Clay mineralogy of the soils in the south Ecuadorian páramo region

Abstract

The páramo soils of the mountainous upper Andean region (>3300 m a. s. l.) of the Rio Paute basin in central Ecuador are characterized by a thick, dark, highly organic epipedon and are classified as Andosols and Histosols. Their high water retention and buffering capacity play a key role in the hydrology of the region, which is subject to land use changes and increased cultivation. In the west (Western Cordillera), the soils are largely formed in the late Miocene and Pliocene volcanoclastic Tarqui formation, while in the east (Central Cordillera) they are formed in an older, mostly intermediate low-grade metamorphic rocks. Ten soil profiles were sampled and studied, using extraction techniques (oxalate and pyrophosphate) and XRD-techniques. Major differences in composition of the clay fractions were found that allow for distinction of three main groups of páramo soils. A first group consists of soils influenced by recent volcanic ashes and dominated by organometallic complexes and with minor but distinct amounts of degraded mica, most probably formed by weathering of primary mica, present in these ashes. The second group comprises soils formed in volcanoclastic material of various Tertiary and earlier formations, containing residual primary and secondary crystalline clay-size minerals, as well as organometallic complexes whose genesis can be linked to the abundant presence of easily weatherable materials in these formations. A third group consists of soils in relicts of Tertiary, highly weathered regolith, formed under humid tropical conditions before the Andean uplift and occurring in the Central Cordillera. These soils contain kaolinite and gibbsite and develop into Histosols in the absence of significant organometallic complexation.

Introduction

Soils of the high altitude tropical grasslands or páramo's of the Northern Andes have thick, dark, highly organic epipedons, largely because of the cold and wet climate and the resulting poor decomposition of organic matter. An additional factor is the widespread occurrence of recent volcanic ashes, giving rise to extensive development of Andosols through weathering of these ashes to specific soil compounds that bind organic matter and thus limit decomposition, notably allophanes and other high alumina secondary minerals. A particularly interesting situation occurs in the Rio Paute basin (Cuenca, middle Ecuador) that lies in the border zone of the recent volcanic ash deposits from the major volcanoes in central Ecuador, including the Sanguay and Tungarahua. In the north of this basin, the páramo soils contain a fair amount of volcanic ash and can largely be classified as non-allophanic Andosols, while to the south and east they grade into volcanic ash free Histosols (Buytaert, 2004), developed on sedimentary and low grade metamorphic rocks.

The páramo region is threatened by both erosion and soil degradation due to land use changes and cultivation practices brought about by increasing population pressure and land expansion (Hofstede, 1995). These risks are studied in a joint program on soil and water management in the Rio Paute basin (PROMAS) executed by the Universities of Cuenca (Ecuador) and Leuven (Belgium). These land use changes have a significant impact on the water regulation capacity of the páramo. Soil structure degradation may lead to changes in water retention and hydraulic conductivity (Buytaert et al., 2002, Buytaert, 2004), as well as increased hydrophobicity and water erosion sensitivity (Poulenard et al., 2001). In the Rio Paute basin, the soil physical properties show regional patterns that appear to be connected with differences in soil parent material (Buytaert, 2004).

Soil physical properties are known to be influenced by the mineralogy of the soils concerned, more particularly that of their clay fraction. For the Rio Paute basin, major differences in composition of these clay fractions can be expected to occur in connection with large differences in parent material. However, as will be elucidated further on, very limited information is available on the clay mineralogy of the soils in the area concerned and, in fact, for Ecuador as a whole; the rare existing studies concentrating on selected Andosols. This paper concerns a first regional study of the clay mineralogy of Ecuadorian páramo soils, among which soils that are not developed in recent volcanic ashes, and aims to provide some insight into the range of clay mineral assemblages that may be encountered in this region and environment.