Multiyear Nutrient Removal Performance of Three Constructed Wetlands Intercepting Tile Drain Flows from Grazed Pastures
- Chris C. Tanner * and
- James P. S. Sukias
Subsurface tile drain flows can be a major source of nutrient loss from agricultural landscapes. This study quantifies flows and nitrogen and phosphorus yields from tile drains at three intensively grazed dairy pasture sites over 3- to 5-yr periods and evaluates the capacity of constructed wetlands occupying 0.66 to 1.6% of the drained catchments to reduce nutrient loads. Continuous flow records are combined with automated flow-proportional sampling of nutrient concentrations to calculate tile drain nutrient yields and wetland mass removal rates. Annual drainage water yields ranged from 193 to 564 mm (16–51% of rainfall) at two rain-fed sites and from 827 to 853 mm (43–51% of rainfall + irrigation) at an irrigated site. Annually, the tile drains exported 14 to 109 kg ha−1 of total N (TN), of which 58 to 90% was nitrate-N. Constructed wetlands intercepting these flows removed 30 to 369 g TN m−2 (7–63%) of influent loadings annually. Seasonal percentage nitrate-N and TN removal were negatively associated with wetland N mass loadings. Wetland P removal was poor in all wetlands, with 12 to 115% more total P exported annually overall than received. Annually, the tile drains exported 0.12 to 1.38 kg ha−1 of total P, of which 15 to 93% was dissolved reactive P. Additional measures are required to reduce these losses or provide supplementary P removal. Wetland N removal performance could be improved by modifying drainage systems to release flows more gradually and improving irrigation practices to reduce drainage losses.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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