Properties and Classification of the Tepui Peats

Abstract

In the Guayana Highlands, peats have formed along an elevation gradient spanning roughly from 600 m to 2,800 m a.s.l. They have been studied earlier in the eastern highlands from a paleogeographic point of view using radiocarbon dating and pollen analysis, but little is known about their morphological, physical, and chemical characteristics, and their taxonomic classification. This study reports on the properties of peatsoils that have developed on sandstone–quartzite mesetas (tepuis) and on igneous–metamorphic massifs in the western Guayana Highlands. Chapter 6 describes and discusses the morphological, physical, and chemical properties of the peatsoils together with their spatial variations. Taxonomic classification of the soils is presented and issues related with the classification are discussed. Most physical and chemical properties show moderate to high spatial variability, controlled by the position on the landscape, the drainage conditions, the nature of and depth to bedrock, and differences in the age of the peat materials. Average thickness of the soil profiles above the lithic substratum, including organic and mineral materials, varies from 40 to 150 cm. Peatsoils consist commonly of an Oo–Oi–Oe layer sequence. Root content decreases with soil depth. Lighter colors in the Oi layers indicate a lower degree of decomposition and higher fiber content than in deeper layers. Dry matter increases with soil depth. Unrubbed and rubbed fibers mostly decrease with depth, except in organic soils rich in mineral matter. The mineral content increases with soil depth and ranges from zero to more than 70%. Wet bulk density values vary from 0.54 to 0.78 Mg m⁻³, while dry bulk density values range from 0.04 to 0.094 Mg m⁻³ in soils with more than 90% organic matter. Field water content is high (average 1,226%) and tends to decrease with soil depth. Average contents of organic matter by loss on ignition and Walkley–Black are 74 and 28%, respectively. Soils are very acid with pH values of 3.1–5.3. Total acidity decreases with soil depth, while salt-replaceable acidity increases from Oi to Oa layers (3.1–5.6 cmol(+) kg⁻¹). The cation exchange capacity by sum of bases plus total acidity varies between 20 and 187 cmol(+) kg⁻¹, and the effective cation exchange capacity determined by sum of bases plus salt-replaceable acidity varies between 2 and 12 cmol(+) kg⁻¹. Both organic and organomineral soils are impoverished in exchangeable Ca, Mg, Na, and K with depth. For the overall organic tier, the relative abundance of exchangeable bases is approximately Ca²⁺ > Na¹⁺ > K¹⁺ > Mg²⁺. Most soils classify as Saprists followed by Hemists and Fibrists.