Mécanismes et maitrise de la pollution diffuse agricole : le cas du phosphore et sa portée générale

Mechanisms and control of agricultural diffuse pollution: the case of phosphorus. The movement of phosphorus is primarily a surface phenomenon closely related to surface runoff and erosion. It is a discontinuous process, set in motion during surface runoff events. The potential for surface runoff begins with any process reducing the infiltration rate of the soil surface. The contribution of cultivated fields to surface runoff varies spatially and temporally in relation to their permeability, soil type, land use, and soil cover. The contribution of these fields to P transfer and export varies in the same way but depends also on the P content of the soils, especially at the soil surface. Subsurface flows general carry in much less phosphorus due to its fixation in the subsoil, except in some conditions such as sandy soils, long-term over-fertilization, and agricultural drainage. Some elements of the landscape are considered as buffers because they can filter and thus attenuate the P fluxes transferred in the hydrologic network (grass filter strips which are purposely constructed but also hedgerows and riparian vegetation). Within the network, particulate forms of P may settle may and dissolved forms of P may sorb onto sediment particles or precipitate or be taken up by biota. In addition, major obstacles such as wetlands can store and/or transform some of the phosphorus emitted from upstream fields. Thus a set of interacting processes moves P through the landscape within a "transfer system", which includes emission from sources (mainly soils or sediments) and transport (including transformation and attenuation). This transfer system comprises a highly diverse and complex set of landscape structures arranged in a complex and diverse hydrologic hierarchy and interacting with agricultural practices and management of the hydrologic network. This paper attempts to characterize the key phenomena and structures of the transfer system that determine the flux, storage, transfer and mass balance of phosphorus in agricultural watersheds. Strategies for controlling diffuse phosphorus pollution are also indicated.