Characteristics of non-allophanic andisols with hydric properties from the Ecuadorian paramos

So far, the occurrence of hydric properties (i.e. water retention values at 1500 kPa matrix potential larger than 1000 g kg(-1)) has generally been reported in Andisols characterised by advanced stages of weathering. Moreover, as these properties are due to the presence of short-range-ordered (SRO) minerals, they are frequently better expressed in the subsurface than in the surface horizons of these Andisols. The present paper describes the characteristics of three Hydric Melanudands whose properties distinctly depart from those briefly summarised above. These soils have been found in some of high-altitude (3200-4000 m a.s.l.) paramos of Ecuador where they are likely to cover several thousands of km(2). They either derived from or have been rejuvenated by volcanic ash deposited more recently than 3000 years BP and they develop under climatic and vegetation conditions favouring exceptionally large accumulations of organic matter. As a consequence, the deep (50-80 cm) epipedons of these Melanudands exhibit an unusual combination of properties. Though rich in primary weatherable minerals and virtually devoid of allophane, these epipedons have nevertheless 1500 kPa water contents either close to or even much larger than 1000 g kg(-1). In these Hydric Melanudands, organic colloids are the constituents responsible for their large porosity. Hydric Andisols are known to experience important irreversible physical changes on drying and this behaviour, in turn, makes them very fragile. As the hydric properties occur here in surface horizons, these paramos soils are thus expected to be especially susceptible to any land use change. (C) 2003 Elsevier Science B.V. All rights reserved.