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TECHNICAL REPORTS

Landscape and Watershed Processes

Nitrogen Removal in Valley Bottom Wetlands

Assessment in Headwater Catchments Distributed throughout a Large Basin

INRA-Agrocampus Rennes, UMR Sol-Agronomie-Spatialisation, 65 rue de Saint-Brieuc, CS 84215, 35042 Rennes Cedex, France

^* Corresponding author (Philippe.merot{at}rennes.inra.fr)

Received for publication March 6, 2006. Although the reduction of nutrient loading between uplands and streams is sometimes considered evidence of the effect of wetlands acting as buffer zones, the influence of valley bottom wetlands (VBWs) on NO₃^– loading has seldom been assessed at the catchment scale. The objective of this study was to quantify the impact of VBWs on NO₃^– concentrations in streams in the Brittany region of France. We analyzed the spatial variation in NO₃–N concentrations in 18 headwater catchments located in a 400-km² basin, with varying topographic, climatic, and agricultural intensity conditions. Approximately every 10 d, water was sampled during the high flow season. We investigated the relationships between the mean NO₃^– concentration and different characteristics of the catchments: (i) the amount of effective rainfall, i.e., the combined effect of precipitation and actual evapotranspiration on discharge and chemical dilution, (ii) the intensity of farming, i.e., the area used for farming in the catchments and the surplus of the agricultural N budget, and (iii) the relative area of VBWs. Although the first two characteristics were the main factors controlling N concentration variability, a step-by-step regression allowed us to attribute a significant part of the NO₃^– concentration decrease to the increase of VBW area in each catchment. For an increase of VBW area from 11 to 16%, the NO₃–N concentration decreased from 5.3 to 4.2 mg L^–1. Therefore in this basin, VBWs reduced the NO₃^– concentrations in streams with sources in agricultural fields by 30%. This work demonstrates the contribution of natural VBWs to NO₃^– removal at the catchment scale compared to other sources of variation, which is a current need for integrating water quality criteria into wetland management.

Abbreviations: Ci, CORPEN index (agricultural nitrogen surplus) • TNI, total nitrogen input • VBW, valley bottom wetland • NO₃–N_cd, nitrate concentration corrected for dilution • NO₃-N_cN, hypothetical NO₃–N concentration for a zero agricultural N surplus