Abstract
Nutrient stoichiometry within a wetland is affected by the surrounding land use, and may play a significant role in the removal of nitrate (NO3–N). Tile-drained, agricultural watersheds experience high seasonal inputs of NO3–N, but low phosphorus (PO4–P) and dissolved organic carbon (DOC) loads relative to surface water dominated systems. This difference may present stoichiometric conditions that limit denitrification within receiving waterways. We investigated how C:N:P ratios affected denitrification rates of sediments from tile-drained mitigation wetlands incubated for: 0, 5, 10, and 20 days. We then tested whether denitrification rates of sediments from surface-water and tile-drained wetlands responded differently to C:N ratios of 2:1 versus 4:1. Ratios of C:N:P (P < 0.05) and incubation length (P < 0.05) had a significant effect on denitrification in tile-drained wetland sediments. Carbon limitation of denitrification became evident at elevated NO3–N concentrations (20 mg L−1). Denitrification measured from tile water and surface water wetland sediments increased significantly (P < 0.05) at the 2:1 and 4:1 C:N treatments. The results from both experiments suggest wetland sediments provide a limiting pool of labile DOC to maintain prolonged NO3–N removal. Also, DOC limitation became more evident at elevated NO3–N concentrations (20 mg L−1). Irrespective of NO3–N concentrations, P did not limit denitrification rates. In addition to wetting period, residence time, and maintenance of anaerobic conditions, the availability of labile DOC is playing an important limiting role in sediment denitrification within mitigation wetlands.
Highlights
Wetlands represent a useful mitigation tool to remove dissolved nutrients, nitrate (NO3–N), in agricultural runoff from tiles or surface waters (Gale et al 1993; Xue et al 1998; Lund et al 2000)
Wetland type carbon study The 2:1 and 4:1 C:N treatments resulted in denitrification rates significantly higher than that of the control (F2,8 = 148.09, P < 0.0001), and increases in denitrification rates did not differ between surface and tile water wetland sediments (F1,4 = 0.55, P = 0.5010; Fig. 4; Table 1)
C:N:P study The results of this laboratory study show that wetland sediments receiving elevated NO3–N levels commonly observed in central Illinois are strongly dissolved organic carbon (DOC) but not P limited
Summary
Wetlands represent a useful mitigation tool to remove dissolved nutrients, nitrate (NO3–N), in agricultural runoff from tiles or surface waters (Gale et al 1993; Xue et al 1998; Lund et al 2000). The effectiveness of these wetlands varies widely, and wetland size relative to drainage area required to effectively reduce nitrate (NO3–N) in runoff ranges from 1:100 to 1:10 (Higgins et al 1993; Woltemade 2000). This wide range of area described for effective NO3–N reduction may be at least in part due to the need for appropriate inputs of carbon, dissolved organic carbon (DOC) and phosphorus (P) in addition to NO3–N. Many Midwestern agricultural fields, in Illinois, have been drained with porous corrugated plastic and clay pipe that drains water from the field directly to the streams, bypassing many land surface mitigation structures (Lemke et al 2011)
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