High Chemical Weathering induced by agricultural pressure. In : Goldschmidt Conference, 5-11 juin 2004, Copenhague, Danemark

Chemical weathering on first-order catchments / The Kerrien (0.095 km2) and Kerbernez (0.120 km2) watersheds, two first-order adjacent catchments, located in south western Brittany (France) are experimental sites for the / quantification of chemical weathering in a catchment dominated by agricultural activities. These watersheds are located on a granitic basement under oceanic temperate / climatic conditions. In order to quantify the chemical weathering, the runoff, / the precipitations and chemical composition of stream waters were measured and calculated. The chemical fluxes are corrected from the atmospheric and the agricultural inputs, as / well as from the water balance,using P-PET for the runoff. The estimate of the atmospheric inputs is based on the chemical composition of local precipitations, sampled and / analyzed monthly from january 2001 to november 2002. This period corresponds to two hydrologically contrasted years. The agricultural inputs consist mostly of organic (cattle / manure and pig slurry, ~4 m3.yr-1 ) and mineral (KCl, NPK and ammonium nitrate, ~160 kg.ha-1.yr-1) fertilizers. A database of the various fertilizers used by local farmers on a / 10-year period, coupled with chemical composition of local fertilizers and data from the literature, has been established. / / High chemical weathering / Major agricultural cation input is potassium,which is almost consumed by local biosphere. The other cation (Ca2+, Mg2+ and Na+) fluxes range from 100 to 7000 mol.ha-1.yr-1, / which is higher, by a factor 2 to 10, than those of small catchments on granitic basement [1]. Agricultural inputs seem to control and increase the chemical weathering by either (1) a preferential mineral dissolution, or (2) release by leaching of cations from old agricultural inputs adsorbed on soil particles. The first major impact of such chemical fluxes is the / chemistry of soil solution and soil composition and thus eventually on soil productivity.