Impact de l'environnement agricole sur une tourbière de maar ; conséquences pour sa protection (Impact of agricultural surroundings on a maar peat bog ; consequences for its conservation)

摘要

Impact of agricultural surroundings on a maar peat bog ; consequences for its conservation. Among the maar peat bogs of Velay (Massif Central Français), only the "Marais de Limagne" has escaped, for historical reasons, the deep deterioration which has affected such places. Protected by an order of conservation of the tope since 1987, on account of its very rich flora (7 protected species by national decree), it is the matter of regular observing. On the periphery of the peat bog, the apparition and development, in a few years, of plant species indicating an enrichment in minerals has led to assess the impact of farming in the site. In 1991, a year of drought, the peat bog waters were more nitrate concentrated on the outskirts than at the centre of it, and richer when facing the cultivated or grazed areas than when facing the wooded areas. In the soils, no trace of accumulated mineral along the slope of the maar was found. In 1992, a rainy year, the gradient of concentration was maintained for the nitrates (more concentrated compared to 1991) and also, for phosphorus. In the meantime, soil nitrate level increased with soil depth and further down the slope. In the meantime, the analysis of numerous data on the farming ecosystem maar has been carried out. It proved that leaching and runoff may become eventually important. Morover, land utilization system generates risks : short lived temporary grasslands, level of fertilization (though low) often exceeding exportation. So, the Marais de Limagne is endangered, at least during the wet years. A solution, having regard to the way of farming and reducing the mineral inputs in the peat bog, could be the belting of the « marais » into permanent reaped grasslands. The DDAF has spotted out this site to benefit from the EEC agrienvironmental measures. Unfortunately, this solution is impaired by : a lack of interest by the farmers in the peat bog ; the underlying rivalry beetween two boroughs on which the "marais" is placed astride.