Phosphorus storage in soil: monitoring and analysis at national and regional scales

Soil is the major storage compartment in the global phosphorus (P) cycle is therefore a potential source of P to surface water.The aim of this paper is to analyse existing French soil test databases in order to evaluate at national regional levels the status of soil P its change over the last decades.Two main databases were considered: i.the French national Soil Test Database, which integrates soil extractable P tests from cultivated fields results from nearly 1 300 000 samples are available for the whole of France with a cantonal (roughly 20-km) resolution ii.the Finistere monitoring network, covers186 fields from western France with slurry loads monitored since 1973.Strong regional disparities are highlighted with in particular large frequencies of high soil content of extractable P in several regions, e.g.Brittany, Nord-Pas-de-Calais Alsace.A detailed study in Brittany corroborates at cantonal level a clear relation between the median Dyer P content of soil the P balance of agricultural practices, with a major influence by the cumulated spreading of animal manure.Nevertheless, the existing databases were designed for agronomic purposes give only indirect indications about the risk to the environment by soil P, as only a fraction of total P is measured.Moreover, to estimate the risk of P transfer to surface water by runoff or lixiviation, soil P retention capacity the saturation level have also to be considered.Such indicators were developed in Canada for regulatory purposes have been tested in an exploratory approach on soil samples from Brittany.Lastly, the influence of farming techniques (minimum tillage, grass strips, spread of animal manure) which strongly impact the concentration of P at the soil surface require better assessment.