Pascal Podwojewski

Runoff and soil erosion under rainfall simulation of Andisols from the Ecuadorian Páramo: effect of tillage and burning

Abstract In northern Ecuador, soils of high altitude grasslands (paramos) are mainly non-allophanic Andisols developed on Holocenic volcanic ash. These soils have a high water retention capacity and are the “water tank” of central Ecuador. To assess the effect of land use (burning and tillage) on soil hydrodynamic properties, rainfall simulation was conducted at two different sites. At Pichincha near Quito, the simulation was conducted on a recent volcanic ash soil comparing natural, tilled and burned plots. At El Angel, the simulation was conducted on a mature non-allophanic Andisol comparing natural, recently tilled and formerly cultivated plots. On natural plots, the infiltration rate was very high and sediment loss very low. Results for infiltration rate and runoff indicated that land use change on paramos increased runoff flow and reduced saturated hydraulic conductivity. Superficial reorganisation of the soil surfaces occurred on tilled plots at both sites. This crusting process was fast and resulted in surfaces with very low conductivity at Pichincha. The same processes seemed to be slower at El Angel. The soil surface of recent Andisols at Pichincha was prone to crusting whereas the mature Andisol, at El Angel, with a lower bulk density, was compacted when the kinetic energy of raindrops was high. Water repellency occurred after burning at Pichincha and following long natural air drying after tillage in the non-allophanic A horizon at El Angel. Water repellency combined with the low bulk density of soil aggregates explain the intensity of sediment losses in the abandoned soils after cultivation (Bare fallow plots). Erosion occurred in these areas through floating hydrophobic and stable aggregates.

Characteristics of non-allophanic Andisols with hydric properties from the Ecuadorian páramos.-

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

The occurrence and interpretation of carbonate and sulfate minerals in a sequence of Vertisols in New Caledonia

Abstract In the Tamoa Valley, on the western coast of New Caledonia, a Vertisol sequence shows a downslope transition from calcimagnesic Vertisols containing gypsum, aragonite, and magnesium-calcite, derived from flysch colluvium, to hypermagnesic Vertisols, rich in dolomite and magnesite, and derived from peridotite and serpentinite alluvium deposited by the Tamoa River. Gypsum, magnesium-calcite and magnesite were probably formed during the dry period of the last glaciation, between 30,000 and 17,000 yr B.P. Magnesite nodules seem to be allochtonous, and were formed upstream by the weathering of serpentinite. During the more humid period of last Flandrian transgression (5000 yr B.P.), floods of the magnesium-rich waters of the Tamoa River over the alluvial plain brought magnesium to the soils and lead to fluctuations of the water table. These conditions favoured the formation of manganese deposits as mangans, calcite pseudomorphs after lenticular gypsum, crystallization of authigenic barite in calcimagnesic Vertisols, and dolomite formation in magnesic and hypermagnesic Vertisols. During the recent period, apparently drier, aragonite is forming in the lower part of calcimagnesic Vertisols.