干旱区生态环境知识资源中心

A large part of Les Garrigues (Western Catalonia) is occupied by stone terraces (locally known as bancals) that were built up in the 18th century. Some soil surveys of the area noted the presence of buried, thick A horizons with a well developed crumb structure and darker colour than the present-day A horizons. The objective of this study was to determine the conditions of the soil cover both at the time of the formation of these buried soils and at the onset of terracing.

The climate is semi-arid and the landscape is defined by platforms and flat-bottomed valleys. Four profiles were described from both slope and valley-floor terraces and sampled for analyses. A buried Ahb horizon appears at various depths between 40 and 180 cm, with thicknesses ranging from 35 to 160 cm, probably due to earthworks since this depends on the position of the profile within the terrace. Many of these buried soils would correspond to Phaeozems: they are identified by an Ahb having low values and chromas (usually 4 or less), a structure due to faunal activity (100% of the structural forms related to fauna either in the form of empty or infilled channels, vermic qualifier), and an organic matter content that is 1.7-3.3%, which is higher but not significantly different than the SOM content of the present day topsoils. P (Olsen) is 1-6 ppm.

The present soils are non-saline and highly calcareous (>40%), they show some calcium carbonate redistribution in the form of pseudomycelia, and biogenic calcite. Charcoal is present in some of these buried horizons, together with small ceramic fragments. These buried horizons have several common micromorphological features: a spongy, highly porous structure due to high faunal activity, frequent silt cappings, charcoal fragments, and biogenic carbonates (queras). In some cases the biogenic calcite has undergone dissolution and reprecipitated as micrite. Pollen assemblages of the buried horizons reveal a large forest cover mainly of oaks and pines, which is also corroborated by the occurrence of Quercus phytoliths; but the presence of pollen of some deciduous crops (such as cereals, Vitis vinifera and Olea europaea.) at the top of the sequences points to a Upper Holocene origin of these soils. The results from the charcoal study support the presence of perennial Quercus and pines as well as shrubs in the period when the terraces were built.

The available information about present day similar soils indicates that the formation of these horizons took place under a moister, milder climate in the past, compared with the present one in the study area. (C) 2014 Elsevier B.V. All rights reserved.