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Nitrogen and phosphorus removal using tile-treatment wetlands: A 12-year study from Midwestern USA.

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. Additionally, long-term data are needed to incorporate variability of weather and farming practices into assessments of wetland performance longevity. Research conducted over 12 years quantified size-effectiveness of wetlands to reduce NO3 -N and dissolved phosphorus (ORP) loadings from subsurface tile systems. Nitrate N export was significantly higher during corn than soybean 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 28-52%. 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 71-85%. Results emphasize how incorporating constructed wetlands into state and watershed-level conservation planning can significantly contribute towards reducing excess nitrogen and phosphorus export to river systems, and ultimately to the Gulf of Mexico. This article is protected by copyright. All rights reserved.