Spatial patterns of soil salinities in old embanked marshlands in western France

Abstract

The spatial pattern of soil salinity was studied in commonly owned grazing meadows, embanked in the distant past (10th century), situated on the Atlantic coast of western France. The meadows were characterized by micro-relief, a relic of former salt marshes, with an amplitude that varied between 15 and 65 cm. The topographic sequence consisted of a mosaic of seasonally flooded depressions, of slopes, and of areas of flat, higher ground. The soils of the various topographic levels that were studied at 6 sites had different electrical conductivities and sodicities. The highest values occurred in slope soils (conductivity between 0.83 and 2.85 mS/cm), especially at the base of slopes, whereas the lowest values were typical of the soils of depressions (conductivity between 0.42 and 0.56 mS/cm). The deepest soil horizon (Cg) always had a higher sodicity and conductivity than that of the surface horizon at all topographic levels. The groundwater specific conductivity showed similar tendencies. A more intensive sampling of the spatial pattern of soil salinity at the Magnils-Reigniers site confirmed these results. In this sampling, only the conductivity of the soil-water extract was measured, as high correlations were shown at the 6 sites between the conductivity, sodicity, the exchangeable sodium content and soluble sodium content of the soil. The spatial pattern of salinity and sodicity can be related to the spatial structure of the vegetation, as salt-tolerant species (Juncus gerardi andHordeum marinum) occur on the slopes. This type of system does not seem to have been described elsewhere and appears to be related, via hydrologic mechanisms and the physical action of grazing animals, to the traditional management of the meadows for grazing.