Le maintien de « zones tampons » telles que les bandes enherbées permet de capter les transferts superficiels de produits phytosanitaires et les Zones Tampons Humides Artificielles (ZTHA), les transferts par les réseaux de drainage. Les premières peuvent être qualifiées d'innovations réussies dans le sens où elles sont passées d'un stade expérimental vers une adoption dans les textes réglementaires et par la profession agricole. Les deuxièmes, encore au stade expérimental, ont nécessité un compromis entre un optimum technique et les requêtes des agriculteurs, afin d'être diffusées une première fois. En nous basant sur une analyse ex-post pour les bandes enherbées et ex-ante pour les ZTHA, nous analysons leur passage depuis la phase de conception vers leur appropriation par les usagers. Le déploiement de ces deux dispositifs suit deux approches différentes mais la mise en place d'une réglementation contraignante, encore non instaurée pour les ZTHA, est le dénominateur commun.

The Orgeval watershed (104 km(2)) is a long-term experimental observatory and research site, representative of rural areas with intensive cereal farming of the temperate world. Since the past few years, we have been carrying out several studies on nitrate source, transformation and transfer of both surface and groundwaters in relation with land use and agriculture practices in order to assess nitrate (NO3(-)) leaching, contamination of aquifers, denitrification processes and associated nitrous oxide (N2O) emissions. A synthesis of these studies is presented to establish a quantitative diagnosis of nitrate contamination and N2O emissions at the watershed scale. Taking this watershed as a practical example, we compare curative management measures, such as pond introduction, and preventive measures, namely conversion to organic farming practices, using model simulations. It is concluded that only preventive measures are able to reduce the NO3(-) contamination level without further increasing N2O emissions, a result providing new insights for future management bringing together water-agro-ecosystems.

There is concern that subsurface drainage, by destroying or by-passing active denitrification areas, may prevent nitrate retention processes and enhance nitrate contamination of surface water by agriculture. To address this question, we studied the flow and concentration signatures of drainage waters and their transformations in a series of 5 nested watersheds, from 1 to 100km2 area, in the Brie region near Paris (France). At all scales, nitrate concentrations are generally higher during the winter drainage season compared to the low flow periods (late spring to early fall). High nitrate concentrations characterizing drainage waters are visible at the 1st, 2nd and 3rd stream order but are “diluted” by surface runoff from forested zones and buffered by groundwater contributions. The analysis of nitrate chemographs and nitrate budgets established for the different nested watersheds show significant nitrogen retention. Isotopic measurements indicate that the nitrate pool is enriched in δ15N–NO3- as its concentration decreases. Direct estimation of benthic denitrification with benthic chambers allowed concluding that benthic denitrification is not the only retention mechanism and that “underground” denitrification, affecting nitrate on its way from the base of the root zone down to the limit of the river bed, may in fact dominate nitrogen retention processes even in this intensively drained watershed.

Étudier les transferts de nitrate dans un contexte de parcelles drainées permet de mettre en évidence les principaux processus de lessivage. La présence du drain à 1 m de profondeur modifie fortement les chemins d'écoulement de l'eau et des nitrates. Les concentrations mesurées en sortie de parcelle drainée (site de La Jaillière) sont liées à la localisation du stock d'azote entre le drain et l'interdrain. Une approche pluriannuelle introduit la notion de cycle de lessivage dont la gestion du reliquat entrée-hiver est une clef. Au niveau du bassin versant (site de l'Orgeval), la compréhension des concentrations en nitrate est plus complexe : les interactions nappe-cours d'eau-ripisylve s'observent à différentes échelles spatiales. Outre l'adaptation des pratiques culturales pour limiter le reliquat d'azote d'entrée-hiver, la mise en valeur des éléments du paysage pour favoriser les processus d'élimination des nitrates (dénitrification benthique et riparienne) fait partie des pistes pour une maîtrise des flux de nitrates dans le bassin versant.

The aim of this research was to characterize the biogeochemical functioning of isolated first order watersheds of forested, agricultural and mixed land use and to locate the compartments where nitrogen retention may occur at this scale. Three first order catchments were studied. An agricultural watershed of 1.3 km, a forested watershed of 1.1 km and a mixed land use watershed of 1.7 km containing the agricultural watershed and a forested area downstream. The forested watershed (named F) served as a reference for the nitrate contribution of a forested area in order to understand the transfer and maybe the retention of nitrogen in the forested area of the mixed land use watershed. Flow rates and nitrate concentrations have been monitored at the outlets of the catchments. A first scheme of the hydrological behavior of the mixed land use watershed has been established. Moreover, the dry winter season 2004-2005 allowed to find and estimate deep contributions, which most likely interfere with the general hydrological functioning of the watershed and hide possible retention processes. In an effort to identify the origin and processes undergone by nitrate molecules, 15N and 18O isotopic analyses will be performed in the future.