title: Constructed wetland attenuation of nitrogen exported in subsurface drainage from irrigated and rain-fed dairy pastures.
abstract: Nitrogen removal performance is reported for constructed wetlands treating subsurface drainage from irrigated and rain-fed dairy pastures in North Island, New Zealand. Flow-proportional sampling of inflow and outflow concentrations were combined with continuous flow records to calculate mass balances for the wetlands. Drainage flows from the irrigated catchment were 2.5-4 fold higher and N exports up to 5 fold higher per unit area than for the rain-fed catchment. Hydraulic and associated N loadings to the wetlands were highly pulsed, associated with rainfall, soil water status, and irrigation events. Transient pulses of organic nitrogen were an important form of N loss from the rain-fed landscape in the first year, and were very effectively removed in the wetland (> 90%). Median nitrate concentrations of approximately 10 g m(-3) in the drainage inflows were reduced by 15-67% during passage through the wetlands and annual nitrate-N loads by 16-61% (38-31 7 g N m(-2)y(-1)). Generation in the wetlands of net ammoniacal-N and organic-N (irrigated site) partially negated reduction in nitrate-N loads. The results show that constructed wetlands comprising 1-2% of catchment area can provide moderate reductions in TN export via pastoral drainage, but performance is markedly influenced by variations in seasonal loading and establishment/maturation factors.
authors.affiliation: National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand. c.tanner@niwa.co.nz
authors.forename: C C
authors.initials: CC
authors.lastname: Tanner
authors.forename: M L
authors.initials: ML
authors.lastname: Nguyen
authors.forename: J P S
authors.initials: JP
authors.lastname: Sukias
mesh_terms: D000818:Animals; D001673:Biodegradation, Environmental; D002417:Cattle; D003612:Dairying; D017753:Ecosystem; D009566:Nitrates / Q000302:isolation & purification; D009584:Nitrogen / Q000302:isolation & purification*; D014865:Waste Disposal, Fluid / Q000379:methods*; D014872:Water Movements; D014881:Water Supply
chemical_list: D009566:Nitrates; D009584:Nitrogen