Effectiveness of Constructed Wetlands in Reducing Nitrogen and Phosphorus Export from Agricultural Tile Drainage

TLDR

Agricultural runoff is the main source of nonpoint nitrogen and phosphorus entering Midwest surface waters. The study aimed to evaluate whether constructed wetlands could reduce nonpoint nitrogen and phosphorus exports from agricultural tile drainage. Three treatment wetlands (0.3–0.8 ha, 1200–5400 m³) were built on Colo soils between maize and soybean fields and the Embarras River, and water and nutrient budgets were monitored from Oct 1994 to Sept 1997. Over three years the wetlands removed 37 % of nitrogen inputs (1697 kg of 4639 kg) and, with a 15.3‑m buffer strip, 46 %, while phosphorus removal was only 2 %, showing that constructed wetlands can effectively reduce agricultural nitrogen loading to surface waters.

Abstract

Abstract Much of the nonpoint N and P entering surface waters of the Midwest is from agriculture. We determined if constructed wetlands could be used to reduce nonpoint N and P exports from agricultural tile drainage systems to surface waters. Three treatment wetlands (0.3 to 0.8 ha in surface area, 1200 to 5400 m 3 in volume) that intercepted subsurface tile drainage water were constructed in 1994 on Colo soils (fine‐silty, mixed, superactive, mesic Cumulic Endoaquoll) between upland maize ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] cropland and the adjacent Embarras River. Water (tile flow, precipitation, evapotranspiration, outlet flow, and seepage) and nutrient (N and P) budgets were determined from 1 Oct. 1994 through 30 Sept. 1997 for each wetland. Wetlands received 4639 kg total N during the 3‐yr period (96% as NO 3 ‐N) and removed 1697 kg N, or 37% of inputs. Wetlands decreased NO 3 −N concentrations in inlet water (annual outlet volume weighted average concentrations of 4.6 to 14.5 mg N L −1 ) by 28% compared with the outlets. When the wetlands were coupled with the 15.3‐m buffer strip between the wetlands and the river, an additional 9% of the tile NO 3 −N was apparently removed, increasing the N removal efficiency to 46%. Overall, total P removal was only 2% during the 3‐yr period, with highly variable results in each wetland and year. Treatment wetlands can be an effective tool in reducing agricultural N loading to surface water and for attaining drinking water standards in the Midwest.