title: Multiyear nutrient removal performance of three constructed wetlands intercepting tile drain flows from grazed pastures.
abstract: Subsurface tile drain flows can be a major s ource of nurient 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 too 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 rangedfrom 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 gTN 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 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.
authors.affiliation: National Institute of Water & Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand. c.tanner@niwa.co.nz
authors.forename: Chris C
authors.initials: CC
authors.lastname: Tanner
authors.forename: James P S
authors.initials: JP
authors.lastname: Sukias
mesh_terms: D000383:Agriculture; D009520:New Zealand; D009584:Nitrogen / Q000378:metabolism; D010758:Phosphorus / Q000378:metabolism; D011891:Rain; D014872:Water Movements; D014881:Water Supply; D053833:Wetlands
chemical_list: D010758:Phosphorus; D009584:Nitrogen