The transfer diffuse phosphorus in agricultural watersheds: orders of magnitude, mechanisms, control

Abstract

Diffuse phosphorus emission from land surfaces includes a combination of forms (inorganic and organic, dissolved and particulate). The impact diffuse P inputs has on the eutrophication of surface waters depends on the biogeochemical properties and nature of the P transferred. A key property is the bioavailability of P, which indicates the proportion of P that is available for biological processes. Most of the P in ecosystems is associated with particles (particulate-P). Particulate-P often accounts for the major part of total phosphorus transferred from terrestrial sources, such as soils, to aquatic ecosystems. The movement of total phosphorus is primarily a surface phenomena closely tied to runoff and erosion and a discontinuous process, in motion during surface runoff events. The potential for this runoff begins with any process reducing the infiltration rate of the soil surface. Cultivated contribution to surface runoff vary spatially and temporally in relationship with their permeability, soil type, land use, and soil cover. Their contribution to P transfer and export vary in the same way but depend also on P content of soils, specially soil surface. Subsurface flow carries in general much less phosphorus due to fixation in the subsoil, except in some conditions including sandy soils, long-term over-fertilization, and agricultural drainage. Some element of the landscape are considered as buffers because they can filtrate and thus attenuate the P flux transferred to the hydrologic network (permanent vegetations e.g., grass strips, hedgerows, riparian vegetation). In the network, particulate forms of P can settle and dissolved P can sorb onto the network surfaces or be taken up by biota. In addition, major obstacles like wetlands can store and/or transform the phosphorus emitted from upstream fields. Thus a set of interacting processes move P through the landscape in a transfer system that includes emission from sources (mainly soils or sediments) and transport (including transformation and attenuation). This transfer system comprise a highly diverse and complex set of landscape structures arrayed in a complex and diverse hydrologic hierarchy and interacting with agricultural practices and management of the hydrologic network. This paper try to characterize the key phenomena and structures of the transfer system that condition the fluxes, storages, transfers and mass balances of phosphorus in agricultural watersheds. Strategies for controlling diffuse phosphorus pollution are also reviewed and discussed based on the experience gained in lake Léman area.