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TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases

Nitrous Oxide Emission and Denitrification in Chronically Nitrate-Loaded Riparian Buffer Zones

Mariet M. Hefting^*, Roland Bobbink and Hannie de Caluwe

Section of Landscape Ecology, Dep. of Geobiology, Faculty of Biology, Utrecht Univ., P.O. Box 80084, 3508TB Utrecht, the Netherlands

^* Corresponding author (M.M.Hefting{at}bio.uu.nl)

Received for publication September 25, 2002. Riparian buffer zones are known to reduce diffuse N pollution of streams by removing and modifying N from agricultural runoff. Denitrification, often identified as the key N removal process, is also considered as a major source of the greenhouse gas nitrous oxide (N₂O). The risks of high N₂O emissions during nitrate mitigation and the environmental controls of emissions have been examined in relatively few riparian zones and the interactions between controls and emissions are still poorly understood. Our objectives were to assess the rates of N₂O emission from riparian buffer zones that receive large loads of nitrate, and to evaluate various factors that are purported to control N emissions. Denitrification, nitrification, and N₂O emissions were measured seasonally in grassland and forested buffer zones along first-order streams in the Netherlands. Lateral nitrate loading rates were high, up to 470 g N m^-2 yr^-1. Nitrogen process rates were determined using flux chamber measurements and incubation experiments. Nitrous oxide emissions were found to be significantly higher in the forested (20 kg N ha^-1 yr^-1) compared with the grassland buffer zone (2–4 kg N ha^-1 yr^-1), whereas denitrification rates were not significantly different. Higher rates of N₂O emissions in the forested buffer zone were associated with higher nitrate concentrations in the ground water. We conclude that N transformation by nitrate-loaded buffer zones results in a significant increase of greenhouse gas emission. Considerable N₂O fluxes measured in this study indicate that Intergovernmental Panel on Climate Change methodologies for quantifying indirect N₂O emissions have to distinguish between agricultural uplands and riparian buffer zones in landscapes receiving large N inputs.

Abbreviations: EF5-g, nitrous oxide emission factor from ground water and agricultural drainage water • PCA, principal component analysis

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