1500 years of soil use reconstructed from the chemical properties of a terraced soil sequence

Colluvial soils can store signals of the environmental conditions occurred during their formation, including the anthropogenic activities they supported. Their study may provide important information for reconstructing and interpreting the evolution of cultural landscapes. We studied the chemical properties of a terraced soil system located in the town of Santiago de Compostela (NW Spain). Aluminum, Fe and Si fractionation was studied by selective dissolution techniques with high vertical resolution, combined with elemental composition and other soil properties such as phosphate retention (Pret) and NaF pH, aiming to identify modifications produced by land use changes and agricultural management practices since Antiquity. The buried epipedon of the paleosol, which is considered to exemplify the pre-terracing soil, showed strong andic character. We argue that its attenuation in the anthropogenic soil layers is, firstly, a consequence of the decreasing amounts of reactive components (organic matter, organo-Al complexes and low ordered aluminosilicates) due to a dilution by mixing superficial and sub-superficial horizons. Secondly, the introduction of agricultural techniques led to modifications in the chemical stability of organo-metal complexes influencing the accumulation of organic matter. Other signals, such as variations in soil acidity, and P and Ca contents, point to management practices as fertilization or liming. The increasing amounts of Fe inorganic compounds in the more recent layers indicate a strong weathering and degradation, probably as a consequence of the intensification of the agricultural use. Our results indicated continued and progressively more intensive agricultural use during the last 15 centuries, linked to the development of the town.