Denitrification ♦ 63 Denitrification in created riverine wetlands: Influence of hydrology and season Maria E. Hernandez and William J. Mitsch Schiermeier Olentangy River Wetland Research Park, Environmental Science Graduate Program and School of Environment and Natural Resources, The Ohio State University Abstract Nitrogen in wetlands is removed from the water by biological transformations. Plant uptake and microbes Seasonal denitrification rates in two created riparian temporary immobilize nitrogen, whereas permanent marshes were investigated under pulsing and steady- nitrogen removal occurs via denitrification (Poe, et al, water flow conditions. Denitrification was measured 2003; Clement et al, 2002). Denitrification is the reduction using the in situ acetylene block technique. Measurements of NO3- to nitrogen gaseous forms such as N2O and N2; this were performed in a transverse gradient with different process is carried out by anaerobic facultative bacteria in hydrologic conditions: low marsh and open water zones anoxic conditions. Denitrification is controlled by oxygen which were permanently flooded, high marsh zones which availability, temperature, nitrogen and organic carbon had permanently saturated soils but standing water during supply (Beauchamp, et al., 1989). While several studies pulses, and edge zones which were normally dry with have investigated how these controlling factors affect standing water during flood pulses. Denitrification in all denitrification rates in riparian buffer zones (Ellman et al, plots was significantly correlated with soil temperature and 2004, Ambus and Lowrance, 1991; Clement et al., 2002, was significantly correlated with the nitrate concentration Willems, et al, 1997; Matheson, et al, 2003, Martin, et al, in the inflow surface water in the growing season. Late 1999), few studies have investigated denitrification in created spring denitrification rates in the high marsh zone were wetlands receiving non point source pollution or river flood significantly higher under flood pulsing (778 ± 92 mg N -2 -1 -2 -1 water (Poe, et al, 1993, Srivedhin and Gray, 2006). m h ) than under steady flow (328 ± 63 mg N m h ). In Created or restored riverine wetlands are expected to the low marsh and edge zones, flood pulses did not affect experience flood pulsing. Flooding facilitates the exchange denitrification. N O/N ratios were higher in intermittently 2 2 of material between rivers and their floodplains (Junk et al., flooded (high marsh and edge) zones than in permanently 1989). The reestablishment of flood pulsing in riverine and flooded (low marsh) zones and highest in the cold seasons. tidal systems is being recognized as an essential step in the Highest mean denitrification rates were observed in the -2 -1 restoration of wetlands (Middleton, 2002). Flood pulses are low marsh zone (800 ± 102 mg N m h ) and they were also nutrient pulses and they often make the wetland area significantly higher (P < 0.05) than in the high marsh (458 -2 -1 -2 -1 larger, changing the oxygen availability of soils and the ± 87 mg N m h ) and edge (315 ± 40 mg N m h ) zones potential area for denitrification to occur. The effect of flood but not significantly different from the open water zone (584 -2 -1 pulses on nitrogen cycling in created riverine wetlands is ± 101 mg N m h ). Denitrification in high marsh zones not completely understood. In a longitudinal gradient, i.e., was not significantly different than in the open water and along the water flow, a decrease in nitrate concentrations edge zones. In permanently flooded areas, denitrification is expected. On the other hand, in a transverse gradient, rates were significantly higher near the wetland inflow than soils have different flooding frequencies and therefore near the outflow. Overall, denitrification in the experimental oxidative–reductive conditions. Since nitrate and oxygen wetlands was 147 ± 54 kg N yr-1 during pulsing year and 112 -1 availability are key factors controlling denitrification ± 41 kg N yr during steady-flow. Denitrification appeared (Ellman et al, 2004, Beauchamp, et al., 1989) it is expected to be nitrogen limited in low marsh, high marsh and edge that denitrification rates vary along these gradients. plots, but both carbon and nitrogen limited in open water. High nitrogen removal in riverine wetlands created or restored for controlling agricultural nitrate loads to rivers Introduction is desirable and denitrification is a desirable mechanism for Agricultural runoff is a main source of nitrogen loading nitrogen removal because the bacterial conversion to gaseous in the Mississippi River and increases of this nitrate loading forms permanently removes nitrogen from the watershed. is cited as the major cause of the extensive hypoxia in the Thus, quantifying and understanding this process in created Gulf of Mexico (Goolsby and Battaglin, 2001; Dagg and wetlands is important for scientists and managers seeking Breed, 2003). To mitigate this problem, the creation and to create long-term improvement of water quality. restoration of wetlands has been recommended in the The objectives of this study were to investigate seasonal Mississippi–Ohio–Missouri (MOM) river basin (Mitsch denitrification rates in zones in longitudinal and transverse et al., 2001, Mitsch et al, 2005a, Mitsch and Day 2006). gradients in two similar 1-ha created wetlands in Midwestern 64 ♦ The Olentangy River Wetland Research Park 2005 USA under both pulsing and steady-flow conditions, and to Gas sampling protocol assess the controlling factors of denitrification in these zones. Hydrology in these wetlands was completely controlled, To evaluate the effect of hydrological pulses on giving the opportunity to design experiments with different denitrification, measurements were taken in zones at different hydrologic regimes. elevations above mean water level (221.10 m AMSL) where the flood frequency would be affected by flood pulses. The edge zone was at +0.18 m, the high marsh zone at 0.03 m, Material and Methods the low marsh zone at -0.09 m, and the open water zone at -0.38 m (Figure 1). The edge zone was usually dry with Site description and hydrologic experiment standing water during flood pulses, the high marsh zone This study was conducted at the Schiermeier Olentangy was saturated with alternate standing water and air exposed River Wetland Research Park (ORWRP) in Columbus, Ohio, conditions, and the low marsh and open water zones USA (latitude N 40.021o, longitude E 83.017o). The ORWRP were permanently flooded. During the pulse year (2004), includes several wetlands that are flooded with different denitrification was measured from May to December; the waters and at different frequencies. Our study was carried frequency of measurements was two times in May, three out in a pair of 1-ha experimental river-diversion wetlands times in June and once per month for the rest of the year. created on alluvial old-field soils adjacent to the third-order During the steady flow conditions (2005), denitrification was Olentangy River in 1993–94. Both wetlands have three measured once per month from January to April, and in the deepwater (>50 cm depth) sections located in the inflow, following months, measurements were made at the same middle and outflow positions of the basins, surrounded by frequency as in 2004. Denitrification in open water zones much shallower sections (20–30 cm deep) dominated by was measured from August 2004 to November 2005, with emergent plants. The hydrology in these wetlands is mostly the same frequency described above, but due to a thick layer controlled by river water pumped from the Olentangy River. of ice, sampling in these plots was not possible in December Water enters to these wetlands at their north side, flows 2004 or January, February, and December 2005. Because southwards through the wetland, and finally returns to the open water zone had hydrologic conditions similar to Olentangy River through an outflow swale (Figure 1). The the low marsh (permanently flooded), denitrification rates primary original soil type at the experimental wetlands is a in this zone were not investigated during flood pulsing. Ross (Rs) series soil, which is a floodplain alluvial soil that Thus, due to the fact that we had fewer measurements in this ranges from silt loam to silty clay loam to loam (Mitsch and area, denitrification rates in the open water zone were only Wu, 1993). These wetland basins were artificially flooded included in the longitudinal spatial analysis. For uniformity, for 10 years prior to the start of this study and had developed all samples were taken between 11:00 am and 3:00 pm. hydric soils over that time (Mitsch et al., 2005c; Anderson Measurement of denitrification in situ et al., 2005). The biogeochemistry and ecology of these wetlands has been described in several other publications The acetylene inhibition technique was utilized to (Mitsch et al., 1998, 2005a,b,c; Nairn and Mitsch, 2000; measure denitrification. Acetylene inhibits the reduction Spieles and Mitsch, 2000; Harter and Mitsch, 2003; Anderson of N2O to N2, during denitrification. Production of N2O et al., 2005; Anderson and Mitsch, 2006; Hernandez and in the presence of acetylene is equivalent to production of Mitsch, in press; Altor and Mitsch, in final review). N2O plus N2 in the absence of acetylene. Variations of this The study period was from May 2004 to December 2005. technique include either 1) in situ treatment of soil with The wetlands were treated as replicates, receiving the same acetylene, followed by determination of N2O emissions, or amount of water under two different hydrologic conditions 2) incubations of soil cores with acetylene followed by N2O (pulsing and steady flow). In spring 2004, the wetlands analysis (Knowles, 1990). We evaluated the advantages and received controlled seasonal hydrologic pulses, and during disadvantages of using the two approaches of the acetylene 2005 they received a steady rate of water inflow. Seasonal technique. Because we were interested in the effects of hydrologic pulses were simulated by pumping river water hydrologic dynamics on denitrification, the incubation at high rates (27–54 cm d-1) during the first week of each of soil cores had constraints.