Effet des caractéristiques pédologiques et des pratiques agricoles sur la disponibilité du phosphore dans les sols de Wallonie | Effect of pedological factors and agricultural practices on P biodisponibility in walloon soils

Essential element for plant growth, phosphorus (P) is also a problematic element, due to the limitation of mineral resources but also the environmental problems which can be caused, as eutrophication. For several years, scientific studies have focused on finding other sources of P and making this element more available for plants. However, improving the bioavailability of P involves a better knowledge of the stocks present in Walloon soils but also a better identification of factors governing its mobility for plants and the environment. This thesis focused on knowing P content and behaviour in Walloon soils and the impact of agricultural practices but also evaluating the P indicators. The first part of the thesis characterized the geochemical background and agricultural practices on soil P stocks and its mobility. Homogeneous and specific groups of soils have been defined. It permitted to characterize the geopedochemical signature of the main Walloon parent materials as well as the influence of the land use. P management should be reflected on the basis of these soil groups. Regression equations have been defined to estimate soil total P, a parameter that is rarely analyzed but necessary. In a second step, the influence of organic and inorganic inputs on the mobility of P was evaluated in the short-term under controlled conditions but also in the long-term thanks to experimental plots. A comparable effect of these different inputs has been observed in silty soils and the potentiality of organic matter in phosphate fertilization demonstrated. Moreover, it permitted to observe release kinetics of P in these soils. Finally, agricultural and environmental indicators that can be used in the Walloon Region have been evaluated according a lot of parameters. The efficiency of the available P indicator currently used as an agronomic indicator in Walloon soils has been demonstrated. This indicator has a good benefit / disadvantage ratio. In addition, it is well correlated with the degree of phosphorus saturation, an environmental indicator expressing the risk of P loss to the environment. Developed in acidic sandy soils, this indicator can also be used in the Walloon soils, but only after adaptations to the Walloon pedological context. To conclude, in view of the important differences between the geopedological regions, it is essential to take into account these specificities in the management of the P in the Walloon territory. In addition, reflections must be carried out to improve the use of organic matters and better enhance soil organic P, which is important in some soils.