Effects of storm Lothar (1999) on the chemical composition of soil solutions and on herbaceous cover,humus and soils (Fougères,France)

Examination of the effect of the 1999 storm on soil solution quality in an 81-year-old monitored plot in Fougères forest has not only made it possible to gain a better understanding of the impact of intensive silviculture in this ecosystem with low mineral fertility, but also to show that the cessation of internal recycling of chemical elements can be prevented by the presence of vegetation. The storm in December, 1999 resulted in several sudden openings in the forest cover, resulting in humus mineralization; the moder humus present before the storm was transformed rapidly into an oligomull. In the absence of any vegetation, the nitrates produced at the surface migrated down the profile by slow convective flux and the concentrations at depths of −10 and −30 cm increased during 2000. Herbaceous vegetation gradually replaced the arborescent strata present before the storm, taking-up the nitrate at the surface and thus limiting leaching into the deeper horizons. As convective transport is a slow process, the concentrations of nitrate in the soil solutions at depths of −55, −80 and −120 cm increased for about a year after the storm; then they decreased and stabilised in 2002. Nitrate leaching was accompanied by the migration of Al, Mg, K and Ca down the soil profile, depending on their relative availability. After the definitive installation of the herbaceous strata, the concentrations of elements in soil solutions returned to levels below those measured before the storm.