Abstract. Wastewater sludges are used in agriculture as soil amendment and fertilizer, with regard to their organic matter and nutrient content. However, availability of nitrogen and phosphorus from sludge-amended soils and their transfer in runoff may lead to eutrophication of downstream surface water. The aim of this study is to establish and compare the effect of two different sludges on these transfers: an anaerobically digested and thermically stabilised sludge (Seine-Aval treatment plant, sludge no. 1), and a limed sludge (Saint-Quentin treatment plant, sludge no. 2). Experiments were performed on 12 sloping micro-plots (1 m × 1 m) submitted to sludge spreading and controlled rainfall simulation. Runoff water was sampled and analysed for concentrations in nitrogen species and phosphorus. Results show that spreading of sludge no. 1 increased both ammonium nitrogen (mean of 1.1 mg L −1 N-NH 4 vs. 0.2 mg L −1 N-NH 4 for control micro-plots) and particulate phosphorus concentrations (mean of 2 mg L −1 P vs. 1.1 mg L −1 P for control micro-plots) in runoff water. On the other hand, sludge no. 2 did not induce any significant effect on nutrient concentrations in runoff. These results are related to chemical composition and physical treatment of sludges. This study underlines the existence of a short-term risk of nutrient mobilisation by runoff after sludge spreading on soil, and the need to check precisely the impact of this practice on water quality.

Changes initiated in the 1970s in the agricultural landscape of the north European plains have resulted in increased water erosion. The spatial redistribution of 137Cs was used to assess the magnitude of long‐term soil movements in a 180 ha watershed located in northern France. The 137Cs base level was estimated at 2000 ± 200 Bq m−2. The point data suggest annual soil movements ranging from a loss of 18 Mg ha−1 to a deposition of 19 Mg ha−1. After spatializing these data, it is concluded that 41% of the watershed area can be considered as stable. Some 45% of the watershed area suffered a net soil loss, with an average rate of 6 Mg ha−1 yr−1. Net deposition occurred over 14% of the area, with an average annual rate of 7 Mg ha−1. From these values, it was estimated that a net output of 1.9 Mg ha−1 yr−1 of eroded material would have left the watershed. This represents a sediment delivery ratio of 59%. These results are consistent with those of other European researchers under comparable environments. They also stress the point that redeposition is an important part of the global erosion process. Finally, these results highlight some of the advantages of the 137Cs technique, which makes possible the development of long‐term sediment budgets at the watershed scale.

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La phytoremédiation compte parmi les méthodes innovantes de dépollution des sols de plus en plus utilisées dans des milieux complexes en fonction de la nature des polluants à traiter. Elle fait partie de la famille des traitements biologiques appliqués aux sols pollués dont l’usage remonte au milieu des années 80, mais dont la généralisation ne date que des années 90. D’abord en pleine expansion en Amérique du Nord (États-Unis et Canada), la phytoremédiation était il y a encore une dizaine d’années très faiblement employée en Europe, en France, et en Afrique, notamment au Gabon. La phytoremédiation offre une réelle alternative, aussi bien écologique que paysagère et financière face aux techniques de dépollution traditionnelles telles que l’excavation, l’épandage ou l’incinération, pour aider à la réhabilitation des terrains contaminés et leur reconversion à des fins d’aménagement agricole, urbain ou de loisirs. La ferme expérimentale d’Auby ou le site Total de Vendin-Le-Vieil dans la région du Nord-Pas-de-Calais et celui des bourbiers de torchage de l’île Mandji au Gabon en sont de bons exemples. Même si cette technique présente des limites, notamment au niveau de la durée des traitements et de la revalorisation des résidus contaminés (cendres), elle demeure moins onéreuse et assez fiable. Elle s’inscrit aussi dans la durabilité, dans la mesure où la reconversion d’un site pollué en espace vert engendre d’une part un paysage verdoyant et d’autre part un système naturel de dépollution des terrains, ce qui de toute évidence facilitera la tâche des décideurs de l’aménagement raisonné du territoire.