Nitrogen removal in buffer strips along a lowland stream in the Netherlands: a pilot study

Abstract

Many surface waters in The Netherlands are polluted with nutrients from agricultural land. Riparian buffer zones have been shown to be very valuable in reducing non-point source pollution from agricultural land to streams. The nitrogen removal in two riparian zones vegetated with alder thicket and grass respectively and the contribution of denitrification activity in nitrate removal were investigated. In the period from September 1996 to June 1997, nitrogen transport and denitrification activity in the headwaters of a small stream, the Hazelbeek in Twente, was measured at 11 dates. At all sampling dates we found clear spatial differences in groundwater quality. High nitrate concentrations (>40 mg N l −1) were measured in groundwater tubes placed at the boundary of the agricultural land (maize) and the riparian forest, and lower nitrate concentrations were measured close to the stream (0.1–2 mg N l −1). Stream water quality measurements indicated that (sub) surface flow from the maize field strongly contributed to the nitrogen load in the stream. Results suggest that nitrate concentrations in groundwater decreased by 95% when it flowed through the riparian buffer zone. Denitrification rates measured in the top soil (0–30 cm) of the buffer zones varied between 9 and 200 kg N ha −1 year −1 in the forested buffer zone and between 1.2 and 32 kg N ha −1 year −1 in the grassland buffer zone. The higher denitrification rates measured in the forest zone compared to the grassland zone could be due to the higher availability of nitrate in the forested soil compared to the grassland zone and/or a higher residence time of the water in the forested zone.