Nitrogen and phosphorus removal using tile-treatment wetlands: A 12-year study from the midwestern United States

Nutrient enrichment from tile-drained agricultural lands to the Mississippi River is a leading cause of hypoxia in the Gulf of Mexico. Small edge-of-field wetlands can effectively treat nitrate-nitrogen (NO₃ -N) export from tiles, although less research exists on their capacity to treat phosphorus (P). Additionally, long-term data are needed to incorporate variability of weather and farming practices into assessments of wetland performance longevity. Research conducted over 12 yr quantified size-effectiveness of wetlands to reduce NO₃ -N and dissolved P (orthophosphate [ORP]) loadings from subsurface tile systems. Nitrate-N export was significantly higher during corn (Zea mays L.) than soybean [Glycine max (L.) Merr.] production years, during which 80-84% of mean annual loadings were exported during spring. Wetlands representing 3% (W1) of tile-drained farmland area reduced 15-38% of NO₃ -N export, with cumulative reductions of 39-49 and 49-57% observed in wetlands representing 6 (W2) and 9% (W3) areas, respectively. Mass NO₃ -N removal ranged from 28 to 52%. Twelve-year total ORP load reductions for W1 ranged from 53 to 81%, with cumulative reductions of 35-91% and 32-95% for W2 and W3 wetlands, respectively. Mass ORP removal ranged from 71 to 85%. Results emphasize how incorporating constructed wetlands into state and watershed-level conservation planning can significantly contribute toward reducing excess N and P export to river systems and ultimately to the Gulf of Mexico.