Nitrate removal processes in a constructed wetland treating drainage from dairy pasture

N-15-labelled NO3- was used in a surface-flow constructed wetland in spring to examine the relative importance of competing NO3- removal processes In situ mesocosms (025 m(2)) were dosed with 21 of (NO3-)-N-15 (NaNO3, 300 mg Nl(-1). 99 atom% N-15) and bromide (Br-) solution (LiBr, 43 gl(-1), as a conservative tracer). Concentrations of NO3-, Br-, dissolved oxygen and N-15(2) were monitored periodically and replicate mesocosms were destructively sampled prior to and 6 clays after N-15 addition. Denitrification, immobilisation, plant uptake and dissimilatory NO3- reduction to NH4+ (DNRA) accounted for 77, 11, 9 and 2% of (NO3-)-N-15 transformed during the experiment Only 6% of denitrification gases were directly measured as atmospheric or dissolved N-15(2): the remainder (71%) was determined via N-15 mass balance This indicated that a large proportion of the denitrification gases were entrapped within the soil matrix and/or plant aerenchyma The floating plant Lemna minor exhibited a significantly higher NO3- uptake rate (221 mg kg(-1) d(-1)) than Typha orientalis (10 mg kg(-1) d(-1)), but periodic harvest of plants would remove <3% of annual NO3- inputs Our results suggest that this 6-year-old constructed wetland functions effectively as a sink for NO3- during the growing season with less than one-quarter of the NO3- processed sequestered into wetland plant, algal and microbial N pools and the balance permanently removed by denitrification. (C) 2010 Elsevier B V All rights reserved