Saltmarsh soil evolution after land reclamation in Atlantic estuaries (Bay of Biscay, North coast of Spain)

Tidal wetland soils exhibit particular physical–chemical properties governed mainly by tidal dynamics. On the Atlantic seaboard of the Iberian Peninsula a great swath of tidal wetlands, which correspond to semi-confined estuaries, represents land reclaimed from the 18th century to the first half of the 20th century and used for urban development or agricultural production. These interventions led to the end of tidal flooding in extensive areas of salt marshes, causing a drop in the water table that triggered pyrite oxidization processes, leaching of cations and salts and soil acidification. In many cases, these transformations have given rise to environmental problems such as the development of acid sulphate soils, a resulting decrease in pH, and the leaching of carbonates. The organic carbon storage capacity has also been altered, and the release of metal cations from sediments has caused toxic waters to appear.In order to investigate the alterations caused by the historic reclamation of estuarine soils on the north coast of Spain, natural and reclaimed lands were mapped in two estuaries (Villaviciosa and San Vicente de la Barquera) and zoned in accordance with flood frequency.The different evolutionary paths of the two studied estuarine sites after reclamation can be assessed through the description and analysis of their soils despite reclamation, some parts (i.e. lower areas) of the reclaimed lands do not remain fully isolated from tidal dynamics, whose influence varies throughout the soil profiles that have developed since. The strength of the morphological (e.g. genetic horizons), chemical (pH, metal cation concentration) and functional (balance of the carbon budget) changes suffered by the original soils seem to depend strongly on the remaining tidal influence as well as the land use of the area.