Factors affecting nitrogen retention in small constructed wetlands treating agricultural non-point source pollution

Nitrogen retention was investigated from 3 to 7 years in four surface-flow constructed wetlands (CWs) in the cold temperate climate of Norway. The constructed wetlands were originally built to mitigate the effects of soil particle and phosphorus runoff from arable land to freshwater recipients. The wetlands were located in first and second order streams, with surface areas of 350 to 900 m(2), corresponding to 0.06 to 0.4% of the watersheds. Each CW had a volume proportional composite sampler in the inlet and outlet. Nitrogen retention was only 3 to 15% of the N-input, due to the high hydraulic load (0.7 to 1.8 m per day) and low temperatures ( - 8 to 18 degreesC). This equals an average retention of 50 to 285 g nitrogen m(-2) per year or a first-order area constant of 31 to 135 m per year. Sedimentation of nitrogen in organic particles was the main retention process., even though denitrification was significant for the wetlands with the lowest hydraulic load. Retention of organic particles (43 to 67%) usually increased with hydraulic load, probably because organic matter was associated with soil particles with a higher sedimentation velocity under storm runoff conditions. Nitrogen retention decreased as the wetlands aged, presumably because trapped organic nitrogen was converted to inorganic forms that were exported from the wetlands. The best statistical prediction model reproduced observed data from two test-CWs with a deviation of less than 3%. (C) 2002 Elsevier Science B.V. All rights reserved.