1. EFFECTS OF EDAPHIC FACTORS AND AGRICULTURAL PRACTICES ON THE MOBILIZATION CAPACITY OF PHOSPHORUS IN SOILS AND SEDIMENTS INTRODUCTION The surface water contaminations by human activities are a major concern in Europe and appropriate P management becomes crucial to meet the Water Framework Directive objectives. Indeed, soil phosphorus faces both environmental and agronomic issues because it is responsible for eutrophication of surface waters and, at the same time, it is an essential element for plants growth. Moreover, an economic issue comes up because the mineral reserves are limited and the cost of fertilizers will continue to increase in the future. 1 University of Liege - Gembloux Agro-Bio Tech – Geopedology Laboratory. Passage des Déportés, 2. B-5030 Gembloux - Malorie.Renneson@ulg.ac.be ² University of Louvain-la-Neuve – Soil sciences unit. Organic and inorganic fertilization Adsorption isotherms, fractionation and forms of P 28 days SOIL SEDIMENTS WATER PLANTS Fertility Eutrophication Fertilization P mobilization Erosion Runoff SOIL SOLUTION Characterization Availability Mobilization capacity Influence of agricultural practices Influence of parental materials Influence of environmental conditions (temperature, biological activity, organic matter content, P content, …) Field conditions Controlled conditions (incubation study) No replacement of water Replacement of water Monitoring of the evolution during 8 weeks, agitation everyday, sampled once a week OBJECTIVES This PhD research aims to contribute to the understanding of the mobilization capacity of phosphorus in soils and sediments. It consists of an assessment of the effects of edaphic factors, fertilization policy and environmental conditions on forms and contents of phosphorus in soils and sediments. This project, focused on the phosphorus cycle, is organized in three issues: a characterization of natural geochemical background, the phosphorus availability for plant growth and the mobilization of the phosphorus from sediments to water. CURRENT RESULTS A characterization of the geochemical background in the major parental materials has been made in Walloon Region. This research has underscored the important role of soil properties and parental material on phosphorus availability. Differences between parental materials suggest to adapt phosphorus management to regional properties. Moreover, land use (crop or grassland) has also an influence on soil phosphorus content. So, agronomic and environmental thresholds have to be different according to soil type and land use. A study has also been realized on the degree of phosphorus sorption (DPS), which is an environmental indicator of the soil phosphorus status. Our results allowed to calibrate specific model for the North of Walloon Region. Differences between parental materials Land use influence Experimental trials with different treatments (organic matter and P fertilization) in loamy soil (CRA-W – Gembloux) ACKNOWLEDGEMENTS This research is conducted with the financial support of the « Fonds pour la formation à la Recherche dans l’Industrie et dans l’Agriculture ». Phosphorus cycle RENNESON 1 M., DUFEY 2 J., BOCK 1 L. and COLINET 1 G.