Studying the origin and evolution of the agricultural landscape in NW Iberia using terraced soils

Abstract

We sampled and analyzed with high-resolution a polycyclic terraced soil located in the surroundings of Santiago de Compostela, looking for the imprints of the different ancient agricultural practices. To this aim a combination of techniques were applied (AAS, XRF, IR-MS) together with selective dissolution extractions (for Al, Si and Fe) and the determination of more traditional physico-chemical properties (soil reaction, pH in NaF, phosphorous retention, etc.). The aim was to identify the signals of land use changes and the different agricultural management techniques applied since the beginning of the agricultural use of the soil. The sequence contained a buried paleosol, with strong andic character, which is considered to exemplify the soil properties prior to the construction of the terraces. In the upper anthropogenic soil layers the relative amounts of soil components responsible for andic properties decrease, presumably as a consequence of the introduction of new agricultural techniques that lead to changes in the chemical stability of organo-metal complexes, thus influencing its accumulation. Variations in acidity, P and Ca contents, and the isotopic composition (13C, 15N) of the organic matter, point to the use of fertilization and liming practices and/or the introduction of new agricultural species. The radiocarbon dating of soil samples indicates that the intensification of agricultural use of this soil and its strong anthropization started in the Early Middle Ages. Our research suggests that the current configuration of this agricultural landscape is mainly inherited from those early modifications, and that the application of several techniques to proper archives (like terraced soils) may help to reveal the history of the build up of the present cultural landscape.