Spatial Variation in Denitrification and N2O Emission in Relation to Nitrate Removal Efficiency in a N-stressed Riparian Buffer Zone

Abstract

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N2O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N2O were measured using flux chambers and incubation experiments. In winter, N2O emissions were significantly higher (12.4 mg N m−2 d−1) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m−2 d−1). In summer a reverse pattern was observed, with higher N2O emissions (13.6 mg N m−2 d−1) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N2O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N2O emission from riparian zones. Furthermore, we conclude that high variability in N2O:N2 ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.