Reduction of nutrient loads from agricultural subsurface drainage water in a small, edge-of-field constructed treatment wetland

Constructed treatment wetlands are a common practice for filtering nitrogen from agricultural sub-surface (i.e. tile) drainage in the Midwest. Wetland size recommendations typically depend on watershed size, but little is known about the effectiveness of wetlands with a pooled area as small as 0.11 ha combined with a high infiltration rate and sub-surface treatment. A small, edge-of-field wetland treating tile drainage from a 10.1-ha row-crop field was constructed in 2013 adjacent to Elm Creek in Martin County, MN. The water, nitrate‑nitrogen (NO₃⁻-N), and phosphorus (P) budgets were determined for 2013 to 2016. The wetland received an annual average of 12.6 m drainage water, 185 kg NO₃⁻-N, and 1.3 kg total phosphorus (TP) each of the four years of this study. It removed 67% of the input NO₃⁻, but TP removal was inconclusive. The majority of NO₃⁻ was removed in the shallow groundwater flow after it infiltrated from the surface. Approximately 3.2 kg-P ha⁻¹ yr⁻¹ was removed by harvesting the vegetation in the wetland. Therefore, it can be concluded that NO₃⁻ removal in the shallow groundwater and TP removal through vegetation harvest were the primary means of removing each nutrient.