Five year water and nitrogen balance for a constructed surface flow wetland treating agricultural drainage waters

The performance of a constructed surface flow wetland in reducing diffuse N pollution coming from croplands is being investigated in an ongoing experiment, begun in 1998 in NE Italy. The 0.32 ha wetland is vegetated with Phragmites australis (Cav.) Trin. and Typha latifolia (L.). It receives drainage water from 6 ha of land managed for an experiment on drainage systems, where maize, sugarbeet, winter wheat and soybean are cultivated. During the period 1998–2002, the wetland received from 4698 to 8412 mm of water per year (on average, about 9 times the environmental rainfall); its water regimen was discontinuous and flooding occurred on a variable number of days per year (from 13 to 126). Nitric nitrogen was the most important form of element load. Its concentration in the inflow water over time was rather discontinuous, with median values ranging from 0.2 (in 2001) to 4.5 (in 2000) mg L−1. Inflow nitric N concentrations were occasionally in the 5–15 mg L−1 range. Concentrations reduced passing through the wetland, with a more evident effect in the last year. Over 5 years, the wetland received slightly more than 2000 kg ha−1 of nitrogen, 87% in nitric form mostly from farmland drainage. The remaining 13% of N was applied as organic slurry directly onto the wetland, with 5 distributions during 1998 to assess wetland performance in treating occasional organic loads. Field drainage loads had a discontinuous time pattern and occurred mostly during autumn–winter, with the exception of the 2001–2002 season which was a very dry. The wetland discharged 206 kg ha−1 of N, over the 5-year period, with an apparent removal efficiency of about 90%. The disappearance was mostly due to plant uptake (1110 kg ha−1) and soil accumulation (570 kg ha−1), with the contribution of denitrification being estimated at around 7%.