolution. The objectives of this study were to: (i) estimate in situ denitrification rates using the ~SN tracer and C2H2 inhibition techniques and compare the denitrification rates with the NO~removal rates in the water column, and (ii) evaluate the significance of denitrification as a sink for NO~-, in constructed wetlands that received agricultural tile drainage. MATERIALS AND METHODS The constructed wetlands were located in Champaign County, Illinois [see David et al. (1997b) and Gentry et al. (1998) for detailed description of the site]. The sediment soil (Colo, a fine-silty, mixed, mesic Cumulic Haplaquolls) conAbbreviations: GC, gas chromatograph; IC, ion chromatograph; MS, mass spectrometer; TKN, total Kjeldahl nitrogen; PVC, polyvinyl chloride.