Contribution of livestock farming systems to the nitrogen cascade and consequences for farming regions

摘要

This article describes the nitrogen flows in the environment and points to the specificities of the livestock production. Till the beginning ofthe 20th century, the symbiotic fixation and the recycling ofanimal excreta supplied the nitrogen necessaryfor the fertility ofsoil. In 1913, the Haber-Bosch process allowed the industrial synthesis of ammonia and made possible the fertilisation without association of crop production with the livestock farming. The efficiency of the nitrogen in livestock farming is low with nearlyhalformoreoftheinputslossestotheenvironment.Theselosseshavediverseimpactsthatinterveneatvariousspatial scales owing to the nitrogen cascade. Quantitative assessment of nitrogen flows at the scale ofregions started in the early 1980sinWesternEuropeandNorthAmerica.Thesestudiesprovidedestimatesofthespatialvariabilityofnitrogendischargewithina region. They confirmed the differences between areas with a high animal densitysuch as Brittany (western region, France) and otherregions.Itwasalsofoundthatthesamenitrogenouslossescouldleadtodifferentlevelsofenvironmentalimpacts according to the sensibility ofa given environment and its capacity to cope with nitrogen excess. Climate, soils characteristics, animaldensity,andproportionsofagriculturallandunderannualandperennialcropsaredriversofthissensibility.